Appendix A to Part 36 - Aircraft Noise Measurement and Evaluation Under § 36.101
14:1.0.1.3.20.14.283.1.36 : Appendix A
Appendix A to Part 36 - Aircraft Noise Measurement and Evaluation
Under § 36.101 Sec. A36.1
Introduction. A36.2
Noise
Certification Test and Measurement Conditions. A36.3
Measurement of Airplane Noise Received on the Ground. A36.4
Calculations of Effective Perceived Noise Level From Measured
Data. A36.5
Reporting of Data to the FAA. A36.6
Nomenclature: Symbols and Units. A36.7
Sound Attenuation
in Air. A36.8 [Reserved] A36.9
Adjustment of Airplane Flight
Test Results. Section A36.1 Introduction
A36.1.1 This appendix prescribes the conditions under which
airplane noise certification tests must be conducted and states the
measurement procedures that must be used to measure airplane noise.
The procedures that must be used to determine the noise evaluation
quantity designated as effective perceived noise level, EPNL, under
§§ 36.101 and 36.803 are also stated.
A36.1.2 The instructions and procedures given are intended to
ensure uniformity during compliance tests and to permit comparison
between tests of various types of airplanes conducted in various
geographical locations.
A36.1.3 A complete list of symbols and units, the mathematical
formulation of perceived noisiness, a procedure for determining
atmospheric attenuation of sound, and detailed procedures for
correcting noise levels from non-reference to reference conditions
are included in this appendix.
A36.1.4 For Stage 4 airplanes, an acceptable alternative for
noise measurement and evaluation is Appendix 2 to ICAO Annex 16,
Volume I, Amendment 7 (incorporated by reference, see § 36.6).
A36.1.5 For Stage 5 airplanes, an acceptable alternative for
noise measurement and evaluation is Appendix 2 to ICAO Annex 16,
Volume 1, Amendment 11-B (incorporated by reference, see §
36.6).
Section A36.2 Noise Certification Test and Measurement Conditions
A36.2.1 General.
A36.2.1.1 This section prescribes the conditions under which
noise certification must be conducted and the measurement
procedures that must be used.
Note:
Many noise certifications involve only minor changes to the
airplane type design. The resulting changes in noise can often be
established reliably without resorting to a complete test as
outlined in this appendix. For this reason, the FAA permits the use
of approved equivalent procedures. There are also equivalent
procedures that may be used in full certification tests, in the
interest of reducing costs and providing reliable results. Guidance
material on the use of equivalent procedures in the noise
certification of subsonic jet and propeller-driven large airplanes
is provided in the current advisory circular for this part.
A36.2.2 Test environment.
A36.2.2.1 Locations for measuring noise from an airplane in
flight must be surrounded by relatively flat terrain having no
excessive sound absorption characteristics such as might be caused
by thick, matted, or tall grass, shrubs, or wooded areas. No
obstructions that significantly influence the sound field from the
airplane must exist within a conical space above the point on the
ground vertically below the microphone, the cone being defined by
an axis normal to the ground and by a half-angle 80° from this
axis.
Note:
Those people carrying out the measurements could themselves
constitute such obstruction.
A36.2.2.2 The tests must be carried out under the following
atmospheric conditions.
(a) No precipitation;
(b) Ambient air temperature not above 95 °F (35 °C) and not
below 14 °F (−10 °C), and relative humidity not above 95% and not
below 20% over the whole noise path between a point 33 ft (10 m)
above the ground and the airplane;
Note:
Care should be taken to ensure that the noise measuring,
airplane flight path tracking, and meteorological instrumentation
are also operated within their specific environmental
limitations.
(c) Relative humidity and ambient temperature over the whole
noise path between a point 33 ft (10 m) above the ground and the
airplane such that the sound attenuation in the one-third octave
band centered on 8 kHz will not be more than 12 dB/100 m
unless:
(1) The dew point and dry bulb temperatures are measured with a
device which is accurate to ±0.9 °F (±0.5 °C) and used to obtain
relative humidity; in addition layered sections of the atmosphere
are used as described in section A36.2.2.3 to compute equivalent
weighted sound attenuations in each one-third octave band; or
(2) The peak noy values at the time of PNLT, after adjustment to
reference conditions, occur at frequencies less than or equal to
400 Hz.;
(d) If the atmospheric absorption coefficients vary over the
PNLTM sound propagation path by more than ±1.6 dB/1000 ft (±0.5
dB/100m) in the 3150Hz one-third octave band from the value of the
absorption coefficient derived from the meteorological measurement
obtained at 33 ft (10 m) above the surface, “layered” sections of
the atmosphere must be used as described in section A36.2.2.3 to
compute equivalent weighted sound attenuations in each one-third
octave band; the FAA will determine whether a sufficient number of
layered sections have been used. For each measurement, where
multiple layering is not required, equivalent sound attenuations in
each one-third octave band must be determined by averaging the
atmospheric absorption coefficients for each such band at 33 ft (10
m) above ground level, and at the flight level of the airplane at
the time of PNLTM, for each measurement;
(e) Average wind velocity 33 ft (10 m) above ground may not
exceed 12 knots and the crosswind velocity for the airplane may not
exceed 7 knots. The average wind velocity must be determined using
a 30-second averaging period spanning the 10 dB-down time interval.
Maximum wind velocity 33 ft (10 m) above ground is not to exceed 15
knots and the crosswind velocity is not to exceed 10 knots during
the 10 dB-down time interval;
(f) No anomalous meteorological or wind conditions that would
significantly affect the measured noise levels when the noise is
recorded at the measuring points specified by the FAA; and
(g) Meteorological measurements must be obtained within 30
minutes of each noise test measurement; meteorological data must be
interpolated to actual times of each noise measurement.
A36.2.2.3 When a multiple layering calculation is required by
section A36.2.2.2(c) or A36.2.2.2(d) the atmosphere between the
airplane and 33 ft (10 m) above the ground must be divided into
layers of equal depth. The depth of the layers must be set to not
more than the depth of the narrowest layer across which the
variation in the atmospheric absorption coefficient of the 3150 Hz
one-third octave band is not greater than ±1.6 dB/1000 ft (±0.5
dB/100m), with a minimum layer depth of 100 ft (30 m). This
requirement must be met for the propagation path at PNLTM. The mean
of the values of the atmospheric absorption coefficients at the top
and bottom of each layer may be used to characterize the absorption
properties of each layer.
A36.2.2.4 The airport control tower or another facility must be
aproved by the FAA for use as the central location at which
measurements of atmospheric parameters are representative of those
conditions existing over the geographical area in which noise
measurements are made.
A36.2.3 Flight path measurement.
A36.2.3.1 The airplane height and lateral position relative to
the flight track must be determined by a method independent of
normal flight instrumentation such as radar tracking, theodolite
triangulation, or photographic scaling techniques, to be approved
by the FAA.
A36.2.3.2 The airplane position along the flight path must be
related to the noise recorded at the noise measurement locations by
means of synchronizing signals over a distance sufficient to assure
adequate data during the period that the noise is within 10 dB of
the maximum value of PNLT.
A36.2.3.3 Position and performance data required to make the
adjustments referred to in section A36.9 of this appendix must be
automatically recorded at an approved sampling rate. Measuring
equipment must be approved by the FAA.
Section A36.3 Measurement of Airplane Noise Received on the Ground
A36.3.1 Definitions.
For the purposes of section A36.3 the following definitions
apply:
A36.3.1.1 Measurement system means the combination of
instruments used for the measurement of sound pressure levels,
including a sound calibrator, windscreen, microphone system, signal
recording and conditioning devices, and one-third octave band
analysis system.
Note:
Practical installations may include a number of microphone
systems, the outputs from which are recorded simultaneously by a
multi-channel recording/analysis device via signal conditioners, as
appropriate. For the purpose of this section, each complete
measurement channel is considered to be a measurement system to
which the requirements apply accordingly.
A36.3.1.2 Microphone system means the components of the
measurement system which produce an electrical output signal in
response to a sound pressure input signal, and which generally
include a microphone, a preamplifier, extension cables, and other
devices as necessary.
A36.3.1.3 Sound incidence angle means in degrees, an
angle between the principal axis of the microphone, as defined in
IEC 61094-3 and IEC 61094-4, as amended and a line from the sound
source to the center of the diaphragm of the microphone
(incorporated by reference, see § 36.6).
Note:
When the sound incidence angle is 0°, the sound is said to be
received at the microphone at “normal (perpendicular) incidence;”
when the sound incidence angle is 90°, the sound is said to be
received at “grazing incidence.”
A36.3.1.4 Reference direction means, in degrees, the
direction of sound incidence specified by the manufacturer of the
microphone, relative to a sound incidence angle of 0°, for which
the free-field sensitivity level of the microphone system is within
specified tolerance limits.
A36.3.1.5 Free-field sensitivity of a microphone system
means, in volts per Pascal, for a sinusoidal plane progressive
sound wave of specified frequency, at a specified sound incidence
angle, the quotient of the root mean square voltage at the output
of a microphone system and the root mean square sound pressure that
would exist at the position of the microphone in its absence.
A36.3.1.6 Free-field sensitivity level of a microphone
system means, in decibels, twenty times the logarithm to the
base ten of the ratio of the free-field sensitivity of a microphone
system and the reference sensitivity of one volt per Pascal.
Note:
The free-field sensitivity level of a microphone system may be
determined by subtracting the sound pressure level (in decibels re
20 µPa) of the sound incident on the microphone from the voltage
level (in decibels re 1 V) at the output of the microphone system,
and adding 93.98 dB to the result.
A36.3.1.7 Time-average band sound pressure level means in
decibels, ten times the logarithm to the base ten, of the ratio of
the time mean square of the instantaneous sound pressure during a
stated time interval and in a specified one-third octave band, to
the square of the reference sound pressure of 20 µPa.
A36.3.1.8 Level range means, in decibels, an operating
range determined by the setting of the controls that are provided
in a measurement system for the recording and one-third octave band
analysis of a sound pressure signal. The upper boundary associated
with any particular level range must be rounded to the nearest
decibel.
A36.3.1.9 Calibration sound pressure level means, in
decibels, the sound pressure level produced, under reference
environmental conditions, in the cavity of the coupler of the sound
calibrator that is used to determine the overall acoustical
sensitivity of a measurement system.
A36.3.1.10 Reference level range means, in decibels, the
level range for determining the acoustical sensitivity of the
measurement system and containing the calibration sound pressure
level.
A36.3.1.11 Calibration check frequency means, in hertz,
the nominal frequency of the sinusoidal sound pressure signal
produced by the sound calibrator.
A36.3.1.12 Level difference means, in decibels, for any
nominal one-third octave midband frequency, the output signal level
measured on any level range minus the level of the corresponding
electrical input signal.
A36.3.1.13 Reference level difference means, in decibels,
for a stated frequency, the level difference measured on a level
range for an electrical input signal corresponding to the
calibration sound pressure level, adjusted as appropriate, for the
level range.
A36.3.1.14 Level non-linearity means, in decibels, the
level difference measured on any level range, at a stated one-third
octave nominal midband frequency, minus the corresponding reference
level difference, all input and output signals being relative to
the same reference quantity.
A36.3.1.15 Linear operating range means, in decibels, for
a stated level range and frequency, the range of levels of steady
sinusoidal electrical signals applied to the input of the entire
measurement system, exclusive of the microphone but including the
microphone preamplifier and any other signal-conditioning elements
that are considered to be part of the microphone system, extending
from a lower to an upper boundary, over which the level
non-linearity is within specified tolerance limits.
Note:
Microphone extension cables as configured in the field need not
be included for the linear operating range determination.
A36.3.1.16 Windscreen insertion loss means, in decibels,
at a stated nominal one-third octave midband frequency, and for a
stated sound incidence angle on the inserted microphone, the
indicated sound pressure level without the windscreen installed
around the microphone minus the sound pressure level with the
windscreen installed.
A36.3.2 Reference environmental conditions.
A36.3.2.1 The reference environmental conditions for specifying
the performance of a measurement system are:
(a) Air temperature 73.4 °F (23 °C);
(b) Static air pressure 101.325 kPa; and
(c) Relative humidity 50%.
A36.3.3. General.
Note:
Measurements of aircraft noise that are made using instruments
that conform to the specifications of this section will yield
one-third octave band sound pressure levels as a function of time.
These one-third octave band levels are to be used for the
calculation of effective perceived noise level as described in
section A36.4.
A36.3.3.1 The measurement system must consist of equipment
approved by the FAA and equivalent to the following:
(a) A windscreen (See A36.3.4.);
(b) A microphone system (See A36.3.5):
(c) A recording and reproducing system to store the measured
aircraft noise signals for subsequent analysis (see A36.3.6);
(d) A one-third octave band analysis system (see A36.3.7);
and
(e) Calibration systems to maintain the acoustical sensitivity
of the above systems within specified tolerance limits (see
A36.3.8).
A36.3.3.2. For any component of the measurement system that
converts an analog signal to digital form, such conversion must be
performed so that the levels of any possible aliases or artifacts
of the digitization process will be less than the upper boundary of
the linear operating range by at least 50 dB at any frequency less
than 12.5 kHz. The sampling rate must be at least 28 kHz. An
anti-aliasing filter must be included before the digitization
process.
A36.3.4 Windscreen.
A36.3.4.1 In the absence of wind and for sinusoidal sounds at
grazing incidence, the insertion loss caused by the windscreen of a
stated type installed around the microphone must not exceed ±1.5 dB
at nominal one-third octave midband frequencies from 50 Hz to 10
kHz inclusive.
A36.3.5 Microphone system.
A36.3.5.1 The microphone system must meet the specifications in
sections A36.3.5.2 to A36.3.5.4. Various microphone systems may be
approved by the FAA on the basis of demonstrated equivalent overall
electroacoustical performance. Where two or more microphone systems
of the same type are used, demonstration that at least one system
conforms to the specifications in full is sufficient to demonstrate
conformance.
Note:
An applicant must still calibrate and check each system as
required in section A36.3.9.
A36.3.5.2 The microphone must be mounted with the sensing
element 4 ft (1.2 m) above the local ground surface and must be
oriented for grazing incidence, i.e., with the sensing
element substantially in the plane defined by the predicted
reference flight path of the aircraft and the measuring station.
The microphone mounting arrangement must minimize the interference
of the supports with the sound to be measured. Figure A36-1
illustrates sound incidence angles on a microphone.
A36.3.5.3 The free-field sensitivity level of the microphone and
preamplifier in the reference direction, at frequencies over at
least the range of one-third-octave nominal midband frequencies
from 50 Hz to 5 kHz inclusive, must be within ±1.0 dB of that at
the calibration check frequency, and within ±2.0 dB for nominal
midband frequencies of 6.3 kHz, 8 kHz and 10 kHz.
A36.3.5.4 For sinusoidal sound waves at each one-third octave
nominal midband frequency over the range from 50 Hz to 10 kHz
inclusive, the free-field sensitivity levels of the microphone
system at sound incidence angles of 30°, 60°, 90°, 120° and 150°,
must not differ from the free-field sensitivity level at a sound
incidence angle of 0° (“normal incidence”) by more than the values
shown in Table A36-1. The free-field sensitivity level differences
at sound incidence angles between any two adjacent sound incidence
angles in Table A36-1 must not exceed the tolerance limit for the
greater angle.
A36.3.6 Recording and reproducing systems.
A36.3.6.1 A recording and reproducing system, such as a digital
or analog magnetic tape recorder, a computer-based system or other
permanent data storage device, must be used to store sound pressure
signals for subsequent analysis. The sound produced by the aircraft
must be recorded in such a way that a record of the complete
acoustical signal is retained. The recording and reproducing
systems must meet the specifications in sections A36.3.6.2 to
A36.3.6.9 at the recording speeds and/or data sampling rates used
for the noise certification tests. Conformance must be demonstrated
for the frequency bandwidths and recording channels selected for
the tests.
A36.3.6.2 The recording and reproducing systems must be
calibrated as described in section A36.3.9.
(a) For aircraft noise signals for which the high frequency
spectral levels decrease rapidly with increasing frequency,
appropriate pre-emphasis and complementary de-emphasis networks may
be included in the measurement system. If pre-emphasis is included,
over the range of nominal one-third octave midband frequencies from
800 Hz to 10 kHz inclusive, the electrical gain provided by the
pre-emphasis network must not exceed 20 dB relative to the gain at
800 Hz.
A36.3.6.3 For steady sinusoidal electrical signals applied to
the input of the entire measurement system including all parts of
the microphone system except the microphone at a selected signal
level within 5 dB of that corresponding to the calibration sound
pressure level on the reference level range, the time-average
signal level indicated by the readout device at any one-third
octave nominal midband frequency from 50 Hz to 10 kHz inclusive
must be within ±1.5 dB of that at the calibration check frequency.
The frequency response of a measurement system, which includes
components that convert analog signals to digital form, must be
within ±0.3 dB of the response at 10 kHz over the frequency range
from 10 kHz to 11.2 kHz.
Note:
Microphone extension cables as configured in the field need not
be included for the frequency response determination. This
allowance does not eliminate the requirement of including
microphone extension cables when performing the pink noise
recording in section A36.3.9.5.
A36.3.6.4 For analog tape recordings, the amplitude fluctuations
of a 1 kHz sinusoidal signal recorded within 5 dB of the level
corresponding to the calibration sound pressure level must not vary
by more than ±0.5 dB throughout any reel of the type of magnetic
tape used. Conformance to this requirement must be demonstrated
using a device that has time-averaging properties equivalent to
those of the spectrum analyzer.
A36.3.6.5 For all appropriate level ranges and for steady
sinusoidal electrical signals applied to the input of the
measurement system, including all parts of the microphone system
except the microphone, at one-third-octave nominal midband
frequencies of 50 Hz, 1 kHz and 10 kHz, and the calibration check
frequency, if it is not one of these frequencies, the level
non-linearity must not exceed ±0.5 dB for a linear operating range
of at least 50 dB below the upper boundary of the level range.
Note 1:
Level linearity of measurement system components may be tested
according to the methods described in IEC 61265 as amended.
Note 2:
Microphone extension cables configured in the field need not be
included for the level linearity determination.
A36.3.6.6 On the reference level range, the level corresonding
to the calibration sound pressure level must be at least 5 dB, but
no more than 30 dB less than the upper boundary of the level
range.
A36.3.6.7 The linear operating ranges on adjacent level ranges
must overlap by at least 50 dB minus the change in attenuation
introduced by a change in the level range controls.
Note:
It is possible for a measurement system to have level range
controls that permit attenuation changes of either 10 dB or 1 dB,
for example. With 10 dB steps, the minimum overlap required would
be 40 dB, and with 1 dB steps the minimum overlap would be 49
dB.
A36.3.6.8 An overload indicator must be included in the
recording and reproducing systems so that an overload indication
will occur during an overload condition on any relevant level
range.
A36.3.6.9 Attenuators included in the measurement system to
permit range changes must operate in known intervals of decibel
steps.
A36.3.7 Analysis systems.
A36.3.7.1 The analysis system must conform to the specifications
in sections A36.3.7.2 to A36.3.7.7 for the frequency bandwidths,
channel configurations and gain settings used for analysis.
A36.3.7.2 The output of the analysis system must consist of
one-third octave band sound pressure levels as a function of time,
obtained by processing the noise signals (preferably recorded)
through an analysis system with the following characteristics:
(a) A set of 24 one-third octave band filters, or their
equivalent, having nominal midband frequencies from 50 Hz to 10 kHz
inclusive;
(b) Response and averaging properties in which, in principle,
the output from any one-third octave filter band is squared,
averaged and displayed or stored as time-averaged sound pressure
levels;
(c) The interval between successive sound pressure level samples
must be 500 ms ±5 milliseconds(ms) for spectral analysis with or
without slow time-weighting, as defined in section A36.3.7.4;
(d) For those analysis systems that do not process the sound
pressure signals during the period of time required for readout
and/or resetting of the analyzer, the loss of data must not exceed
a duration of 5 ms; and
(e) The analysis system must operate in real time from 50 Hz
through at least 12 kHz inclusive. This requirement applies to all
operating channels of a multi-channel spectral analysis system.
A36.3.7.3 The minimum standard for the one-third octave band
analysis system is the class 2 electrical performance requirements
of IEC 61260 as amended, over the range of one-third octave nominal
midband frequencies from 50 Hz through 10 kHz inclusive
(incorporated by reference, see § 36.6).
Note:
IEC 61260 specifies procedures for testing of one-third octave
band analysis systems for relative attenuation, anti-aliasing
filters, real time operation, level linearity, and filter
integrated response (effective bandwidth).
A36.3.7.4 When slow time averaging is performed in the analyzer,
the response of the one-third octave band analysis system to a
sudden onset or interruption of a constant sinusoidal signal at the
respective one-third octave nominal midband frequency, must be
measured at sampling instants 0.5, 1, 1.5 and 2 seconds(s) after
the onset and 0.5 and 1s after interruption. The rising response
must be −4 ±1 dB at 0.5s, −1.75 ±0.75 dB at 1s, −1 ±0.5 dB at 1.5s
and −0.5 ±0.5 dB at 2s relative to the steady-state level. The
falling response must be such that the sum of the output signal
levels, relative to the initial steady-state level, and the
corresponding rising response reading is −6.5 ±1 dB, at both 0.5
and 1s. At subsequent times the sum of the rising and falling
responses must be −7.5 dB or less. This equates to an exponential
averaging process (slow time-weighting) with a nominal 1s time
constant (i.e., 2s averaging time).
A36.3.7.5 When the one-third octave band sound pressure levels
are determined from the output of the analyzer without slow
time-weighting, slow time-weighting must be simulated in the
subsequent processing. Simulated slow time-weighted sound pressure
levels can be obtained using a continuous exponential averaging
process by the following equation:
Ls (i,k) = 10 log [(0.60653) 100.1 Ls[i, (k−1)] + (0.39347) 100.1 L
(i, k)] where Ls(i,k) is the simulated slow time-weighted sound
pressure level and L(i,k) is the as-measured 0.5s time average
sound pressure level determined from the output of the analyzer for
the k-th instant of time and i-th one-third octave band. For k = 1,
the slow time-weighted sound pressure Ls[i, (k − 1 = 0)] on the
right hand side should be set to 0 dB. An approximation of the
continuous exponential averaging is represented by the following
equation for a four sample averaging process for k ≥4: Ls (i,k) =
10 log [(0.13) 100.1 L[i,(k−3)] + (0.21) 100.1 L[i, (k−2)] + (0.27)
100.1 L[i, (k−1)] + (0.39) 100.1 L[i, k]] where Ls (i, k) is the
simulated slow time-weighted sound pressure level and L (i, k) is
the as measured 0.5s time average sound pressure level determined
from the output of the analyzer for the k-th instant of time and
the i-th one-third octave band.
The sum of the weighting factors is 1.0 in the two equations.
Sound pressure levels calculated by means of either equation are
valid for the sixth and subsequent 0.5s data samples, or for times
greater than 2.5s after initiation of data analysis.
Note:
The coefficients in the two equations were calculated for use in
determining equivalent slow time-weighted sound pressure levels
from samples of 0.5s time average sound pressure levels. The
equations do not work with data samples where the averaging time
differs from 0.5s.
A36.3.7.6 The instant in time by which a slow time-weighted
sound pressure level is characterized must be 0.75s earlier than
the actual readout time.
Note:
The definition of this instant in time is needed to correlate
the recorded noise with the aircraft position when the noise was
emitted and takes into account the averaging period of the slow
time-weighting. For each 0.5 second data record this instant in
time may also be identified as 1.25 seconds after the start of the
associated 2 second averaging period.
A36.3.7.7 The resolution of the sound pressure levels, both
displayed and stored, must be 0.1 dB or finer.
A36.3.8 Calibration systems.
A36.3.8.1 The acoustical sensitivity of the measurement system
must be determined using a sound calibrator generating a known
sound pressure level at a known frequency. The minimum standard for
the sound calibrator is the class 1L requirements of IEC 60942 as
amended (incorporated by reference, see § 36.6).
A36.3.9 Calibration and checking of system.
A36.3.9.1 Calibration and checking of the measurement system and
its constituent components must be carried out to the satisfaction
of the FAA by the methods specified in sections A36.3.9.2 through
A36.3.9.10. The calibration adjustments, including those for
environmental effects on sound calibrator output level, must be
reported to the FAA and applied to the measured one-third-octave
sound pressure levels determined from the output of the analyzer.
Data collected during an overload indication are invalid and may
not be used. If the overload condition occurred during recording,
the associated test data are invalid, whereas if the overload
occurred during analysis, the analysis must be repeated with
reduced sensitivity to eliminate the overload.
A36.3.9.2 The free-field frequency response of the microphone
system may be determined by use of an electrostatic actuator in
combination with manufacturer's data or by tests in an anechoic
free-field facility. The correction for frequency response must be
determined within 90 days of each test series. The correction for
non-uniform frequency response of the microphone system must be
reported to the FAA and applied to the measured one-third octave
band sound pressure levels determined from the output of the
analyzer.
A36.3.9.3 When the angles of incidence of sound emitted from the
aircraft are within ±30° of grazing incidence at the microphone
(see Figure A36-1), a single set of free-field corrections based on
grazing incidence is considered sufficient for correction of
directional response effects. For other cases, the angle of
incidence for each 0.5 second sample must be determined and applied
for the correction of incidence effects.
A36.3.9.4 For analog magnetic tape recorders, each reel of
magnetic tape must carry at least 30 seconds of pink random or
pseudo-random noise at its beginning and end. Data obtained from
analog tape-recorded signals will be accepted as reliable only if
level differences in the 10 kHz one-third-octave-band are not more
than 0.75 dB for the signals recorded at the beginning and end.
A36.3.9.5 The frequency response of the entire measurement
system while deployed in the field during the test series,
exclusive of the microphone, must be determined at a level within 5
dB of the level corresponding to the calibration sound pressure
level on the level range used during the tests for each one-third
octave nominal midband frequency from 50 Hz to 10 kHz inclusive,
utilizing pink random or pseudo-random noise. Within six months of
each test series the output of the noise generator must be
determined by a method traceable to the U.S. National Institute of
Standards and Technology or to an equivalent national standards
laboratory as determined by the FAA. Changes in the relative output
from the previous calibration at each one-third octave band may not
exceed 0.2 dB. The correction for frequency response must be
reported to the FAA and applied to the measured one-third octave
sound pressure levels determined from the output of the
analyzer.
A36.3.9.6 The performance of switched attenuators in the
equipment used during noise certification measurements and
calibration must be checked within six months of each test series
to ensure that the maximum error does not exceed 0.1 dB.
A36.3.9.7 The sound pressure level produced in the cavity of the
coupler of the sound calibrator must be calculated for the test
environmental conditions using the manufacturer's supplied
information on the influence of atmospheric air pressure and
temperature. This sound pressure level is used to establish the
acoustical sensitivity of the measurement system. Within six months
of each test series the output of the sound calibrator must be
determined by a method traceable to the U.S. National Institute of
Standards and Technology or to an equivalent national standards
laboratory as determined by the FAA. Changes in output from the
previous calibration must not exceed 0.2 dB.
A36.3.9.8 Sufficient sound pressure level calibrations must be
made during each test day to ensure that the acoustical sensitivity
of the measurement system is known at the prevailing environmental
conditions corresponding with each test series. The difference
between the acoustical sensitivity levels recorded immediately
before and immediately after each test series on each day may not
exceed 0.5 dB. The 0.5 dB limit applies after any atmospheric
pressure corrections have been determined for the calibrator output
level. The arithmetic mean of the before and after measurements
must be used to represent the acoustical sensitivity level of the
measurement system for that test series. The calibration
corrections must be reported to the FAA and applied to the measured
one-third octave band sound pressure levels determined from the
output of the analyzer.
A36.3.9.9 Each recording medium, such as a reel, cartridge,
cassette, or diskette, must carry a sound pressure level
calibration of at least 10 seconds duration at its beginning and
end.
A36.3.9.10 The free-field insertion loss of the windscreen for
each one-third octave nominal midband frequency from 50 Hz to 10
kHz inclusive must be determined with sinusoidal sound signals at
the incidence angles determined to be applicable for correction of
directional response effects per section A36.3.9.3. The interval
between angles tested must not exceed 30 degrees. For a windscreen
that is undamaged and uncontaminated, the insertion loss may be
taken from manufacturer's data. Alternatively, within six months of
each test series the insertion loss of the windscreen may be
determined by a method traceable to the U.S. National Institute of
Standards and Technology or an equivalent national standards
laboratory as determined by the FAA. Changes in the insertion loss
from the previous calibration at each one-third-octave frequency
band must not exceed 0.4 dB. The correction for the free-field
insertion loss of the windscreen must be reported to the FAA and
applied to the measured one-third octave sound pressure levels
determined from the output of the analyzer.
A36.3.10 Adjustments for ambient noise.
A36.3.10.1 Ambient noise, including both an acoustical
background and electrical noise of the measurement system, must be
recorded for at least 10 seconds at the measurement points with the
system gain set at the levels used for the aircraft noise
measurements. Ambient noise must be representative of the
acoustical background that exists during the flyover test run. The
recorded aircraft noise data is acceptable only if the ambient
noise levels, when analyzed in the same way, and quoted in PNL (see
A36.4.1.3 (a)), are at least 20 dB below the maximum PNL of the
aircraft.
A36.3.10.2 Aircraft sound pressure levels within the 10 dB-down
points (see A36.4.5.1) must exceed the mean ambient noise levels
determined in section A36.3.10.1 by at least 3 dB in each one-third
octave band, or must be adjusted using a method approved by the
FAA; one method is described in the current advisory circular for
this part.
Section A36.4 Calculation of Effective Perceived Noise Level From
Measured Data
A36.4.1 General.
A36.4.1.1 The basic element for noise certification criteria is
the noise evaluation measure known as effective perceived noise
level, EPNL, in units of EPNdB, which is a single number evaluator
of the subjective effects of airplane noise on human beings. EPNL
consists of instantaneous perceived noise level, PNL, corrected for
spectral irregularities, and for duration. The spectral
irregularity correction, called “tone correction factor”, is made
at each time increment for only the maximum tone.
A36.4.1.2 Three basic physical properties of sound pressure must
be measured: level, frequency distribution, and time variation. To
determine EPNL, the instantaneous sound pressure level in each of
the 24 one-third octave bands is required for each 0.5 second
increment of time during the airplane noise measurement.
A36.4.1.3 The calculation procedure that uses physical
measurements of noise to derive the EPNL evaluation measure of
subjective response consists of the following five steps:
(a) The 24 one-third octave bands of sound pressure level are
converted to perceived noisiness (noy) using the method described
in section A36.4.2.1 (a). The noy values are combined and then
converted to instantaneous perceived noise levels, PNL(k).
(b) A tone correction factor C(k) is calculated for each
spectrum to account for the subjective response to the presence of
spectral irregularities.
(c) The tone correction factor is added to the perceived noise
level to obtain tone-corrected perceived noise levels PNLT(k), at
each one-half second increment:
PNLT(k) = PNL(k) + C(k) The instantaneous values of tone-corrected
perceived noise level are derived and the maximum value, PNLTM, is
determined.
(d) A duration correction factor, D, is computed by integration
under the curve of tone-corrected perceived noise level versus
time.
(e) Effective perceived noise level, EPNL, is determined by the
algebraic sum of the maximum tone-corrected perceived noise level
and the duration correction factor:
EPNL = PNLTM + D
A36.4.2 Perceived noise level.
A36.4.2.1 Instantaneous perceived noise levels, PNL(k), must be
calculated from instantaneous one-third octave band sound pressure
levels, SPL(i, k) as follows:
(a) Step 1: For each one-third octave band from 50 through
10,000 Hz, convert SPL(i, k) to perceived noisiness n(i, k), by
using the mathematical formulation of the noy table given in
section A36.4.7.
(b) Step 2: Combine the perceived noisiness values, n(i, k),
determined in step 1 by using the following formula:
where n(k) is the largest of the 24 values of
n(i, k) and N(k) is the total perceived noisiness.
(c) Step 3: Convert the total perceived noisiness, N(k),
determined in Step 2 into perceived noise level, PNL(k), using the
following formula:
Note:
PNL(k) is plotted in the current advisory circular for this
part.
A36.4.3 Correction for spectral irregularities.
A36.4.3.1 Noise having pronounced spectral irregularities (for
example, the maximum discrete frequency components or tones) must
be adjusted by the correction factor C(k) calculated as
follows:
(a) Step 1: After applying the corrections specified under
section A36.3.9, start with the sound pressure level in the 80 Hz
one-third octave band (band number 3), calculate the changes in
sound pressure level (or “slopes”) in the remainder of the
one-third octave bands as follows:
s(3,
k) = no value s(4,
k) =
SPL(4,
k)−SPL(3,
k) • • s(
i,k) =
SPL(
i,k)−SPL(
i−1,
k) • • s(24,
k) =
SPL(24,
k)−SPL(23,
k)
(b) Step 2: Encircle the value of the slope, s(i, k), where the
absolute value of the change in slope is greater than five; that is
where:
|Δ
s(
i,k)| =
|
s(
i,k)−
s(
i−1,
k)|>5
(c) Step 3:
(1) If the encircled value of the slope s(i, k) is positive and
algebraically greater than the slope s(i−1, k) encircle SPL(i,
k).
(2) If the encircled value of the slope s(i, k) is zero or
negative and the slope s(i−1, k) is positive, encircle SPL(i−1,
k).
(3) For all other cases, no sound pressure level value is to be
encircled.
(d) Step 4: Compute new adjusted sound pressure levels SPL′(i,
k) as follows:
(1) For non-encircled sound pressure levels, set the new sound
pressure levels equal to the original sound pressure levels,
SPL′(i, k) = SPL(i, k).
(2) For encircled sound pressure levels in bands 1 through 23
inclusive, set the new sound pressure level equal to the arithmetic
average of the preceding and following sound pressure levels as
shown below:
SPL′(
i,k) = 1/2[SPL(
i−1,
k) + SPL(
i +
1,
k)]
(3) If the sound pressure level in the highest frequency band (i
= 24) is encircled, set the new sound pressure level in that band
equal to:
SPL′(24,
k) = SPL(23,
k) +
s(23,
k)
(e) Step 5: Recompute new slope s′(i, k), including one for an
imaginary 25th band, as follows:
s′(3,
k) =
s′(4,
k)
s′(4,
k)
= SPL′(4,
k)−SPL′(3,
k) • •
s′(
i,k) =
SPL′(
i,k)−SPL′(
i−1,
k) • •
s′(24,
k) = SPL′(24,
k)−SPL′(23,
k)
s′(25,
k) =
s′(24,
k)
(f) Step 6: For i, from 3 through 23, compute the arithmetic
average of the three adjacent slopes as follows:
s (
i,k) = 1/3[
s′(
i,k) +
s′(
i + 1,
k) +
s′(
i +
2,
k)]
(g) Step 7: Compute final one-third octave-band sound pressure
levels, SPL′ (i,k), by beginning with band number 3 and proceeding
to band number 24 as follows:
SPL′(3,k) = SPL(3,k)
SPL′(4,k) = SPL′(3,k) + s (3,k)
•
•
SPL′(i,k) = SPL′(i−1,k) + s (i−1,k)
•
•
SPL′(24,k) = SPL′(23,k) + s (23,k)
(h) Setp 8: Calculate the differences, F (i,k), between the
original sound pressure level and the final background sound
pressure level as follows:
F(
i,k) = SPL(
i,k)-SPL′(
i,k) and note
only values equal to or greater than 1.5.
(i) Step 9: For each of the relevant one-third octave bands (3
through 24), determine tone correction factors from the sound
pressure level differences F (i, k) and Table A36-2.
(j) Step 10: Designate the largest of the tone correction
factors, determined in Step 9, as C(k). (An example of the tone
correction procedure is given in the current advisory circular for
this part). Tone-corrected perceived noise levels PNLT(k) must be
determined by adding the C(k) values to corresponding PNL(k)
values, that is:
PNLT(
k) = PNL(
k) +
C(
k) For any i-th
one-third octave band, at any k-th increment of time, for which the
tone correction factor is suspected to result from something other
than (or in addition to) an actual tone (or any spectral
irregularity other than airplane noise), an additional analysis may
be made using a filter with a bandwidth narrower than one-third of
an octave. If the narrow band analysis corroborates these
suspicions, then a revised value for the background sound pressure
level SPL′(i,k), may be determined from the narrow band analysis
and used to compute a revised tone correction factor for that
particular one-third octave band. Other methods of rejecting
spurious tone corrections may be approved.
A36.4.3.2 The tone correction procedure will underestimate EPNL
if an important tone is of a frequency such that it is recorded in
two adjacent one-third octave bands. An applicant must demonstrate
that either:
(a) No important tones are recorded in two adjacent one-third
octave bands; or
(b) That if an important tone has occurred, the tone correction
has been adjusted to the value it would have had if the tone had
been recorded fully in a single one-third octave band.
A36.4.4 Maximum tone-corrected perceived noise level
A36.4.4.1 The maximum tone-corrected perceived noise level,
PNLTM, must be the maximum calculated value of the tone-corrected
perceived noise level PNLT(k). It must be calculated using the
procedure of section A36.4.3. To obtain a satisfactory noise time
history, measurements must be made at 0.5 second time
intervals.
Note 1:
Figure A36-2 is an example of a flyover noise time history where
the maximum value is clearly indicated.
Note 2:
In the absence of a tone correction factor, PNLTM would equal
PNLM.
A36.4.4.2 After the value of PNLTM is obtained, the frequency
band for the largest tone correction factor is identified for the
two preceding and two succeeding 500 ms data samples. This is
performed in order to identity the possibility of tone suppression
at PNLTM by one-third octave band sharing of that tone. If the
value of the tone correction factor C(k) for PNLTM is less than the
average value of C(k) for the five consecutive time intervals, the
average value of C(k) must be used to compute a new value for
PNLTM.
A36.4.5 Duration correction.
A36.4.5.1 The duration correction factor D determined by the
integration technique is defined by the expression:
where T
is a normalizing time constant, PNLTM is the maximum value of PNLT,
t(1) is the first point of time after which PNLT becomes greater
than PNLTM-10, and t(2) is the point of time after which PNLT
remains constantly less than PNLTM-10.
A36.4.5.2 Since PNLT is calculated from measured values of sound
pressure level (SPL), there is no obvious equation for PNLT as a
function of time. Consequently, the equation is to be rewritten
with a summation sign instead of an integral sign as follows:
where Δt
is the length of the equal increments of time for which PNLT(k) is
calculated and d is the time interval to the nearest 0.5s during
which PNLT(k) remains greater or equal to PNLTM-10.
A36.4.5.3 To obtain a satisfactory history of the perceived
noise level use one of the following:
(a) Half-Second time intervals for Δt; or
(b) A shorter time interval with approved limits and
constants.
A36.4.5.4 The following values for T and Δt must be used in
calculating D in the equation given in section A36.4.5.2:
T = 10 s, and Δt = 0.5s (or the approved sampling time interval).
Using these values, the equation for D becomes:
where d is the duration time defined by the points corresponding to
the values PNLTM-10.
A36.4.5.5 If in using the procedures given in section A36.4.5.2,
the limits of PNLTM-10 fall between the calculated PNLT(k) values
(the usual case), the PNLT(k) values defining the limits of the
duration interval must be chosen from the PNLT(k) values closest to
PNLTM-10. For those cases with more than one peak value of PNLT(k),
the applicable limits must be chosen to yield the largest possible
value for the duration time.
A36.4.6 Effective perceived noise level.
The total subjective effect of an airplane noise event,
designated effective perceived noise level, EPNL, is equal to the
algebraic sum of the maximum value of the tone-corrected perceived
noise level, PNLTM, and the duration correction D. That is:
EPNL = PNLTM + D where PNLTM and D are calculated using the
procedures given in sections A36.4.2, A36.4.3, A36.4.4. and
A36.4.5.
A36.4.7 Mathematical formulation of noy tables.
A36.4.7.1 The relationship between sound pressure level (SPL)
and the logarithm of perceived noisiness is illustrated in Figure
A36-3 and Table A36-3.
A36.4.7.2 The bases of the mathematical formulation are:
(a) The slopes (M(b), M(c), M(d) and M(e)) of the straight
lines;
(b) The intercepts (SPL(b) and SPL(c)) of the lines on the SPL
axis; and
(c) The coordinates of the discontinuities, SPL(a) and log n(a);
SPL(d) and log n = −1.0; and SPL(e) and log n = log (0.3).
A36.4.7.3 Calculate noy values using the following
equations:
(a)
SPL ≥SPL (a) n = antilog {(c)[SPL−SPL(c)]}
(b)
SPL(b) ≤SPL <SPL(a) n = antilog {M(b)[SPL−SPL(b)]}
(c)
SPL(e) ≤SPL <SPL(b) n = 0.3 antilog {M(e)[SPL−SPL(e)]}
(d)
SPL(d) ≤SPL <SPL(e) n = 0.1 antilog {M(d)[SPL−SPL(d)]}
A36.4.7.4 Table A36-3 lists the values of the constants
necessary to calculate perceived noisiness as a function of sound
pressure level.
Section A36.5
Reporting of Data to the FAA
A36.5.1 General.
A36.5.1.1 Data representing physical measurements and data used
to make corrections to physical measurements must be recorded in an
approved permanent form and appended to the record.
A36.5.1.2 All corrections must be reported to and approved by
the FAA, including corrections to measurements for equipment
response deviations.
A36.5.1.3 Applicants may be required to submit estimates of the
individual errors inherent in each of the operations employed in
obtaining the final data.
A36.5.2 Data reporting.
An applicant is required to submit a noise certification
compliance report that includes the following.
A36.5.2.1 The applicant must present measured and corrected
sound pressure levels in one-third octave band levels that are
obtained with equipment conforming to the standards described in
section A36.3 of this appendix.
A36.5.2.2 The applicant must report the make and model of
equipment used for measurement and analysis of all acoustic
performance and meteorological data.
A36.5.2.3 The applicant must report the following atmospheric
environmental data, as measured immediately before, after, or
during each test at the observation points prescribed in section
A36.2 of this appendix.
(a) Air temperature and relative humidity;
(b) Maximum, minimum and average wind velocities; and
(c) Atmospheric pressure.
A36.5.2.4 The applicant must report conditions of local
topography, ground cover, and events that might interfere with
sound recordings.
A36.5.2.5 The applicant must report the following:
(a) Type, model and serial numbers (if any) of airplane,
engine(s), or propeller(s) (as applicable);
(b) Gross dimensions of airplane and location of engines;
(c) Airplane gross weight for each test run and center of
gravity range for each series of test runs;
(d) Airplane configuration such as flap, airbrakes and landing
gear positions for each test run;
(e) Whether auxiliary power units (APU), when fitted, are
operating for each test run;
(f) Status of pneumatic engine bleeds and engine power take-offs
for each test run;
(g) Indicated airspeed in knots or kilometers per hour for each
test run;
(h) Engine performance data:
(1) For jet airplanes: engine performance in terms of net
thrust, engine pressure ratios, jet exhaust temperatures and fan or
compressor shaft rotational speeds as determined from airplane
instruments and manufacturer's data for each test run;
(2) For propeller-driven airplanes: engine performance in terms
of brake horsepower and residual thrust; or equivalent shaft
horsepower; or engine torque and propeller rotational speed; as
determined from airplane instruments and manufacturer's data for
each test run;
(i) Airplane flight path and ground speed during each test run;
and
(j) The applicant must report whether the airplane has any
modifications or non-standard equipment likely to affect the noise
characteristics of the airplane. The FAA must approve any such
modifications or non-standard equipment.
A36.5.3 Reporting of noise certification reference
conditions.
A36.5.3.1 Airplane position and performance data and the noise
measurements must be corrected to the noise certification reference
conditions specified in the relevant sections of appendix B of this
part. The applicant must report these conditions, including
reference parameters, procedures and configurations.
A36.5.4 Validity of results.
A36.5.4.1 Three average reference EPNL values and their 90
percent confidence limits must be produced from the test results
and reported, each such value being the arithmetical average of the
adjusted acoustical measurements for all valid test runs at each
measurement point (flyover, lateral, or approach). If more than one
acoustic measurement system is used at any single measurement
location, the resulting data for each test run must be averaged as
a single measurement. The calculation must be performed by:
(a) Computing the arithmetic average for each flight phase using
the values from each microphone point; and
(b) Computing the overall arithmetic average for each reference
condition (flyover, lateral or approach) using the values in
paragraph (a) of this section and the related 90 percent confidence
limits.
A36.5.4.2 For each of the three certification measuring points,
the minimum sample size is six. The sample size must be large
enough to establish statistically for each of the three average
noise certification levels a 90 percent confidence limit not
exceeding ±1.5 EPNdB. No test result may be omitted from the
averaging process unless approved by the FAA.
Note:
Permitted methods for calculating the 90 percent confidence
interval are shown in the current advisory circular for this
part.
A36.5.4.3 The average EPNL figures obtained by the process
described in section A36.5.4.1 must be those by which the noise
performance of the airplane is assessed against the noise
certification criteria.
Section A36.6 Nomenclature: Symbols and Units
Symbol |
Unit |
Meaning |
antilog |
|
Antilogarithm to the base
10. |
C(k) |
dB |
Tone correction factor. The
factor to be added to PNL(k) to account for the presence of
spectral irregularities such as tones at the k-th increment of
time. |
d |
s |
Duration time. The time
interval between the limits of t(1) and t(2) to the nearest 0.5
second. |
D |
dB |
Duration correction. The
factor to be added to PNLTM to account for the duration of the
noise. |
EPNL |
EPNdB |
Effective perceived noise
level. The value of PNL adjusted for both spectral irregularities
and duration of the noise. (The unit EPNdB is used instead of the
unit dB). |
EPNLr |
EPNdB |
Effective perceived noise
level adjusted for reference conditions. |
f(i) |
Hz |
Frequency. The geometrical
mean frequency for the i-th one-third octave band. |
F (i, k) |
dB |
Delta-dB. The difference
between the original sound pressure level and the final background
sound pressure level in the i-th one-third octave band at the k-th
interval of time. In this case, background sound pressure level
means the broadband noise level that would be present in the
one-third octave band in the absence of the tone. |
h |
dB |
dB-down. The value to be
subtracted from PNLTM that defines the duration of the noise. |
H |
Percent |
Relative humidity. The ambient
atmospheric relative humidity. |
i |
|
Frequency band index. The
numerical indicator that denotes any one of the 24 one-third octave
bands with geometrical mean frequencies from 50 to 10,000 Hz. |
k |
|
Time increment index. The
numerical indicator that denotes the number of equal time
increments that have elapsed from a reference zero. |
Log |
|
Logarithm to the base 10. |
log n(a) |
|
Noy discontinuity coordinate.
The log n value of the intersection point of the straight lines
representing the variation of SPL with log n. |
M(b), M(c),
etc |
|
Noy inverse slope. The
reciprocals of the slopes of straight lines representing the
variation of SPL with log n. |
n |
noy |
The perceived noisiness at any
instant of time that occurs in a specified frequency range. |
n(i,k) |
noy |
The perceived noisiness at the
k-th instant of time that occurs in the i-th one-third octave
band. |
n(k) |
noy |
Maximum perceived noisiness.
The maximum value of all of the 24 values of n(i) that occurs at
the k-th instant of time. |
N(k) |
noy |
Total perceived noisiness. The
total perceived noisiness at the k-th instant of time calculated
from the 24-instantaneous values of n (i, k). |
p(b), p(c),
etc |
|
Noy slope. The slopes of
straight lines representing the variation of SPL with log n. |
PNL |
PNdB |
The perceived noise level at
any instant of time. (The unit PNdB is used instead of the unit
dB). |
PNL(k) |
PNdB |
The perceived noise level
calculated from the 24 values of SPL (i, k), at the k-th increment
of time. (The unit PNdB is used instead of the unit dB). |
PNLM |
PNdB |
Maximum perceived noise level.
The maximum value of PNL(k). (The unit PNdB is used instead of the
unit dB). |
PNLT |
TPNdB |
Tone-corrected perceived noise
level. The value of PNL adjusted for the spectral irregularities
that occur at any instant of time. (The unit TPNdB is used instead
of the unit dB). |
PNLT(k) |
TPNdB |
The tone-corrected perceived
noise level that occurs at the k-th increment of time. PNLT(k) is
obtained by adjusting the value of PNL(k) for the spectral
irregularities that occur at the k-th increment of time. (The unit
TPNdB is used instead of the unit dB). |
PNLTM |
TPNdB |
Maximum tone-corrected
perceived noise level. The maximum value of PNLT(k). (The unit
TPNdB is used instead of the unit dB). |
PNLTr |
TPNdB |
Tone-corrected perceived noise
level adjusted for reference conditions. |
s (i, k) |
dB |
Slope of sound pressure level.
The change in level between adjacent one-third octave band sound
pressure levels at the i-th band for the k-th instant of time. |
Δs (i, k) |
dB |
Change in slope of sound
pressure level. |
s′ (i, k) |
dB |
Adjusted slope of sound
pressure level. The change in level between adjacent adjusted
one-third octave band sound pressure levels at the i-th band for
the k-th instant of time. |
s (i, k) |
dB |
Average slope of sound
pressure level. |
SPL |
dB re
20 µPa |
Sound pressure level. The
sound pressure level that occurs in a specified frequency range at
any instant of time. |
SPL(a) |
dB re
20 µPa |
Noy discontinuity coordinate.
The SPL value of the intersection point of the straight lines
representing the variation of SPL with log n. |
SPL(b)
SPL (c) |
dB re
20 µPa |
Noy intercept. The intercepts
on the SPL-axis of the straight lines representing the variation of
SPL with log n. |
SPL (i, k) |
dB re
20 µPa |
The sound pressure level at
the k-th instant of time that occurs in the i-th one-third octave
band. |
SPL′ (i, k) |
dB re
20 µPa |
Adjusted sound pressure level.
The first approximation to background sound pressure level in the
i-th one-third octave band for the k-th instant of time. |
SPL(i) |
dB re
20 µPa |
Maximum sound pressure level.
The sound pressure level that occurs in the i-th one-third octave
band of the spectrum for PNLTM. |
SPL(i)r |
dB re
20 µPa |
Corrected maximum sound
pressure level. The sound pressure level that occurs in the i-th
one-third octave band of the spectrum for PNLTM corrected for
atmospheric sound absorption. |
SPL′ (i, k) |
dB re
20 µPa |
Final background sound
pressure level. The second and final approximation to background
sound pressure level in the i-th one-third octave band for the k-th
instant of time. |
t |
s |
Elapsed time. The length of
time measured from a reference zero. |
t(1), t(2) |
s |
Time limit. The beginning and
end, respectively, of the noise time history defined by h. |
Δt |
s |
Time increment. The equal
increments of time for which PNL(k) and PNLT(k) are
calculated. |
T |
s |
Normalizing time constant. The
length of time used as a reference in the integration method for
computing duration corrections, where T = 10s. |
t(°F) (°C) |
°F, °C |
Temperature. The ambient air
temperature. |
α(i) |
dB/1000ft db/100m |
Test atmospheric absorption.
The atmospheric attenuation of sound that occurs in the i-th
one-third octave band at the measured air temperature and relative
humidity. |
α(i)o |
dB/1000ft db/100m |
Reference atmospheric
absorption. The atmospheric attenuation of sound that occurs in the
i-th one-third octave band at a reference air temperature and
relative humidity. |
A1 |
Degrees |
First constant climb angle
(Gear up, speed of at least V2 + 10 kt (V2 + 19 km/h), takeoff
thrust). |
A2 |
Degrees |
Second constant climb angle
(Gear up, speed of at least V2 + 10 kt (V2 + 19 km/h), after
cut-back). |
δ
ε |
Degrees |
Thrust cutback angles. The
angles defining the points on the takeoff flight path at which
thrust reduction is started and ended respectively. |
η |
Degrees |
Approach angle. |
ηr |
Degrees |
Reference approach angle. |
θ |
Degrees |
Noise angle (relative to
flight path). The angle between the flight path and noise path. It
is identical for both measured and corrected flight paths. |
ψ |
Degrees |
Noise angle (relative to
ground). The angle between the noise path and the ground. It is
identical for both measured and corrected flight paths. |
μ |
|
Engine noise emission
parameter. |
μr |
|
Reference engine noise
emission parameter. |
Δ1 |
EPNdB |
PNLT correction. The
correction to be added to the EPNL calculated from measured data to
account for noise level changes due to differences in atmospheric
absorption and noise path length between reference and test
conditions. |
Δ2 |
EPNdB |
Adjustment to duration
correction. The adjustment to be made to the EPNL calculated from
measured data to account for noise level changes due to the noise
duration between reference and test conditions. |
Δ3 |
EPNdB |
Source noise adjustment. The
adjustment to be made to the EPNL calculated from measured data to
account for noise level changes due to differences between
reference and test engine operating conditions. |
Section A36.7 Sound Attenuation in Air
A36.7.1 The atmospheric attenuation of sound must be determined
in accordance with the procedure presented in section A36.7.2.
A36.7.2 The relationship between sound attenuation, frequency,
temperature, and humidity is expressed by the following
equations.
A36.7.2(a) For calculations using the English System of
Units:
and
where η(δ) is listed in
Table A36-4 and f0 in Table A36-5; α(i) is the attenuation
coefficient in dB/1000 ft; θ is the temperature in °F; and H is the
relative humidity, expressed as a percentage.
A36.7.2(b) For calculations using the International System of
Units (SI):
and
where η(δ) is listed in
Table A36-4 and f0 in Table A36-5; α(i) is the attenuation
coefficient in dB/100 m; θ is the temperature in °C; and H is the
relative humidity, expressed as a percentage.
A36.7.3 The values listed in table A36-4 are to be used when
calculating the equations listed in section A36.7.2. A term of
quadratic interpolation is to be used where necessary.
Section A36.8 [Reserved]
Section A36.9
Adjustment of Airplane Flight Test Results.
A36.9.1 When certification test conditions are not identical to
reference conditions, appropriate adjustments must be made to the
measured noise data using the methods described in this
section.
A36.9.1.1 Adjustments to the measured noise values must be made
using one of the methods described in sections A36.9.3 and A36.9.4
for differences in the following:
(a) Attenuation of the noise along its path as affected by
“inverse square” and atmospheric attenuation
(b) Duration of the noise as affected by the distance and the
speed of the airplane relative to the measuring point
(c) Source noise emitted by the engine as affected by the
differences between test and reference engine operating
conditions
(d) Airplane/engine source noise as affected by differences
between test and reference airspeeds. In addition to the effect on
duration, the effects of airspeed on component noise sources must
be accounted for as follows: for conventional airplane
configurations, when differences between test and reference
airspeeds exceed 15 knots (28 km/h) true airspeed, test data and/or
analysis approved by the FAA must be used to quantify the effects
of the airspeed adjustment on resulting certification noise
levels.
A36.9.1.2 The “integrated” method of adjustment, described in
section A36.9.4, must be used on takeoff or approach under the
following conditions:
(a) When the amount of the adjustment (using the “simplified”
method) is greater than 8 dB on flyover, or 4 dB on approach;
or
(b) When the resulting final EPNL value on flyover or approach
(using the simplified method) is within 1 dB of the limiting noise
levels as prescribed in section B36.5 of this part.
A36.9.2 Flight profiles.
As described below, flight profiles for both test and reference
conditions are defined by their geometry relative to the ground,
together with the associated airplane speed relative to the ground,
and the associated engine control parameter(s) used for determining
the noise emission of the airplane.
A36.9.2.1 Takeoff Profile.
Note:
Figure A36-4 illustrates a typical takeoff profile.
(a) The airplane begins the takeoff roll at point A, lifts off
at point B and begins its first climb at a constant angle at point
C. Where thrust or power (as appropriate) cut-back is used, it is
started at point D and completed at point E. From here, the
airplane begins a second climb at a constant angle up to point F,
the end of the noise certification takeoff flight path.
(b) Position K1 is the takeoff noise measuring station and AK1
is the distance from start of roll to the flyover measuring point.
Position K2 is the lateral noise measuring station, which is
located on a line parallel to, and the specified distance from, the
runway center line where the noise level during takeoff is
greatest.
(c) The distance AF is the distance over which the airplane
position is measured and synchronized with the noise measurements,
as required by section A36.2.3.2 of this part.
A36.9.2.2 Approach Profile.
Note:
Figure A36-5 illustrates a typical approach profile.
(a) The airplane begins its noise certification approach flight
path at point G and touches down on the runway at point J, at a
distance OJ from the runway threshold.
(b) Position K3 is the approach noise measuring station and K3O
is the distance from the approach noise measurement point to the
runway threshold.
(c) The distance GI is the distance over which the airplane
position is measured and synchronized with the noise measurements,
as required by section A36.2.3.2 of this part.
The airplane reference point for approach
measurements is the instrument landing system (ILS) antenna. If no
ILS antenna is installed an alternative reference point must be
approved by the FAA.
A36.9.3 Simplified method of adjustment.
A36.9.3.1 General. As described below, the simplified
adjustment method consists of applying adjustments (to the EPNL,
which is calculated from the measured data) for the differences
between measured and reference conditions at the moment of
PNLTM.
A36.9.3.2 Adjustments to PNL and PNLT.
(a) The portions of the test flight path and the reference
flight path described below, and illustrated in Figure A36-6,
include the noise time history that is relevant to the calculation
of flyover and approach EPNL. In figure A36-6:
(1) XY represents the portion of the measured flight path that
includes the noise time history relevant to the calculation of
flyover and approach EPNL; XrYr represents the corresponding
portion of the reference flight path.
(2) Q represents the airplane's position on the measured flight
path at which the noise was emitted and observed as PNLTM at the
noise measuring station K. Qr is the corresponding position on the
reference flight path, and Kr the reference measuring station. QK
and QrKr are, respectively, the measured
and reference noise propagation paths, Qr being
determined from the assumption that QK and QrKr form the same angle
θ with their respective flight paths.
(b) The portions of the test flight path and the reference
flight path described in paragraph (b)(1) and (2), and illustrated
in Figure A36-7(a) and (b), include the noise time history that is
relevant to the calculation of lateral EPNL.
(1) In figure A36-7(a), XY represents the portion of the
measured flight path that includes the noise time history that is
relevant to the calculation of lateral EPNL; in figure A36-7(b),
XrYr represents the corresponding portion of the reference flight
path.
(2) Q represents the airplane position on the measured flight
path at which the noise was emitted and observed as PNLTM at the
noise measuring station K. Qr is the corresponding position on the
reference flight path, and Kr the reference measuring station. QK
and QrKr are, respectively, the measured and reference noise
propagation paths. In this case Kr is only specified as being on a
particular Lateral line; Kr and Qr are therefore determined from
the assumptions that QK and QrKr:
(i) Form the same angle θ with their respective flight paths;
and
(ii) Form the same angle ψ with the ground.
Note:
For the lateral noise measurement, sound propagation is affected
not only by inverse square and atmospheric attenuation, but also by
ground absorption and reflection effects which depend mainly on the
angle ψ.
A36.9.3.2.1 The one-third octave band levels SPL(i) comprising
PNL (the PNL at the moment of PNLTM observed at K) must be adjusted
to reference levels SPL(i)r as follows:
A36.9.3.2.1(a) For calculations using the English System of
Units:
SPL(
i)r = SPL(
i) + 0.001[α(
i)−α(
i)0]QK
+ 0.001α(
i)0(QK−QrKr) + 20log(QK/QrKr)
In this expression,
(1) The term 0.001[α(i)−α(i)0]QK is the adjustment
for the effect of the change in sound attenuation coefficient, and
α(i) and α(i)0 are the coefficients for the test and reference
atmospheric conditions respectively, determined under section A36.7
of this appendix;
(2) The term 0.001α(i)0(QK − QrKr) is the adjustment for the
effect of the change in the noise path length on the sound
attenuation;
(3) The term 20 log(QK/QrKr) is the adjustment for the effect of
the change in the noise path length due to the “inverse square”
law;
(4) QK and QrKr are measured in feet and α(i) and α(i)0 are
expressed in dB/1000 ft.
A36.9.3.2.1(b) For calculations using the International System
of Units:
SPL(i)r = SPL(i) + 0.01[α(i)−α(i)0]QK + 0.01α(i)0 (QK − QrKr) + 20
log(QK/QrKr) In this expression,
(1) The term 0.01[α(i) − α(i)0]QK is the adjustment for the
effect of the change in sound attenuation coefficient, and α(i) and
α(i)0 are the coefficients for the test and reference atmospheric
conditions respectively, determined under section A36.7 of this
appendix;
(2) The term 0.01α(i)0(QK − QrKr) is the adjustment for the
effect of the change in the noise path length on the sound
attenuation;
(3) The term 20 log(QK/QrKr) is the adjustment for the effect of
the change in the noise path length due to the inverse square
law;
(4) QK and QrKr are measured in meters and α(i) and α(i)0 are
expressed in dB/100 m.
A36.9.3.2.1.1 PNLT Correction.
(a) Convert the corrected values, SPL(i)r, to PNLTr;
(b) Calculate the correction term Δ1 using the following
equation:
Δ1 = PNLTr − PNLTM
A36.9.3.2.1.2 Add Δ1 arithmetically to the EPNL calculated from
the measured data.
A36.9.3.2.2 If, during a test flight, several peak values of
PNLT that are within 2 dB of PNLTM are observed, the procedure
defined in section A36.9.3.2.1 must be applied at each peak, and
the adjustment term, calculated according to section A36.9.3.2.1,
must be added to each peak to give corresponding adjusted peak
values of PNLT. If these peak values exceed the value at the moment
of PNLTM, the maximum value of such exceedance must be added as a
further adjustment to the EPNL calculated from the measured
data.
A36.9.3.3 Adjustments to duration correction.
A36.9.3.3.1 Whenever the measured flight paths and/or the ground
velocities of the test conditions differ from the reference flight
paths and/or the ground velocities of the reference conditions,
duration adjustments must be applied to the EPNL values calculated
from the measured data. The adjustments must be calculated as
described below.
A36.9.3.3.2 For the flight path shown in Figure A36-6, the
adjustment term is calculated as follows:
Δ2 = −7.5 log(QK/QrKr) + 10 log(V/Vr)
(a) Add Δ2 arithmetically to the EPNL calculated from the
measured data.
A36.9.3.4 Source noise adjustments.
A36.9.3.4.1 To account for differences between the parameters
affecting engine noise as measured in the certification flight
tests, and those calculated or specified in the reference
conditions, the source noise adjustment must be calculated and
applied. The adjustment is determined from the manufacturer's data
approved by the FAA. Typical data used for this adjustment are
illustrated in Figure A36-8 that shows a curve of EPNL versus the
engine control parameter μ, with the EPNL data being corrected to
all the other relevant reference conditions (airplane mass, speed
and altitude, air temperature) and for the difference in noise
between the test engine and the average engine (as defined in
section B36.7(b)(7)). A sufficient number of data points over a
range of values of μr is required to calculate the source noise
adjustments for lateral, flyover and approach noise
measurements.
A36.9.3.4.2 Calculate adjustment term Δ3 by subtracting the EPNL
value corresponding to the parameter μ from the EPNL value
corresponding to the parameter μr. Add Δ3 arithmetically to the
EPNL value calculated from the measured data.
A36.9.3.5 Symmetry adjustments.
A36.9.3.5.1 A symmetry adjustment to each lateral noise value
(determined at the section B36.4(b) measurement points), is to be
made as follows:
(a) If the symmetrical measurement point is opposite the point
where the highest noise level is obtained on the main lateral
measurement line, the certification noise level is the arithmetic
mean of the noise levels measured at these two points (see Figure
A36-9(a));
(b) If the condition described in paragraph (a) of this section
is not met, then it is assumed that the variation of noise with the
altitude of the airplane is the same on both sides; there is a
constant difference between the lines of noise versus altitude on
both sides (see figure A36-9(b)). The certification noise level is
the maximum value of the mean between these lines.
A36.9.4 Integrated method of adjustment
A36.9.4.1 General. As described in this section, the
integrated adjustment method consists of recomputing under
reference conditions points on the PNLT time history corresponding
to measured points obtained during the tests, and computing EPNL
directly for the new time history obtained in this way. The main
principles are described in sections A36.9.4.2 through
A36.9.4.4.1.
A36.9.4.2 PNLT computations.
(a) The portions of the test flight path and the reference
flight path described in paragraph (a)(1) and (2), and illustrated
in Figure A36-10, include the noise time history that is relevant
to the calculation of flyover and approach EPNL. In figure
A36-10:
(1) XY represents the portion of the measured flight path that
includes the noise time history relevant to the calculation of
flyover and approach EPNL; XrYr represents the corresponding
reference flight path.
(2) The points Q0, Q1, Qn represent airplane positions on the
measured flight path at time t0, t1 and tn respectively. Point Q1
is the point at which the noise was emitted and observed as
one-third octave values SPL(i)1 at the noise measuring station K at
time t1. Point Qr1 represents the corresponding position on the
reference flight path for noise observed as SPL(i)r1 at the
reference measuring station Kr at time tr1. Q1K and Qr1Kr are
respectively the measured and reference noise propagation paths,
which in each case form the angle θ1 with their respective flight
paths. Qr0 and Qrn are similarly the points on the reference flight
path corresponding to Q0 and Qn on the measured flight path. Q0 and
Qn are chosen so that between Qr0 and Qrn all values of PNLTr
(computed as described in paragraphs A36.9.4.2.2 and A36.9.4.2.3)
within 10 dB of the peak value are included.
(b) The portions of the test flight path and the reference
flight path described in paragraph (b)(1) and (2), and illustrated
in Figure A36-11(a) and (b), include the noise time history
that is relevant to the calculation of lateral EPNL.
(1) In figure A36-11(a) XY represents the portion of the
measured flight path that includes the noise time history that is
relevant to the calculation of lateral EPNL; in figure A36-11(b),
XrYr represents the corresponding portion of the reference flight
path.
(2) The points Q0, Q1 and Qn represent airplane positions on the
measured flight path at time t0, t1 and tn respectively. Point Q1
is the point at which the noise was emitted and observed as
one-third octave values SPL(i)1 at the noise measuring station K at
time t1. The point Qr1 represents the corresponding position on the
reference flight path for noise observed as SPL(i)r1 at the
measuring station Kr at time tr1. Q1K and Qr1Kr are respectively
the measured and reference noise propagation paths. Qr0 and Qrn are
similarly the points on the reference flight path corresponding to
Q0 and Qn on the measured flight path.
Q0 and Qn are chosen to that between Qro and Qrn all values of
PNLTr (computed as described in paragraphs A36.9.4.2.2 and
A36.9.4.2.3) within 10 dB of the peak value are included. In this
case Kr is only specified as being on a particular lateral line.
The position of Kr and Qr1 are determined from the following
requirements.
(i) Q1K and Qr1Kr form the same angle θ1 with their respective
flight paths; and
(ii) The differences between the angles 1 and r1 must be
minimized using a method, approved by the FAA. The differences
between the angles are minimized since, for geometrical reasons, it
is generally not possible to choose Kr so that the condition
described in paragraph A36.9.4.2(b)(2)(i) is met while at the same
time keeping 1 and r1 equal.
Note:
For the lateral noise measurement, sound propagation is affected
not only by “inverse square” and atmospheric attenuation, but also
by ground absorption and reflection effects which depend mainly on
the angle.
A36.9.4.2.1 In paragraphs A36.9.4.2(a)(2) and (b)(2) the time
tr1 is later (for Qr1Kr >Q1K) than t1 by two separate
amounts:
(1) The time taken for the airplane to travel the distance
Qr1Qr0 at a speed Vr less the time taken for it to travel Q1Q0 at
V;
(2) The time taken for sound to travel the distance
Qr1Kr-Q1K.
Note:
For the flight paths described in paragraphs A36.9.4.2(a) and
(b), the use of thrust or power cut-back will result in test and
reference flight paths at full thrust or power and at cut-back
thrust or power. Where the transient region between these thrust or
power levels affects the final result, an interpolation must be
made between them by an approved method such as that given in the
current advisory circular for this part.
A36.9.4.2.2 The measured values of SPL(i)1 must be adjusted to
the reference values SPL(i)r1 to account for the differences
between measured and reference noise path lengths and between
measured and reference atmospheric conditions, using the methods of
section A36.9.3.2.1 of this appendix. A corresponding value of
PNLr1 must be computed according to the method in section A36.4.2.
Values of PNLr must be computed for times t0 through tn.
A36.9.4.2.3 For each value of PNLr1, a tone correction factor C1
must be determined by analyzing the reference values SPL(i)r using
the methods of section A36.4.3 of this appendix, and added to PNLr1
to yield PNLTr1. Using the process described in this paragraph,
values of PNLTr must be computed for times t0 through tn.
A36.9.4.3 Duration correction.
A36.9.4.3.1 The values of PNLTr corresponding to those of PNLT
at each one-half second interval must be plotted against time
(PNLTr1 at time tr1). The duration correction must then be
determined using the method of section A36.4.5.1 of this appendix,
to yield EPNLr.
A36.9.4.4 Source Noise Adjustment.
A36.9.4.4.1 A source noise adjustment, Δ3, must be determined
using the methods of section A36.9.3.4 of this appendix.
A36.9.5 Flight Path Identification
Positions
Position |
Description |
A |
Start of Takeoff roll. |
B |
Lift-off. |
C |
Start of first constant
climb. |
D |
Start of thrust
reduction. |
E |
Start of second constant
climb. |
F |
End of noise certification
Takeoff flight path. |
G |
Start of noise certification
Approach flight path. |
H |
Position on Approach path
directly above noise measuring station. |
I |
Start of level-off. |
J |
Touchdown. |
K |
Noise measurement point. |
Kr |
Reference measurement
point. |
K1 |
Flyover noise measurement
point. |
K2 |
Lateral noise measurement
point. |
K3 |
Approach noise measurement
point. |
M |
End of noise certification
Takeoff flight track. |
O |
Threshold of Approach end of
runway. |
P |
Start of noise certification
Approach flight track. |
Q |
Position on measured Takeoff
flight path corresponding to apparent PNLTM at station K See
section A36.9.3.2. |
Qr |
Position on corrected Takeoff
flight path corresponding to PNLTM at station K. See section
A36.9.3.2. |
V |
Airplane test speed. |
Vr |
Airplane reference speed. |
A36.9.6 Flight Path Distances
Distance |
Unit |
Meaning |
AB |
Feet (meters) |
Length of takeoff roll. The
distance along the runway between the start of takeoff roll and
lift off. |
AK |
Feet (meters) |
Takeoff measurement distance.
The distance from the start of roll to the takeoff noise
measurement station along the extended center line of the
runway. |
AM |
Feet (meters) |
Takeoff flight track distance.
The distance from the start of roll to the takeoff flight track
position along the extended center line of the runway after which
the position of the airplane need no longer be recorded. |
QK |
Feet (meters) |
Measured noise path. The
distance from the measured airplane position Q to station K. |
QrKr |
Feet (meters) |
Reference noise path. The
distance from the reference airplane position Qr to station
Kr. |
K3H |
Feet (meters) |
Airplane approach height. The
height of the airplane above the approach measuring station. |
OK3 |
Feet (meters) |
Approach measurement distance.
The distance from the runway threshold to the approach measurement
station along the extended center line of the runway. |
OP |
Feet (meters) |
Approach flight track
distance. The distance from the runway threshold to the approach
flight track position along the extended center line of the runway
after which the position of the airplane need no longer be
recorded. |
[Amdt. 36-54, 67 FR 45212, July 8, 2002; Amdt. 36-24, 67 FR 63195,
63196, Oct. 10, 2002; 68 FR 1512, Jan. 10, 2003; Amdt. 36-26, 70 FR
38749, July 5, 2005; FAA Doc. No. FAA-2015-3782, Amdt. No. 36-31,
82 FR 46131, Oct. 4, 2017]