FAR 36, APPENDIX H2: Part B -- Noise Measurement Under §36.801 -- FAA FARS, 14 CFR

PART B -- NOISE MEASUREMENT UNDER §36.801

Section H36.101 Noise certification test and measurement conditions.

(a) General. This section prescribes the conditions under which aircraft noise certification tests must be conducted and the measurement procedures that must be used to measure helicopter noise during each test.

(b) Test site requirements. (1) Tests to show compliance with established helicopter noise certification levels must consist of a series of takeoffs, level flyovers, and approaches during which measurement must be taken at noise measuring stations located at the measuring points prescribed in this section.

(2) Each takeoff test, flyover test, and approach test includes simultaneous measurements at the flight-track noise measuring station vertically below the reference flight path and at two sideline noise measuring stations, one on each side of the reference flight track 492 feet (150m) from, and on a line perpendicular to, the flight track of the noise measuring station.

(3) The difference between the elevation of either sideline noise measuring station may not differ from the flight-track noise measuring station by more than 20 feet.

(4) Each noise measuring station must be surrounded by terrain having no excessive sound absorption characteristics, such as might be caused by thick, matted, or tall grass, shrubs, or wooded areas.

(5) During the period when the takeoff, flyover, or approach noise/time record indicates the noise measurement is within 10 dB of PNLTM, no obstruction that significantly influences the sound field from the aircraft may exist --

(i) For any flight-track or sideline noise measuring station, within a conical space above the measuring position (the point on the ground vertically below the microphone), the cone being defined by an axis normal to the ground and by half-angle 80° from this axis; and

(ii) For any sideline noise measuring station, above the line of sight between the microphone and the helicopter.

(6) If a takeoff or flyover test series is conducted at weights other than the maximum takeoff weight for which noise certification is requested, the following additional requirements apply:

(i) At least one takeoff test must be conducted at a weight at, or above, the maximum certification weight.

(ii) Each test weight must be within +5 percent or −10 percent of the maximum certification weight.

(iii) FAA-approved data must be used to determine the variation of EPNL with weight for takeoff test conditions.

(7) Each approach test must be conducted with the aircraft stabilized and following a 6.0 degree ±0.5 degree approach angle and must meet the requirements of section H36.107 of this part.

(8) If an approach test series is conducted at weights other than the maximum landing weight for which certification is requested, the following additional requirements apply:

(i) At least one approach test must be conducted at a weight at, or above, the maximum landing weight.

(ii) Each test weight must exceed 90 percent of the maximum landing weight.

(iii) FAA-approved data must be used to determine the variation of EPNL with weight for approach test conditions.

(9) Aircraft performance data sufficient to make the corrections required under section H36.205 of this appendix must be recorded at an FAA-approved sampling rate using FAA approved equipment.

(1) No rain or other precipitation.

(2) Ambient air temperature between 36 °F and 95 °F (2.2 °C and 35 °C), inclusively, over that portion of the sound propagation path between the aircraft and a point 10 meters above the ground at the noise measuring station. The temperature and relative humidity measured at aircraft altitude and at 10 meters above ground shall be averaged and used to adjust for propagation path absorption.

(3) Relative humidity and ambient temperature over the portion of the sound propagation path between the aircraft and a point 10 meters above the ground at the noise measuring station is such that the sound attenuation in the one-third octave band centered at 8 kHz is not greater than 12 dB/100 meters and the relative humidity is between 20 percent and 95 percent, inclusively.

(4) Wind velocity as measured at 10 meters above ground does not exceed 10 knots (19 km/h) and the crosswind component does not exceed 5 knots (9 km/h). The wind shall be determined using a continuous thirty-second averaging period spanning the 10dB down time interval.

(5) No anomalous wind conditions (including turbulence) which will significantly affect the noise level of the aircraft when the noise is recorded at each noise measuring station.

(6) The wind velocity, temperature, and relative humidity measurements required under the appendix must be measured in the vicinity of noise measuring stations 10 meters above the ground. The location of the meteorological measurements must be approved by the FAA as representative of those atmospheric conditions existing near the surface over the geographical area which aircraft noise measurements are made. In some cases, a fixed meteorological station (such as those found at airports or other facilities) may meet this requirement.

(7) Temperature and relative humidity measurements must be obtained within 25 minutes of each noise test measurement. Meteorological data must be interpolated to actual times of each noise measurement.

(d) Aircraft testing procedures. (1) The aircraft testing procedures and noise measurements must be conducted and processed in a manner which yields the noise evaluation measure designated as Effective Perceived Noise Level (EPNL) in units of EPNdB, as prescribed in appendix A of this part.

(2) The aircraft height and lateral position relative to the centerline of the reference flight-track (which passes through the noise measuring point) must be determined by an FAA approved method which is independent of normal flight instrumentation, such as radar tracking, theodolite triangulation, laser trajectography, or photographic scaling techniques.

(3) The aircraft position along the flight path must be related to the noise recorded at the noise measuring stations by means of synchronizing signals at an approved sampling rate. The position of the aircraft must be recorded relative to the runway during the entire time period in which the recorded signal is within 10 dB of PNLTM. Measuring and sampling equipment must be approved by the FAA.

Section H36.103 Takeoff test conditions.

(a) This section, in addition to the applicable requirements of sections H36.101 and H36.205(b) of this appendix, applies to all takeoff noise tests conducted under this appendix to show compliance with Part 36.

(b) A test series must consist of at least six flights over the flight-track noise measuring station (with simultaneous measurements at all three noise measuring stations) as follows:

(1) An airspeed of either V_y±5 knots or the lowest approved speed ±5 knots for the climb after takeoff, whichever speed is greater, must be established during the horizontal portion of each test flight and maintained during the remainder of the test flight.

(2) The horizontal portion of each test flight must be conducted at an altitude of 65 feet (20 meters) above the ground level at the flight-track noise measuring station.

(3) Upon reaching a point 1,640 feet (500 meters) from the noise measuring station, the helicopter shall be stabilized at:

(i) The torque used to establish the takeoff distance for an ambient temperature at sea level of 25 °C for helicopters for which the determination of takeoff performance is required by airworthiness regulations; or

(ii) The torque corresponding to minimum installed power available for an ambient temperature at sea level of 25 °C for all other helicopters.

(4) The helicopter shall be maintained throughout the takeoff reference procedure at:

(i) The speed used ±5 knots to establish takeoff distance for an ambient temperature at sea level of 25 °C for helicopters for which the determination of takeoff performance is required by airworthiness regulations; or

(ii) The best rate of climb speed V_y±5 knots, or the lowest approved speed for climb after takeoff, whichever is greater, for an ambient temperature at sea level of 25 °C for all other helicopters.

(5) The rotor speed must be stabilized at the normal operating RPM (±1%) during the entire period of the test flight when the measured helicopter noise level is within 10 dB of PNLTM.

(6) The helicopter must pass over the flight-track noise measuring station within ±10° from the zenith.

Section H36.105 Flyover test conditions.

(a) This section, in addition to the applicable requirements of sections H36.101 and H36.205(c) of this appendix, applies to all flyover noise tests conducted under this appendix to show compliance with Part 36.

(b) A test series must consist of at least six flights (three in each direction) over the flight-track noise measuring station (with simultaneous measurements at all three noise measuring stations) --

(1) In level flight;

(2) At a height of 492 feet ±30 feet (150±9 meters) above the ground level at the flight-track noise measuring station; and

(3) Within ±5° from the zenith.

(1) At a speed of 0.9 V_H or 0.45 V_H+120 km/hr (0.45 V_H+65 kt), whichever is less, maintained throughout the measured portion of the flyover;

(2) At rotor speed stabilized at the normal operating rotor RPM (±1 percent); and

(3) With the power stabilized during the period when the measured helicopter noise level is within 10 dB of PNLTM.

(d) The airspeed shall not vary from the reference airspeed by more than ±5 knots (9 km/hr).

Section H36.107 Approach test conditions.

(a) This section, in addition to the requirements of sections H36.101 and H36.205(d) of this appendix, applies to all approach tests conducted under this appendix to show compliance with Part 36.

(b) A test series must consist of at least six flights over the flight-track noise measuring station (with simultaneous measurements at the three noise measuring stations) --

(1) On an approach slope of 6°±0.5°;

(2) At a height of 394±30 feet (120±9 meters) above the ground level at the flight-track noise measuring station;

(3) Within ±10° of the zenith;

(4) At stabilized airspeed equal to the certificated best rate of climb V_y, or the lowest approved speed for approach, whichever is greater, with power stabilized during the approach and over the flight path reference point, and continued to a normal touchdown; and

(5) At rotor speed stabilized at the maximum normal operating rotor RPM (±1 percent).

Section H36.109 Measurement of helicopter noise received on the ground.

(a) General. (1) The measurements prescribed in this section provide the data needed to determine the one-third octave band noise produced by an aircraft during testing, at specific noise measuring stations, as a function of time.

(2) Sound pressure level data for aircraft noise certification purposes must be obtained with FAA-approved acoustical equipment and measurement practices.

(3) Paragraphs (b), (c), and (d) of this section prescribe the required equipment specifications. Paragraphs (e) and (f) prescribe the calibration and measurement procedures required for each certification test series.

(b) Measurement system. The acoustical measurement system must consist of FAA-approved equipment equivalent to the following:

(1) A microphone system with frequency response and directivity which are compatible with the measurement and analysis system accuracy prescribed in paragraph (c) of this section.

(2) Tripods or similar microphone mountings that minimize interference with the sound energy being measured.

(3) Recording and reproducing equipment, the characteristics, frequency response, and dynamic range of which are compatible with the response and accuracy requirements of paragraph (c) of this section.

(4) Calibrators using sine wave, or pink noise, of known levels. When pink noise (defined in paragraph (e)(1) of this section) is used, the signal must be described in terms of its root-mean-square (rms) value.

(5) Analysis equipment with the response and accuracy which meets or exceeds the requirements of paragraph (d) of this section.

(6) Attenuators used for range changing in sensing, recording, reproducing, or analyzing aircraft sound must be capable of being operated in equal-interval decibel steps with no error between any two settings which exceeds 0.2 dB.

(c) Sensing, recording, and reproducing equipment. (1) The sound produced by the aircraft must be recorded in such a way that the complete information, including time history, is retained. A magnetic tape recorder is acceptable.

(2) The microphone must be a pressure-sensitive capacitive type, or its FAA-approved equivalent, such as a free-field type with incidence corrector.

(i) The variation of microphone and preamplifier system sensitivity within an angle of ±30 degrees of grazing (60-120 degrees from the normal to the diaphragm) must not exceed the following values:

------------------------------------------------------------------------
Change in
Frequency (Hz)                        sensitivity
erru                                                 (dB)
------------------------------------------------------------------------
45 to 1,120................................................           1
1,120 to 2,240.............................................         1.5
2,240 to 4,500.............................................         2.5
4,500 to 7,100.............................................           4
7,100 to 11,200............................................           5
------------------------------------------------------------------------

With the windscreen in place, the sensitivity variation in the plane of the microphone diaphragm shall not exceed 1.0 dB over the frequency range 45 to 11,200 Hz.

(ii) The overall free-field frequency response at 90 degrees (grazing incidence) of the combined microphone (including incidence corrector, if applicable) preamplifier, and windscreen must be determined by using either (A) an electrostatic calibrator in combination with manufacturer-provided corrections, or (B) an anechoic free-field facility. The calibration unit must include pure tones at each preferred one-third octave frequency from 50 Hz to 10,000 Hz. The frequency response (after corrections based on that determination) must be flat and within the following tolerances: 44-3,549 Hz ±0.25 dB 3,550-7,099 Hz ±0.5 dB 7,100-11,200 Hz ±1.0 dB

(iii) Specifications concerning sensitivity to environmental factors such as temperature, relative humidity, and vibration must be in conformity with the recommendations of International Electrotechnical Commission (IEC) Publication No. 179, entitled "Precision Sound Level Meters", as incorporated by reference under §36.6 of this part.

(iv) If the wind speed exceeds 6 knots, a windscreen must be employed with the microphone during each measurement of aircraft noise. Correction for any insertion loss produced by the windscreen, as a function of frequency, must be applied to the measured data and any correction applied must be reported.

(3) If a magnetic tape recorder is used to store data for subsequent analysis, the record/replay system (including tape) must conform to the following:

(i) The electric background noise produced by the system in each one-third octave must be at least 35 dB below the standard recording level, which is defined as the level that is either 10 dB below the 3 percent harmonic distortion level for direct recording or ±40 percent deviation for frequency modulation (FM) recording.

(ii) At the standard recording level, the corrected frequency response in each selected one-third octave band between 44 Hz and 180 Hz must be flat and within ±0.75 dB, and in each band between 180 Hz and 11,200 Hz must be flat and within ±0.25 dB.

(iii) If the overall system satisfies the requirements of paragraph (c)(2)(ii) of this section, and if the limitations of the dynamic range of the equipment are insufficient to obtain adequate spectral information, high frequency pre-emphasis may be added to the recording channel with the converse de-emphasis on playback. If pre-emphasis is added, the instantaneously recorded sound-pressure level between 800 Hz and 11,200 Hz of the maximum measured noise signal must not vary more than 20 dB between the levels of the maximum and minimum one-third octave bands.

(d) Analysis equipment. (1) A frequency analysis of the acoustic signal must be performed using one-third octave filters which conform to the recommendations of International Electrotechnical Commission (IEC) Publication No. 225, entitled "Octave, Half-Octave, and Third-Octave Band Filters Intended for Analysis of Sound and Vibrations," as incorporated by reference under §36.6 of this part.

(2) A set of 24 consecutive one-third octave filters must be used. The first filter of the set must be centered at a geometric mean frequency of 50 Hz and the last filter at 10,000 Hz. The output of each filter must contain less than 0.5 dB ripple.

(3) The analyzer indicating device may be either analog or digital, or a combination of both. The preferred sequence of signal processing is:

(i) Squaring the one-third octave filter outputs;

(ii) Averaging or integrating; and

(iii) Converting linear formulation to logarithmic.

(4) Each detector must operate over a minimum dynamic range of 60 dB and perform as a root-mean-square device for sinusoidal tone bursts having crest factors of at least 3 over the following dynamic range:

(i) Up to 30 dB below full-scale reading must be accurate within ±0.5 dB;

(ii) Between 30 dB and 40 dB below full-scale reading must be accurate within ±1.0 dB; and

(iii) In excess of 40 dB below full-scale reading must be accurate within ±2.5 dB.

(5) The averaging properties of the integrator must be tested as follows:

(i) White noise must be passed through the 200 Hz one-third octave band filter and the output fed in turn to each detector/integrator. The standard deviation of the measured levels must then be determined from a statistically significant number of samples of the filtered white noise taken at intervals of not less than 5 seconds. The value of the standard deviation must be within the interval 0.48±0.06 dB for a probability limit of 95 percent. An approved equivalent method may be substituted for this test on those analyzers where the test signal cannot readily be fed directly to each detector/integrator.

(ii) For each detector/integrator, the response to a sudden onset or interruption of a constant amplitude sinusoidal signal at the respective one-third octave band center frequency must be measured at sampling times 0.5, 1.0, 1.5, and 2.0 seconds after the onset or interruption. The rising responses must be in the following amounts before the steady-state level:

0.5 seconds, 4.0±1.0 dB

1.0 seconds, 1.75±0.5 dB

1.5 seconds, 1.0±0.5 dB

2.0 seconds, 0.6±0.25 dB

(iii) The falling response must be such that the sum of the decibel readings below the initial steady-state level, and the corresponding rising response reading is 6.5± 1.0 dB, at both 0.5 and 1.0 seconds and, on subsequent records, the sum of the onset plus decay must be greater than 7.5 decibels.

Note 1: For analyzers with linear detection, an approximation of this response would be given by:

SPL (i, k)-10 log  [0.17 (10[SU]0.1(Li,k-3)[/
SU])
+10.21 (0[SU]0.1(Li,k-2)[/
SU])
+0.24 (10[SU]0.1(Li,k-1)[/
SU])
+0.33 (10[SU]0.1(Li,k)[/
SU])]

When this approximation is used, the calibration signal should be established without this weighting.

Note 2: Some analyzers have been shown to have signal sampling rates that are insufficiently accurate to detect signals with crest factor ratios greater than three which is common to helicopter noise. Preferably, such analyzers should not be used for helicopter certification. Use of analysis systems with high signal sampling rates (greater than 40KHz) or those with analog detectors prior to digitization at the output of each one-third octave filter is encouraged.

(iv) Analyzers using true integration cannot meet the requirements of (i), (ii), and (iii) directly, because their overall average time is greater than the sampling interval. For these analyzers, compliance must be demonstrated in terms of the equivalent output of the data processor. Further, in cases where readout and resetting require a dead-time during acquisition, the percentage loss of the total data must not exceed one percent.

(6) The sampling interval between successive readouts shall not exceed 500 milliseconds and its precise value must be known to within ±1 one percent. The instant in time by which a readout is characterized shall be the midpoint of the average period where the averaging period is defined as twice the effective time constant of the analyzer.

(7) The amplitude resolution of the analyzer must be at least 0.25 dB.

(8) After all systematic errors have been eliminated, each output level from the analyzer must be accurate within ±1.0 dB of the level of the input signal. The total systematic errors for each of the output levels must not exceed ±3.0 dB. For contiguous filter systems, the systematic corrections between adjacent one-third octave channels must not exceed 4.0 dB.

(9) The dynamic range capability of the analyzer to display a single aircraft noise event, in terms of the difference between full-scale output level and the maximum noise level of the analyzer equipment, must be at least 60 dB.

(e) Calibrations. (1) Within five days prior to beginning each test series, the complete electronic system, as installed in field including cables, must be electronically calibrated for frequency and amplitude by the use of a pink noise signal of known amplitudes covering the range of signal levels furnished by the microphone. For purposes of this section, "pink noise" means a noise whose noise-power/unit-frequency is inversely proportional to frequency at frequencies within the range of 44 Hz to 11,200 Hz. The signal used must be described in terms of its average root-mean-square (rms) values for a nonoverload signal level. This system calibration must be repeated within five days of the end of each test series, or as required by the FAA.

(2) Immediately before and after each day's testing, a recorded acoustic calibration of the system must be made in the field with an acoustic calibrator to check the system sensitivity and provide an acoustic reference level for the sound level data analysis. The performance of equipment in the system will be considered satisfactory if, during each day's testing, the variation in the calibration value does not exceed 0.5 dB.

(3) A normal incidence pressure calibration of the combined microphone/preamplifier must be performed with pure tones at each preferred one-third octave frequency from 50 Hz to 10,000 Hz. This calibration must be completed within 90 days prior to the beginning of each test series.

(4) Each reel of magnetic tape must:

(i) Be pistonphone calibrated; and

(ii) At its beginning and end, carry a calibration signal consisting of at least a 15 second burst of pink noise, as defined in paragraph (e)(1) of this section.

(5) Data obtained from tape recorded signals are not considered reliable if the difference between the pink noise signal levels, before and after the tests in each one-third octave band, exceeds 0.75 dB.

(6) The one-third octave filters must have been demonstrated to be in conformity with the recommendations of IEC Publication 225 as incorporated by reference under §36.6 of this part, during the six calendar months preceding the beginning of each test series. However, the correction for effective bandwidth relative to the center frequency response may be determined for each filter by:

(i) Measuring the filter response to sinusoidal signals at a minimum of twenty frequencies equally spaced between the two adjacent preferred one-third octave frequencies; or

(ii) Using an FAA approved equivalent technique.

(7) A performance calibration analysis of each piece of calibration equipment, including pistonphones, reference microphones, and voltage insert devices, must have been made during the six calendar months preceding the beginning of each day's test series. Each calibration must be traceable to the National Bureau of Standards.

(f) Noise measurement procedures. (1) Each microphone must be oriented so that the diaphragm is substantially in the plane defined by the flight path of the aircraft and the measuring station. The microphone located at each noise measuring station must be placed so that its sensing element is approximately 4 feet above ground.

(2) Immediately before and immediately after each series of test runs and each day's testing, acoustic calibrations of the system prescribed in this section of this appendix must be recorded in the field to check the acoustic reference level for the analysis of the sound level data. Ambient noise must be recorded for at least 10 seconds and be representative of the acoustical background, including system noise, that exists during the flyover test run. During that recorded period, each component of the system must be set at the gain-levels used for aircraft noise measurement.

(3) The mean background noise spectrum must contain the sound pressure levels, which, in each preferred third octave band in the range of 50 Hz to 10,000 Hz, are the averages of the energy of the sound pressure levels in every preferred third octave. When analyzed in PNL, the resulting mean background noise level must be at least 20 PNdB below the maximum PNL of the helicopter.

(4) Corrections for recorded levels of background noise are allowed, within the limits prescribed in section H36.111(c)(3) of this appendix.

Section H36.111 Reporting and correcting measured data

(a) General. Data representing physical measurements, and corrections to measured data, including corrections to measurements for equipment response deviations, must be recorded in permanent form and appended to the record. Each correction must be reported and is subject to FAA approval. An estimate must be made of each individual error inherent in each of the operations employed in obtaining the final data.

(b) Data reporting. (1) Measured and corrected sound pressure levels must be presented in one-third octave band levels obtained with equipment conforming to the standards prescribed in section H36.109 of this appendix.

(2) The type of equipment used for measurement and analysis of all acoustic, aircraft performance, and meteorological data must be reported.

(3) The atmospheric environmental data required to demonstrate compliance with this appendix, measured throughout the test period, must be reported.

(4) Conditions of local topography, ground cover, or events which may interfere with sound recording must be reported.

(5) The following aircraft information must be reported:

(i) Type, model, and serial numbers, if any, of aircraft engines and rotors.

(ii) Gross dimensions of aircraft and location of engines.

(iii) Aircraft gross weight for each test run.

(iv) Aircraft configuration, including landing gear positions.

(v) Airspeed in knots.

(vi) Helicopter engine performance as determined from aircraft instruments and manufacturer's data.

(vii) Aircraft flight path, above ground level in feet, determined by an FAA approved method which is independent of normal flight instrumentation, such as radar tracking, theodolite triangulation, laser trajectography, or photographic scaling techniques.

(6) Aircraft speed, and position, and engine performance parameters must be recorded at an approved sampling rate sufficient to correct to the noise certification reference test conditions prescribed in section H36.3 of this appendix. Lateral position relative to the reference flight-track must be reported.

(c) Data corrections. (1) Aircraft position, performance data and noise measurement must be corrected to the noise certification reference conditions as prescribed in sections H36.3 and H36.205 of this appendix.

(2) The measured flight path must be corrected by an amount equal to the difference between the applicant's predicted flight path for the certification reference conditions and the measured flight path at the test conditions. Necessary corrections relating to aircraft flight path or performance may be derived from FAA-approved data for the difference between measured and reference engine conditions, together with appropriate allowances for sound attenuation with distance. The Effective Perceived Noise Level (EPNL) correction must be less than 2.0 EPNdB for any combination of the following:

(i) The aircraft's not passing vertically above the measuring station.

(ii) Any difference between the reference flight-track and the actual minimum distance of the aircraft's ILS antenna from the approach measuring station.

(iii) Any difference between the actual approach angle and the noise certification reference approach flight path.

(iv) Any correction of the measured level flyover noise levels which accounts for any difference between the test engine thrust or power and the reference engine thrust or power.

Detailed correction requirements are prescribed in section H36.205 of this appendix.

(3) Aircraft sound pressure levels within the 10 dB-down points must exceed the mean background sound pressure levels determined under section A36.3.10.1 by at least 5 dB in each one-third octave band or be corrected under an FAA approved method to be included in the computation of the overall noise level of the aircraft. An EPNL may not be computed or reported from data from which more than four one-third octave bands in any spectrum within the 10 dB-down points have been excluded under this paragraph.

(d) Validity of results. (1) The test results must produce three average EPNL values within the 90 percent confidence limits, each value consisting of the arithmetic average of the corrected noise measurements for all valid test runs at the takeoff, level flyovers, and approach conditions. The 90 percent confidence limit applies separately to takeoff, flyover, and approach.

(2) The minimum sample size acceptable for each takeoff, approach, and flyover certification measurements is six. The number of samples must be large enough to establish statistically for each of the three average noise certification levels a 90 percent confidence limit which does not exceed ±1.5 EPNdB. No test result may be omitted from the averaging process, unless otherwise specified by the FAA.

(3) To comply with this appendix, a minimum of six takeoffs, six approaches, and six level flyovers is required. To be counted toward this requirement, each flight event must be validly recorded at all three noise measuring stations.

(4) The approved values of V_H and V_y used in calculating test and reference conditions and flight profiles must be reported along with measured and corrected sound pressure levels.

Section H36.113 Atmospheric attenuation of sound.

(a) The values of the one-third octave band spectra measured during helicopter noise certification tests under this appendix must conform, or be corrected, to the reference conditions prescribed in section H36.3(a). Each correction must account for any differences in the atmospheric attenuation of sound between the test-day conditions and the reference-day conditions along the sound propagation path between the aircraft and the microphone. Unless the meteorological conditions are within the test window prescribed in this appendix, the test data are not acceptable.

(b) Attenuation rates. The atmospheric attenuation rates of sound with distance for each one-third octave band from 50 Hz to 10,000 Hz must be determined in accordance with the formulations and tabulations of SAE ARP 866A, entitled "Standard Values of Atmospheric Absorption as a Function of Temperatures and Humidity for Use in Evaluating Aircraft Flyover Noise", as incorporated by reference under §36.6 of this part.

(i) The ambient atmospheric conditions of temperature and relative humidity do not conform to the reference conditions, 77 °F and 70%, respectively, or

(ii) The measured flight paths do not conform to the reference flight paths.

(iii) The temperature and relative humidity measured at aircraft altitude and at 10 meters above the ground shall be averaged and used to adjust for propagation path absorption.

(2) The mean attenuation rate over the complete sound propagation path from the aircraft to the microphone must be computed for each one-third octave band from 50 Hz to 10,000 Hz. These rates must be used in computing the corrections required in section H36.111(d) of this appendix.

FAA Federal Aviation Regulations (FARS, 14 CFR)

FARs > Part 36 > Appendix H2 - Part B -- Noise Measurement Under §36.801

PART B -- NOISE MEASUREMENT UNDER §36.801

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