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
dynamic range is further decreased. Significant improv ement of amplitude linearity can be obtained via the implementation of techniques such as oversampling and dithering. Therefore, testing must be performed to determine the actual limits for each digital recording system. Note that assumptions based on experience with analog systems do not always apply.
401(c)(6) Pre-emphasis AMC §A36.3.6.2(a) [ETM AMC A2 3.6.2]
(1) Pre-emphasis systems Use of pre-emphasis systems will be allowed only if the system also employs complementary de-emphasis.
Attempts to compensate for the effects of a pre-emphasis filter by applying one-third octave band de-emphasis adjustments, either numerically to analyzed data via a pink noise adjustment, or on a band-by-band basis using separate gain stages for each one-third octave band filter, are not allowed. In addition, use of a pre-emphasis/deemphasis system will require testing and documentation of all filters and gain stages involved to ensure that any errors are quantified and minimized and that the system performs predictably and reliably.
401(c)(7) Attenuator specifications AMC §A36.3.6.9 [ETM AMC A2 3.6.9]
(1) Attenuator specifications Attenuator specifications allow for the use of switchable voltage input range settings, now commonplace on DAT recorders, as controllable attenuation steps for gain-settin g purposes. In all cases, attenuators should have fixed repeatable steps. Any devices in the measurement system that use vernier or continuously-adjustable gain controls should also have some demonstrable means of being fixed, or locked at a specific setting, to eliminate non-traceable gain errors.
401(c)(8) Linear integrating analyzer specifications GM §A36.3.7.2(c) [ETM GM A2 3.7.2]
(1) Externally controlled linear-integrating analysers In cases where a computer or other external device is used to control and/or communicate with an analyser performing linear integration, extra care should be taken to ensure that the integration period requirements are met. Some analysers from major manufacturers have required a factory modification in order to provide an integration time within 5 ms of the specified 500-ms integration period.
401(c)(9) Analyzer perf ormance specifications GM §A36.3.7.4 [ETM GM A2 3.7.3]
(1) Analyzer specifications Reference T2 specifies the electrical performance requi rements of one-third octave band filters, including tolerances for the attenuation in the transition bands (i.e ., “skirts”) adjacent to the one-third octave passbands.
Most digital one-third octave band analysis systems o ffer only hardwired filtering algorithms that emulate the response of a traditional third-order analysis filter havi ng a maximally flat passband. However, some analysis systems allow the selection of other filtering algorithms which might not provide equivalent performance.
Applicants should demonstrate the effects that alternate filter design response characteristics might have on noise certification EPNL values.
(2) Determination of bandwidth error adjustments The manufacturer can establish the geometric center frequen cies of one-third octave band filters using either Page 127
Base 2 or Base 10 systems. While the use of either method results in frequencies close to the nominal center frequencies referred to in Table A36-3, it is important to note which system is used so that the bandwidth error adjustment can be properly determined. Use of test frequencies calculated by a different base-number system than that for which the analyzer was designed can result in erroneous values for these adjustments.
401(c)(10) Microphone incidence adjustments AMC §A36.3.9.3 [ETM AMC A2 3.9.3]
(1) Applications of adjustments for incidence When using microphones whose frequency response is nearly flat at grazing incidence, and when the angles of incidence of sound emitted from the aircraft are within ±30 of grazing incidence, a single set of data adjustments for free-field response and windscreen insertion loss, based on grazing incidence, is considered sufficient to account for incidence effects. When it is impractical to orient the micr ophone properly to maintain grazing incidence, provided that a continuous record of TSPI is available, free-field and windscreen insertion-loss incidence data adjustments can be applied to the noise da ta on a spectral-record by spectral-record basis. These adjustments are obtained by calculating the angle of in cidence for each record, usin g the point of time which characterizes the 2-second averaging period (see A36.3.7.6 of part 36) and determining the aircraft’s sound emission coordinates and angle of incidence for the sound measured at that time.
401(c)(11) Pink noise specification GM §A36.3.9.4 [ETM GM A2 3.9.4]
(1) Pink noise Pink noise contains equal energy in each octave band or fractional octave band (e.g., the octave from 100 Hz to 200 Hz contains the same amount of energy as the octave from 1 kHz to 2 kHz, although for the lower-frequency octave, it is distributed over a frequency range 10 times narrower).
(2) Pink noise usage Because of the dynamic nature of the pink noise signal, longer samples produce statistically better measurements.
A minimum duration of 30 seconds of pink noise should be recorded.
401(c)(12) Measurement sy stem field calibration AMC §A36.3.9.5 [ETM AMC A2 3.9.5]
(1) Measurement system field calibration (all components of the measurement system, except microphones) All components of the measur ement system, except microphones, should be tested while deployed in the field, using pink noise at a level within 5 dB of the calibration level (see A36.3.9.5 of part 36). The signal should be recorded for a duration of at least 30 seconds so that one-third octave band system frequency response adjustments can be determined and applied during analysis. The pink noise generator should be calibrated within 6 months of the measurement and is acceptable for certification use only if its output in each one-t hird octave band does not change by more than 0.2 dB between calibrations.
401(c)(13) Field acoustical calibrations AMC §A36.3.9.8 [ETM AMC A2 3.9.8]
(1) Field acoustical calibrations All components of the system, excluding the windscreen, should be in place at this time, including cables, attenuators, gain and signal-conditioning amplifiers, filters (including pre-emphasis) and power supplies. During Page 128
calibration, attenuators and gain stages should be set to prevent overload and to maintain the calibration signal level on the reference level range within the limits specified in A36.3.6.6 of part 36. If any switchable filters that could affect the calibration signal are utilized during measurements, then calibrations should be performed both with and without these filters enabled. Components of the electrical system should not be added, removed or replaced without re-calibrating the entire system immediately before and after each change.
401(c)(14) Windscreen loss adjustment AMC §A36.3.9.10 [ETM AMC A2 3.9.10]
(1) Determination of windscreen data adjustments The physical condition of a windscreen can significantly affect its performance, and manufacturer-provided windscreen data adjustments for insertion loss are valid only for new, or clean, dry windscreens. For these adjustments, a single set of values based upon windscreen insertion loss tests at grazing incidence may be used when the angles of incidence of sound emitted from an aircraft are within +30° of grazing incidence. For other cases, the windscreen insertion loss adjustments should be determined and applied on the basis of intervals between angles tested not exceeding 30°.
When the windscreen data adjustments provided by the manufacturer are presented in the form of curves, care should be taken to include the insertion loss throughout each one-third octave band, rather than just at the nominal midband frequency. Windscreen insertion loss can vary substantially within the frequency range of a single band and must be averaged or faired to more accurately correct one-third octa ve band data for the presence of the windscreen. Windscreen data adjustments may also be obtained by free-field calibration in an anechoic chamber.
401(c)(15) Measurement sy stem background noise AMC §A36.3.10.1 [ETM AMC A2 3.10.1]
(1) Measurement system noise Since measurement system noise can add energy to m easured aircraft noise levels, the background noise measurement described in A36.3.10.1 of part 36 should be made with all gain stages and attenuators set as they would be used during the aircraft noise certification measurements. If it is expected that multiple settings will be required during the measurements, background noise data should be collected at each of these settings. Care should be taken to ensure that the background noise is truly representative of that present during the aircraft noise certification tests.
(2) Mean background noise assessments At least 30 seconds of background noise data must be time-averaged to determine the mean level for each onethird octave band. The PNL value for this averaged spectrum should then be calculated using the procedures defined in A36.4.1.3 a) of part 36. The aircraft noise level data should also be analyzed and PNL values calculated for each spectral record. The maximum aircraft PNL value should be at least 20 dB above the PNL of the averaged background noise spectrum for the data to be considered acceptable.
401(d) Calculation of EPNL from Measured Data §A36.4 [ETM 4.1.4] 401(d)(1) Instantaneous sound pressure levels GM §A36.4.1.2 [ETM GM A2 4.2]
(1) Instantaneous sound pressure levels For the purposes of this procedure, “instantaneous” sound pressure levels are considered to be one-third octave band sound pressure levels for each one-half second record obtained using a continuous exponential averaging Page 129
process, as described in A36.3.7.5 of part 36 or its equivalent.
401(d)(2) Tone correction calculation AMC §A36.4.3.1 [ETM AMC A2 4.3.1]
(1) Data precision for tone correction computation Prior to Step 1, it is recommended that all one-third octave band sound pressure levels be temporarily rounded to
0.1 dB resolution. The tone correcti on procedure presented here includes several steps that utilize decibel level criteria to characterize the significance of tonal content. These criteria can become artificially sensitive to small variations in level if a resolution finer than 0.1 dB is used in the computations.
401(d)(3) Adjustments relati ng to background noise AMC §A36.4.3.1(d) & (e) [ETM AMC A2 4.3.1 (Steps 4, 5)]
(1) Data adjustments for background noise When the technical procedure presente d in 306(c)(2) of this AC is used for adjustment for the effects of background noise, Steps 4 and 5 of this tone correction procedure should be modified as follows:
— Step 4. The LGB should be used in place of the highest frequency band (i = 24); and — Step 5. A new slope, s’(25,k), should be calculated for the band beyond LGB as described for an imaginary 25th band. This slope should be used in place of the slope derived from the actual level of the band beyond LGB.
401(d)(4) Data resolution afte r tone correction calculation AMC §A36.4.3.1(j) [ETM AMC A2 4.3.1 (Step 10)]
(1) Data precision (after calculation of tone correction factor) At this point, the original sound pressure level resolu tion of 0.01 dB should be restored. Although the required precision of reported EPNL is 0.1 dB, all other intermedia te calculations external to the tone correction process should maintain a precision of at least 0.01 dB.
(2) Identification of pseudo-tones Section 403(b)(2) presents guidance material on methods for identifying pseudo-tones. Note that the use of ground plane or 33 ft (10 m) microphones is supplemental to the required 4 ft (1.2 m) microphones and is allowed only for identification of frequency bands within which pseudo-tones might occur and not for the determination of aircraft certification noise levels.
(3) Tone correction factor adjustment When tone correction f actors result from false or fictitious tones, recalculation is allowed using revised sound pressure level values, based on narrow-band analysis, of the smoothed spectral levels obtained in Step 7. Once the levels have been revised, the tone correction factor should be recomputed for the revised one-third octave band spectrum. This recomputed maximum tone correction factor should be applied, even if it occurs at or near the band associated with an artific ial tone, and approval by the FAA should be obtained for the methodology used.
401(d)(5) Band sharing adjustment GM §A36.4.4.2 [ETM GM A2 4.4.2] Page 130
(1) Band-sharing adjustment concept The one-third octave band filtering process specified for analysis of aircraft noise certification data in A36.4.3.2 of part 36 may allow the tone correction procedure to und er-predict a tone correction factor when the frequency of a tone is located at or near the edge of one or more one-third octave bands. To account for this phenomenon, a band-sharing adjustment is computed that takes advantage of the fact that, as a result of the Doppler effect, a tone that is suppressed at PNLTM will probably appear normally in the spectra that occur before or after PNLTM. By averaging the tone correction factors calculated for the spectra within a 2-second period around PNLTM, the tone correction factor that would have occurred at PNLTM if it were not suppressed can be reasonably estimated.
401(d)(6) Calculation of band sharing adjustment AMC §A36.4.4.2 [ETM AMC A2 4.4.2]
(1) Computation of band-sharing adjustment Although part 36 refers to identification of the frequency bands in which maximum tone corrections occur for the records near PNLTM, the presence or absence of band-sharing cannot be established merely by observing these frequencies. Even though the maximum tone that occurs in a one-third octave band spectrum may not be related to the band of maximum tone correction in the PNLTM spectrum, a related tone may still be present. Therefore, the average of the tone corrections of all spectra within one second (i.e., five, one-half-second data records) of PNLTM should be used regardless of the bands in which maximum tones are found. If the band-sharing adjustment is believed to result from effects other than band-sharing, the applicant should demonstrate its absence for each event.
(2) Adjustment of PNLTM for band-sharing The band-sharing adjustment should be computed before the determination of the 10 dB-down period and should be included in the reported PNLTM and EPNL values for the test condition data.
(3) Application of band-sharing adjustment for simplified procedure When the simplified procedure is used to adjust data to reference conditions, the band-sharing adjustment should be applied to the PNLTr at time of PNLTM before “∆1” and EPNLr are calculated.
(4) Application of band-sharing adjustment for integrated procedure When the integrated procedure is used to adjust data to reference conditions, a new band-sharing adjustment should be calculated as in A36.4.4.2 of part 36. This new band-sharing adjustment uses the average of the tone correction factors of the PNLTMr spectrum and the tw o preceding and two succeeding spectra, after adjusting them to reference conditions, and should be applied to the PNLTMr value prior to identification of the reference condition 10 dB-down points and calculation of EPNLr.
401(d)(7) Noise Duration AMC §A36.4.5 [ETM AMC A2 4.5]
(1) Noise duration (10 dB-down period) This period is the portion of the aircraft flyover in wh ich the measured noise level is within 10 dB of PNLTM
(i.e., the period to be used for the calculation of EPNL). To ensure an adequate duration of recorded noise, recording systems should be activated, and the aircraft maintaining a stable condition, when the noise level at the first microphone location is estimated to be approximately 20 dB(A) below what is expected to be L Amax. Care should be taken during use of the flight path intercept method (see 402(a)(1)(i)) to ensure that noise levels have fallen 20 dB(A) below LAmax before flight path go-around procedures are initiated.
Note.— If recorded data do not encompass the entire 10 dB-down period, an EPNL cannot be calculated from those data, and the event should not be used for aircraft noise certification purposes.
(2) Identification of the first and last records within the noise duration When identifying the records that define the limits of the noise duration, those records having PNLT values closest to the actual value of PNLTM – 10 dB should be used. As a result, the PNLT values for the PNLTM – 10 dB points may not always be greater than or equal to PNLTM – 10 dB.
In order to illustrate the correct identification of the 10 dB-down points, Figure 9 provides examples of PNLT time-histories made up of records calculated from measured one-half-second values of SPL in accordance with the procedures specified in A36.4.2 of part 36. The shaded record k M represents the reco rd associated with PNLTM. Shaded records kF and kL represent respectively the first and last 10 dB-down points.
In the first example the PNLT value associated with kF is greater than PNLTM – 10. The PNLT value associated with kL is less than PNLTM – 10.
In the second example there are two records after kM with a value equal to PNLTM – 10. In this case kL is the last of the two records. The first 10 dB-down point, k F, is the record closest in valu e to PNLTM – 10, ignoring any records that precede it with greater values but which are less in value than PNLTM – 10.
Note.— In all cases in the calculation of EPNL, the contribution of all the records from kF to kL inclusive should be included.
401(d)(8) Equations for co mputing duration correction GM §A36.4.5.4 [ETM GM A2 4.6.2]
(1) Duration correction factor The equation for the duration correction factor, D, in A36.4.5.4 of part 36 is valid only for records of one-half second in length. The constant value 13 is used to normalize the one-half second values to the 10 seconds standard duration
(i.e., 10 seconds duration comprising twenty 0.5-second data records and 10 log (20) = 13.01).