402(a)(3)(D)(ix) Airframe noise181
402(a)(3)(D)(x) Airplane flight path considerations181
402(a)(3)(D)(xi) Total noise spectra181
402(a)(3)(D)(xii) EPNL computations182
402(a)(3)(D)(xiii) Changes to noise levels182
402(b) Propeller-Driven Airplanes Over 19,000 lb182
402(b)(1) Flight test procedures182
402(b)(1)(A) Flight path intercept procedures182
402(b)(1)(B) Generalized flight test procedures182
402(b)(1)(C) Determination of the lateral certification noise level183
402(b)(1)(D) Measurements at non-reference points184
402(b)(2) Analytical procedures185
402(b)(3) Ground static testing procedures186
402(b)(3)(A) General186
402(b)(3)(B) Guidance on the test-site characteristics186
402(b)(3)(C) Static tests of the gas generator186
402(c) Helicopters186
402(c)(1) Flight test procedures187
402(c)(1)(A) Helicopter test speed187
402(c)(1)(B) Atmospheric test conditions187
402(c)(1)(C) Temperature and relative humidity measurements187
402(c)(1)(D) Modifications or upgrades involving aerodynamic drag changes188
402(c)(1)(E) Anomalous test conditions188
402(c)(1)(F) Helicopter test rotor speed189
402(c)(1)(G) Helicopter test weight189
402(c)(1)(H) Helicopter approach189
402(c)(2) Analytical procedures189
402(c)(2)(A) Helicopter test window for zero adjustment for atmospheric attenuation189
402(c)(2)(B) Procedure for the determination of source noise adjustment191
403 TECHNICAL PROCEDURES INFORMATION192
403(a) Jet and Propeller-Driven Airplanes and Helicopters192
403(a)(1) Adjustment of measured noise data to reference conditions192
403(a)(2) Determination of reference-condition noise geometry192
403(a)(2)(A) Assumptions193
403(a)(2)(B) Steps involved193
403(a)(2)(C) Characterization of a straight-line average flight path based on descriptors193
403(a)(2)(D) Determination of test aircraft position at time of sound emission195
403(a)(2)(E) Calculation of the geometric minimum distance between the test flight path197
403(a)(2)(F) Determination of test aircraft noise geometry200
403(a)(2)(G) Determination of reference flight path201
403(a)(2)(H) Determination of reference sound propagation distance202
403(a)(2)(I) Determination of effective duration for each acoustic spectrum203
403(a)(3) Computation of EPNL by the simplified method of adjustment205
403(a)(3)(A) Inputs required for performing the simplified method205
403(a)(3)(B) Adjustment of sound pressure levels to reference conditions206
403(a)(3)(C) Determination of reference-condition tone-corrected perceived noise levels206
403(a)(3)(D) Application of test-day bandsharing adjustment to reference-condition206
403(a)(3)(E) Determination of the ΔPeak simplified adjustment term to account for the effects207
403(a)(3)(G) Determination of the Δ3 simplified adjustment term to account for differences207
403(a)(3)(H) Determination of the simplified reference-condition effective perceived noise level208
403(a)(3)(I) Worked example calculations illustrating the simplified method208
403(a)(4) Computation of EPNL by the integrated method of adjustment208
403(a)(4)(A) Inputs required for performing the integrated method208
403(a)(4)(B) Adjustment of sound pressure levels to reference conditions209
403(a)(4)(C) Determination of reference-condition tone-corrected perceived noise209
403(a)(4)(D) Identification of integrated reference-condition maximum tone-corrected209
403(a)(4)(E) Identification of integrated reference-condition EPNL Noise Duration210
403(a)(4)(F) Summation of integrated reference-condition EPNL210
403(a)(4)(G) Worked example calculations illustrating the integrated method210
403(b) Jet Airplanes211
403(b)(1) Control of noise certification computer program software and documentation211
403(b)(1)(A) General211
403(b)(1)(B) Software control procedures — four key elements212
403(b)(1)(B)(i) Configuration index212
403(b)(1)(B)(ii) Software control plan212
403(b)(1)(B)(iii) Design description212
403(b)(1)(B)(iv) Verification process212
403(b)(1)(C) Applicability212
403(b)(2) Identification of spectral irregularities212
403(b)(2)(A) Introduction213
403(b)(2)(B) Methods for identifying false tones213
403(b)(2)(B)(i) Frequency tracking213
403(b)(2)(B)(ii) Narrow-band analysis214
403(b)(2)(B)(iii) Microphone mounting height214
403(b)(2)(B)(iv) Inspection of noise time-histories214
403(b)(2)(C) Treatment of false tones214
403(b)(3) Noise data adjustments for tests at high altitude test sites214
403(b)(3)(A) Introduction214
403(b)(3)(B) Jet source noise adjustment214
403(b)(3)(B)(i) Criteria215
403(b)(3)(B)(ii) Adjustment procedures215
EXPLANATORY INFORMATION [ETM 4.1] Noise Measurements 401(a) §A36.2 [ETM 4.1.1] See 301 for technical procedures generally applicable for noise certification tests of all aircraft types including those evaluated under the provisions of Appendices A or H of part 36. Procedures specific to Appendix A or H are presented in the following section.
Noise Certification Test and Measurement Conditions 401(b) §A36.2 [ETM 4.1.2] 401(b)(1) Ambient temperature GM §A36.2.2.2(b) [ETM GM A2 2.2.2.4.1 b)]
(1) Criteria for measuring atmospheric conditions Experience has shown that proper measurement of non-r eference meteorological conditions and the associated adjustment of noise data for these conditions are crucial to obtaining accurate, consistent and repeatable test results. For airplanes, meteorological observations of the temperature and relative humidity are required over the whole sound propagation path from the aircraft to the vicinity of the noise measurem ent points. For helicopters, temperature and relative humidity measurements are require d at 33 ft (10 m) in the vicinity of the noise measurement points.
AMC §A36.2.2.2(b) [ETM AMC A2 2.2.2.4.1 b)] Page 116
(1) Atmospheric measurements Several methods have been approved for the measurement of atmospheric conditions from 33 ft (10 m) above the ground to the altitude of the test airplane. Some appli cants have used instrument ed balloons. Another method consists of a meteorological airplane, manned or unmanned, flown in a spiral flight path in the vicinity of the noise measurement points to measure the dry bulb temperature and dew point along the sound propagation path.
401(b)(2) Calculation of sound attenuation coefficients for the ef fects of atmospheric absorption AMC §A36.2.2.2(d) [ETM AMC A2 2.2.2.2.1)]
(1) Basic data Measurements of the ambient temperature and relative humidity should be made at 33 ft (10 m) above the ground.
The ambient temperature and relative humidity should also be determined with vertical height increments not greater than 100 ft (30 m) over the sound propagation path. All measurements of ambient temperature and relative humidity must be obtained within 30 minutes of each airplane test run.
(2) Determination of the average sound attenuation coefficient Table 11 is an example of calculation of sound attenuation coefficients in the 3150 Hz one-third octave band for an airplane approach noise certification when multiple layering is not required. Temperature and humidity values obtained from atmospheric soundings performed before and after a series of airplane test runs are interpolated to the time of PNLTM.
Table 11. Basic data (layering not required) Height Temperature Relative humidity α (3150 Hz)
(m) (°C) (%) (dB/100 m) 10 14.1 50 2.45 30 13.4 53 2.38 60 12.9 56 2.30 90 12.2 57 2.33 120 11.5 58 2.37 150 11.3 61 2.27 The individual coefficients shown in Table 11are calculated at vertical height increments of 30 m from 10 m to a height of 150 m. The ambient conditions from the ground to 10 m are assumed to be those measured at 10 m.
The individual sound attenuation coefficients for the 3150 Hz one-third octave band shown in Table 11 vary by less than 0.5 dB/100 m relative to the value determined at 10 m (33 ft). In this case the coefficient to be used for adjustment of sound pressure levels fr om test to reference conditions is the average of the coefficients at 10 m (33 ft) and at the height of the airplane at the time of PNLTM.
For this example, the height of the test airplane at the time of PNLTM is 125 m. The associated attenuation Page 117
coefficient is calculated by linear interpolation as follows:
ݕൌ ݕ ሺݔെݔ ሻ ∗ሺ ݕ െݕ ሻሺݔ െݔ ሻ ߙሺ3150ሻଵଶହ ൌ2 . 3 7 ሺ125 െ 120ሻ ∗ ሺ2.27 െ 2.37ሻሺ150 െ 120ሻ ߙሺ3150ሻଵଶହ ൌ 2.35/ܤ݀100݉ .
Then the average attenuation coefficient for the 3150 Hz one-third octave band used for adjustment of the airplane sound pressure levels is calculated as follows:
ߙሺ3150ሻ ൌ ߙሺ3150ሻଵ ߙሺ3150ሻଵଶହ 2 ߙሺ3150ሻ ൌ 2.45 2.352 ߙሺ3150ሻ ൌ 2.40/ܤ݀100݉ .
The coefficients for the other one-third octave band s are determined in a similar manner. These average coefficients are then used in the adjustment of airplane SPLs to reference conditions. The same general procedure would be used if no layering was required for determin ing the average coefficients during flyover and lateral noise certification measurements.
(3) Determination of the cumulative sound attenuation coefficients Table 12 is an example of calculation of sound attenuation coefficients in the 3150 Hz one-third octave band for an airplane flyover noise certification when multiple layering is required. Temperature and humidity values obtained from atmospheric soundings performed before and after a series of airplane test runs are interpolated to the time of PNLTM.
Table 12. Basic data (layering required) Height Temperature Relative humidity α (3 150 Hz)
(m) (°C) (%) (dB/100 m) 10 7.2 80 2.09 30 7.2 75 2.23 60 8.9 73 2.11 90 10.0 67 2.19 120 10.6 63 2.27 150 10.6 62 2.31 180 10.6 61 2.34 210 10.6 59 2.43 240 11.1 55 2.57 270 11.7 53 2.59 Page 118
Height
(m) 300 330 360 390 420 Temperature (°C)
11.7
11.1
11.1
11.1
11.1 Relative humidity (%) 51 51 50 47 46 α (3 150 Hz) (dB/100 m)
2.70
2.79
2.84
3.04
3.10 The individual coefficients shown in Table 12 are calculated at vertical height increments of 30 m from 10 m to a height of 420 m. The ambient conditions from the ground to 10 m are assumed to be those measured at 10 m.
The individual sound attenuation coefficients for the 3150 Hz one-third octave band shown in Table 12 vary by more than 0.5 dB/100 m. In this case the coefficient to be used for adjustment of sound pressure levels from test to reference conditions is the cumulative sound attenuation from the ground to the height of the airplane at the time of PNLTM.
In the absence of extreme or anomalous conditions (e.g., large variations in, or inversions of, temperature and/or humidity), which will generally be the case, it is acceptable, subject to the approval by the FAA, to determine the cumulative sound attenuation coefficients for each one-third octave band from a simple average of the coefficients at the boundaries of each layer.
Where extreme or anomalous conditions are present (e.g., large variations in, or inversions of, temperature and/or humidity) the cumulative sound attenuation coefficients for each one-third octave band should be determined by apportioning the sound attenuation coefficients for each layer. Table 13 illustrates an example of such a method.
Table 13. Determination of the cumulative sound attenuation Average layer Apportioned Effective sound sound Effective layer Sound attenuation attenuation attenuation layer depth coefficients, coefficients, coefficients, Layer depth proportion α (3 150 Hz) α (3 150 Hz) α (3 150 Hz) boundaries (m) (%) (dB/100 m) (dB/100 m) (dB/100 m) 0–30 28.8 7.03 2.09–2.23 2.16 0.1518 30–60 30.0 7.32 2.23–2.11 2.17 0.1589 60–90 30.0 7.32 2.11–2.19 2.15 0.1574 90–120 30.0 7.32 2.19–2.27 2.23 0.1633 120–150 30.0 7.32 2.27–2.31 2.29 0.1676 150–180 30.0 7.32 2.31–2.34 2.32 0.1698 180–210 30.0 7.32 2.34–2.43 2.39 0.1750 210–240 30.0 7.32 2.43–2.57 2.50 0.1830 240–270 30.0 7.32 2.57–2.59 2.58 0.1889 270–300 30.0 7.32 2.59–2.70 2.65 0.1940 300–330 30.0 7.32 2.70–2.79 2.74 0.2006 330–360 30.0 7.32 2.79–2.84 2.82 0.2064 Page 119
Average layer Apportioned Effective sound sound Effective layer Sound attenuation attenuation attenuation layer depth coefficients, coefficients, coefficients, Layer depth proportion α (3 150 Hz) α (3 150 Hz) α (3 150 Hz) boundaries (m) (%) (dB/100 m) (dB/100 m) (dB/100 m) 360–390 30.0 7.32 2.84–3.04 2.94 0.2152 390–411 21.0 5.12 3.04–3.08 3.06 0.1568 Cumulative 2.49 sound attenuation coefficient, α (3150 Hz) (dB/100 m):
The atmosphere is first divided into layers from the ground to the airplane height. For this example the height of the airplane at the time of PNLTM is 411 m.
The sound attenuation coefficient at the height of the test airplane is calculated by linear interpolation of the sound attenuation coefficients at the upper and lower boundaries of the uppermost layer.
The effective layer depth is determined as follows: for all layers between the airplane and the microphone, except the lowest layer containing the microphone and the upperm ost layer containing the airplane, the effective layer depth is the full 30 m; for the lowest layer containing the microphone, the effective layer depth is 30 m minus the
1.2 m height of the microphone; for the uppermost layer containing the airplane, the effective layer depth is the height of the airplane minus the height of the lower boundary of the layer.
The effective layer depth proportion for each layer is determined as the ratio of that layer’s effective depth relative to the total vertical component of the sound propagation distance from the microphone to the height of the airplane at the time of PNLTM.
The average sound attenuation coefficient for each layer is obtained by averaging the coefficients at the upper and lower boundaries of the layer.
The apportioned sound attenuation coefficient for each layer is obtained by multiplying the average layer sound attenuation coefficient by the effective layer depth proportion.
The summation of all apportioned sound attenuation coefficients results in the cumulative sound attenuation coefficient. In this example the cumulative coefficient is calculated for the 3 150 Hz one-third octave band. The same general procedure would be used to obtain the cumulative sound attenuation coefficient for each one-third octave band. These coefficients are then used in the adjustment of airplane SPLs to reference conditions.
401(b)(3) Wind speed AMC §A36.2.2.2(e) [ETM AMC A2 2.2.2.4.1 e) f) g) h)]
(1) Real-time crosswind component measurements Applicants are advised to provide ap proved real-time crosswind component measurement systems such that the crosswind component speeds can be verified after each aircraft test run. When the applicant uses a wind measurement system that is remotely located and not readily accessible, such as chart recorders that simultaneously and independently measure and record wind speed and direction, it may not be practical to determine the real-time crosswind component for each test run. For airplanes, if the applicant does not provide an acceptable real-time crosswind component measurement system, the 10 kt (5.1 m/s) maximum crosswind component and the 7 kt (3.6 m/s) average crosswind component become the maximum wind limitations regardless of wind direction.
401(b)(4) Anomalous meteorological conditions GM §A36.2.2.2(f) [ETM GM A2 2.2.2.4.1 i)]
(1) Anomalous winds For airplanes, compliance of measured wind speeds with the requirements of A36.2.2.2 e) of part 36 may not be sufficient to ensure that the wind speeds at the airplane height or along the sound propagation path are not excessive. Such conditions may exist as a steady headwind, tailwind or crosswind, or as a wind from varying directions with increasing height. Anomalous winds may affect the handling characteristics of an aircraft during the noise duration. They also may affect the transmitted noise. Anomalous winds include not only gusts and turbulent winds, but also wind shear, strong vertical winds, and high crosswinds at the aircraft height and along the sound propagation path. An applicant may be required to measure winds aloft and provide FAA with the information. Acceptability of the wind conditions over the propagation path will be determined by the (see A36.
2.2.2(f) of part 36).
(2) Winds aloft measurement Modern INS and DGPS can provide on-board aircraft data that can be used to quantify winds aloft. The measurement of winds aloft can further be processed to provide a permanent record of wind speed and direction.
(3) Effects of wind on airplane control FAA has permitted a ±20 percent tolerance in overhead test height and a ±10° lateral tolerance relative to the extended runway centerline. If the flight crew cannot fly within the pre-test-approved flight path tolerance limits, or experiences major variations in airspeed, or the airplane crabs or yaws significantly during the flight, adverse or anomalous wind conditions aloft are often the cause.
(4) Effects of wind on helicopter control If the test helicopter cannot be flown within the pre-test -approved flight path tolerance limits, or experiences major variations in airspeed, or the aircraft yaws or sideslips excessively during the flight, adverse or anomalous wind conditions aloft are often the cause. Normally such issues arise only with gusty wind conditions, high crosswinds or in the presence of strong thermals.
AMC §A36.2.2.2(f) [ETM AMC A2 2.2.2.4 i)]
(1) Flight path The flight crew should observe and record any occurrence where conditions aloft cause difficulty in maintaining the flight path or airspeeds, or when rough air in general makes the flight unacceptable.
In the context of determining whether such conditions are present for airplanes, B36.8(g) of part 36 specifies that “for take-off, lateral, and approach conditions, the vari ation in instantaneous IAS of the airplane must be maintained within ±3 percent of the average airspeed between the 10 dB-down points. This shall be determined by reference to the pilot’s airspeed indicator. However, when the instantaneous IAS varies from the average airspeed over the 10 dB-down points by more than ±3 kt (±5.5 km/h), and this is judged by the FAA representative on the flight deck to be due to atmospheric turbulence, then the flight so affected shall be rejected for noise certification purposes.”
401(b)(5) Time of meteorological measurements GM §A36.2.2.2(g) [ETM GM A2 2.2.2.2.1]
(1) Upper atmospheric condition measurements Page 121
Atmospheric conditions affect sound propagation. Therefore, measurements of temperature and relative humidity must be made before and after each aircraft test run, at least one of which must be made within 30 minutes of the test run. To avoid the possibility that the meteorological conditions might change significantly over time, both measurements must be representative of the prevailing conditions during the test run. The measurements must be made using an approved method at 33 ft (10 m) above the ground surface and, for airplanes only, from 33 ft (10 m) above the ground surface to the airplane test he ight at time of PNLTM. These measurements must be obtained and validated through out the test period to en sure acceptable meteorological data for the noise data evaluation process.
AMC §A36.2.2.2(g) [ETM AMC A2 2.2.2.2.1]
(1) Atmospheric measurements Applicants should consider the maximum height that will be attained within the next 60 minutes, or less, of airplane test runs to ensure that adequate upper at mospheric measurements are acq uired. Interpolations of atmospheric data for all test runs are made to the airp lane height at the time of PNLTM. To have sufficient meteorological data to perform the interpolation to the ac tual time of each test run, the first meteorological measurement flight of the day should be made not earlier than 30 minutes before the first test run, and the last meteorological measurement flight of the day should be made not later than 30 minutes after the last test run flight of the day.
(2) Atmospheric data interpolation The temperature and relative humidity data at the actual time of the test run must be interpolated over time and height, as necessary, from the measured meteorological data. The interpolation time of the test run may be taken to be either the time the aircraft flew overhead or abeam the noise measurement point, or the time of PNLTM.
401(b)(6) Aircraft position measurement GM §A36.2.3.1 [ETM GM A2 2.3.1]
(1) Independent aircraft position determination The FAA will approve only those aircraft position and height indicating and recording systems that are independent from the direct aircraft flight path indicating systems. The data from such independent systems should be recorded to produce a time-coordinated permanent record of each test.
The independent system restriction does not prohibit use of real-time flight guidance systems (e.g., CDI or GDI) on-board the aircraft to assist the flight crew during noise certification tests. Systems such as microwave space position systems, INS, precision distance measuring uni t (DMU), and DGPS can also provide guidance to the flight crew by providing the direct, real-time aircraft position relative to the extended runway centerline.
401(b)(7) Time space position information (TSPI) measurement system characteristics GM §A36.2.3.2 [ETM GM A2 2.3.2]
(1) Measurement system synchronization Approved aircraft position and height measurement systems must be time-synchronized with the noise and meteorological measurement systems. The time synchronization between noise measurements and aircraft position should be precise. A common time base should be used to synchronize noise, aircraft tracking and meteorological measurements (see §A36.2.3.2(b)(1)(3)(3)) of part 36 for details).
TSPI should be determined at intervals no greater than one-half second throughout the sound-measuring period
(i.e., within 10 dB of PNLTM) by an approved method that is independent from systems installed aboard, and Page 122
normally used to control, the aircraft. During processing, measured TSPI data must be interpolated over time to the time of sound emission of each one-half second noise data record within the 10 dB-down period. The time associated with each one-half second record is 0.75 seconds before the end of each 2-second exponential averaging period (see A36.3.7.6 of part 36).
Although the simplified procedure requir es adjustment of only the (PNLT) maximum record to the reference track, sound emission coordinates should be determined for each one-half second record for use in background noise adjustment procedures and/or for determination of incidence-dependent free-field microphone and windscreen adjustments.
(2) Measurement system component approval Some off-the-shelf TSPI equipment may require software enhancement to accommodate the specific installation.
Each applicant should submit information to the FAA about the software used. The FAA will determine whether the software yields results that satisfy the part 36 Standards. All TSPI equipment and software should be demonstrated to, and approved by, the FAA to ensure the system’s operational accuracy.
(3) Methods of time synchronization Special care should be taken to properly synchronize noise data recordings with TSPI data (see 304 of this AC for details of specific methods).
401(b)(8) Aircraft performance GM §A36.2.3.3 [ETM GM A2 2.3.3]
(1) Aircraft and engine performance parameters Examples of parameters needed for measurement of aircraft and engine performance include aircraft height, climb angle, airspeed, gross weight, flap position, landing gear position, engine thrust (power) setting parameters (e.g., compressor rotor speed, engine pressure ratio, exhaust ga s temperature), and aircraft accessory condition (e.g., air conditioning and auxiliary power unit (APU) “on” or “off”). Any other parameters that may affect measurement or adjustment of noise data and/or aircraft or engine performance should also be recorded throughout the 10 dB-down period (e.g., the status of surge bleed valves (SBV) and the center of gravity (CG) position).
AMC §A36.2.3.3 [ETM AMC A2 2.3.3]
(1) Aircraft performance measurements Calibrated instrumentation is required to determine aircraft performance. Adequate aircraft and engine parameters are to be recorded during all certification testing to ensure that aircraft performance can be accurately determined.
For example, for transport airplanes this may necessitate measurement and recording of flap position, landing gear position, speed brake position, APU operation, and normal engine thrust (power) setting and associated flight parameters. Determination and recording of adequate information enables validation of the test configuration and adjustment of performance and engine performance from test conditions to reference conditions specified in B36.7(a)(5) of part 36.
(2) Recorder sampling rate The measurements of aircraft position, airspeed, perf ormance and engine performance parameters are to be recorded at an approved sampling rate sufficient to permit adjustments from test to reference conditions throughout the 10 dB-down period. An acceptable recording sampling rate for transport category airplanes is two to five samples per second.
401(c) Measurement of Aircraft Noise Received on the Ground §A36.3 [ETM 4.1.3] 401(c)(1) Environmental specifications GM §A36.3.2.1 [ETM GM A2 3.2]
(1) Measurement system performance The environmental conditions for specifying the performance of a measurement system are specified in A36.3.2.1 of part 36.
401(c)(2) Measurement sy stem specification GM §A36.3.3.1 [ETM GM A2 3.3.1]
(1) Measurement system criteria The specifications for a measurement system allow flexibility in the procurement of measurement system components by the applicant. While on-site EPNL analysis may be useful for estimation of recording levels or for other diagnostic purposes, a true acoustical analysis requires that data be recorded in the field. This will allow for later re-analysis or auditing of acoustical data. A recording also facilitates later offline processing of acoustic data, including application of adjustments for items such as system frequency response, microphone pressure response and analyzer bandwidth error. Recording simplifies synchronization with other pertinent data, such as tracking and meteorological measurements. Such synchronization is necessary for proper application of many of the required adjustments to noise data, such as adjustments for microphone free-field response, windscreen incidence-dependent insertion loss, the influence of ambient noise, high altitude jet noise effects, non-reference flight performance, and non-reference meteorological conditions.
(2) Approval of measurement system The approval by the FAA should be obtained for systems used for measurement, recording and analysis of aircraft noise. Most of the currently available system components that are appropriate for aircraft noise certification have already been approved, but implementation of new technology and variants or upgrades of existing components may require approval by the FAA. Of special concern is the potential for a digital component’s functionality to change as a result of firmware or operating system upgrades or modifications. Applicants should be aware that approval of a particular component might be version-dependent.
401(c)(3) Microphone orientation AMC §A36.3.5.2 [ETM AMC A2 3.5.2]
(1) Microphone orientation Figure 8 shows the orientations relative to a microphone sensing unit for grazing and normal incidence. For microphones located directly under the flight path, an orientation angle of 90° from vertical is appropriate regardless of target height. For noise measurements to the side of the flight path, applicants may wish to reorient the microphones for grazing incidence for each target height in order to maintain substantially grazing incidence throughout the 10 dB-down periods. In many cases, this reorientation can eliminate the need to apply data adjustments for varying incidence, since the incidence angles will be more likely to be contained within ±30º of grazing incidence. Figure 8 provides illustrations of microphones positioned for grazing incidence under the flight path and to the side of the flight path of an airplane.
401(c)(4) Microphone specification GM §A36.3.5.4(b) [ETM GM A2 3.5.4]
(1) Microphone specifications Page 124
Table A36-1 specifies the maximum permitted differences between the free-field sensitivity of a microphone at normal incidence and the free-field sensitivity at specified sound incidence angles for sinusoidal sound waves at each one-third octave band nominal midband frequency over the range of 50 Hz to 10 kHz. These differences are larger at higher frequencies, allowing for the effect of the microphone body in a free-field environment.
(2) Microphone characteristics The specifications of Table A36-1 are based on the performance characteristics of typical one-half inch condenser microphones designed for nearly uniform frequency response at grazing incidence (see Figure 8). Other microphones may be used, provided they meet the specified performance requirements. For example, prepolarized (i.e., electret condenser) free -field microphones greatly minimize the possibility of arcing in humid environments and do not require an external polarization voltage. Although many of these microphones are intended primarily for use in normal-incidence free-fiel d applications, they can be used in aircraft noise certification testing if their performance at grazing incidence meets the requirements of A36.3.5 of part 36.
401(c)(5) Recorder specifications GM §A36.3.6.1 [ETM GM A2 3.6.1]
(1) Recorder types An applicant has a choice of recorder types that will satisfy the requirement for recording “the complete acoustic signal” during certification testing. In addition to a magnetic tape recorder, other means of attaining a “true” acoustic recording include digital audiotape (DAT), reco rdable compact disc (CD-R) and direct-to-hard-disk recording. The applicant should be aware that systems that use data compression techniques that result in substantial data loss, such as mini-disc (MD) or digital compact cassette (DCC), are not acceptable.
AMC §A36.3.6.1 [ETM AMC A2 3.6.1]
(1) Frequency range for recordings The time-varying waveform produced by the microphone response to noise signals during certification tests should be recorded. If there are questions about the data observed during the tests, the recording can be replayed, multiple times if necessary, to verify the results. Recorded data, whether digital or analog in nature, should allow reproduction and reprocessing of an analog signal over the frequency range of 40 Hz to 12.6 kHz. A dynamic range of at least 60 dB is recommended.
Many typical instrumentation DAT recorders feature a nominal 10 kHz bandwidth operating mode in which the attenuating response of the anti-aliasing filter intrudes within the 10 kHz one-third octave passband. In such cases, the recorder should be operated in a nominal 20 kHz bandwidth mode, which may reduce the number of available channels or the duration of available time per tape.
Note.— Although the one-third octave bands of interest are those with nominal center frequencies of 50 Hz through 10 kHz, to ensure that the entire actual band width of the uppermost and lowermost bands is included, the center frequencies of the one-third octave bands immediately outside this range are specified.
0° normal incidence Sensing element 180° 120° 60° 90° grazing incidence Figure 8. Illustration of sound incidence angles on a microphone
(2) Digital recording levels The overload characteristic of a digita l system is determined primarily by the limits of the analog-to-digital conversion. Since such an overload co ndition is characterized by an abrupt, catastrophic type of distortion, the level range should be set so that the anticipated maximu m signal level is at least 10 dB, and preferably 20 dB, below the upper boundary of the linear operating range.
(3) Dynamic range limits for digital recorders The lower limit of a digital recording system’s usable dynamic range is more often determined by amplitude nonlinearity due to “quantization error”, rather than by the presence of a noise floor. Digital devices such as recorders or analyzers that are to be used for aircraft noise certif ication purposes should be tested to determine the extent of such non-linearity.
(4) 16-bit quantization systems The theoretical dynamic range of such a system is usually assumed to be near 96 dB (i.e., 20 log (2 16)). At the lower limit of this range, there is a potential for a 6 dB error in the digitized signal versus the analog input signal that it represents. Reference T1 imposes a ±0.4 dB limit on acceptable linearity error in the reference level range and ±0.5 dB for a linear operating range of at least 50 dB. As amplitude levels are increased above the lower quantization limit, the linearity error is reduced. If the guidance for setting the level range is followed, the usable Page 126