306 Table 7. Example of confidence interval calculation Description Reference thrust Function ܨ ߜ ே Datum Hard wall 1 600 Silenced 1 600 90% confidence interval about the mean CI90 0.3183 0.4817 0.2128 Number of data points n 6 23 6 Degree of curve fit k 0 2 1 Number of independent variables K 0 2 1 Number of degrees of freedom N – K - 1 5 20 4 Student’s t-distribution Z Z2 Z2t t ܫܥଽ ݐ ሺܫܥଽ ݐ ሻଶ ሺܫܥଽ ݐ ሻଶݐ
2.015
0.1580
2.4953 × 10–2
5.0280 × 10–2
1.725
0.2792
7.7979 × 10–2
0.1345
2.132
0.09981
9.9625 × 10 –3
2.1240 × 10–2 ∑Z2 0.1129 ∑(Z2t) T √∑ܼଶ CI ∑ሺܼଶݐሻ ∑ ܼଶ ܶ√∑ܼଶ
0.2060
1.8248
0.3360
0.6131
ADJUSTMENT OF AIRCRAFT NOISE LEVELS FOR THE EFFECTS
OF BACKGROUND NOISE [ETM 3.6] 306(a) Introduction [ETM 3.6.1] The following is provided as guidance material on procedures for adjusting measured aircraft noise levels for the effects of background noise.
The presence of background noise during aircraft noise certification tests can influence measured aircraft sound levels and, in some cases, obscure portions of the spectral time history used to obtain EPNL values. Adjustment procedures should include the following components:
a) testing to determine which portions of the spectral time history, if any, are obscured;
b) adjustment of unobscured levels to determine the aircraft sound levels that would have been measured in the absence of background noise; and
c) replacement or reconstruction of obscured levels by frequency extrapolation, time extrapolation or other means.
Definitions of the terms used in this section are provid ed in 306(b). Although some of the terms have generally accepted meanings, the specific meanings as defined apply herein.
A detailed step-by-step procedure is presented in 306(c) including equations and descriptions of time and frequency extrapolation methods (see 306(c)(2)(J)). Other procedures may be used provided that they have been approved by the FAA.
General considerations that apply to any background noise adjustment procedure are listed in 306(d), including limitations and requirements (see 306(d)(1)) and other special considerations (see 306(d)(2) through 306(d)(4)).
306(b) Definitions [ETM 3.6.2] For the purposes of 306, the following definitions apply:
Adjusted level. A valid one-third octave band level which has been adjusted for measurement conditions, including:
a) the energy contribution of pre-detection noise; and
b) frequency-dependent adjustments such as system frequency response, microphone pressure response and free-field response, and windscreen incidence-dependent insertion loss.
Ambient noise. The acoustical noise from sources other than the te st aircraft present at the microphone site during aircraft noise measurements. Ambient noise is one component of background noise.
Background noise. The combined noise present in a measurement system from sources other than the test aircraft, which can influence or obscure the aircraft noise levels being measured. Typical elements of background noise include, but are not limited to, ambient noise from sour ces around the microphone s ite, thermal electrical noise generated by components in the measurement system, magnetic flux noise (“tape hiss”) from analog tape recorders and digitization noise caused by quantization error in digital converters. Some elements of background noise, such as digitization noise, can obscure the aircraft noise signal, while others, such as ambient noise, can also contribute energy to the measured aircraft noise signal.
Energy-subtraction. Subtraction of one sound pressure level from another, on an energy basis, in the form of the following:
10݈݃ଵሾ10ሺଵಲሻ െ1 0ሺଵಳሻሿ where L A and LB are two sound pressure levels in decibels, with LB being the value subtracted from LA.
Frequency extrapolation. A method for reconstruction of high frequency masked data, based on unmasked data in a lower-frequency one-third octave band from the same spectrum.
High frequency bands. The twelve bands from 800 Hz through 10 kHz inclusive (also see “low frequency bands”).
Last good band (LGB). In the adjustment methodology presented in 3.6.3, for any aircraft one-third band spectrum, Page 98
the LGB is the highest frequency unmasked band within the range of 630 Hz to 10 kHz inclusive, below which there are no masked high frequency bands.
Low frequency bands. The twelve bands from 50 Hz through 630 Hz inclusive (also see “high frequency bands”).
Masked band. Within a single spectrum, any one-third octave band containing a masked level.
Masked level. Any one-third octave band level which is less than or equal to the masking criterion for that band. When a level is identified as being masked, the actual level of aircraft noise in that band has been obscured by background noise and cannot be determined. Masked levels can be reconstructed using frequency extrapolation, time extrapolation or other methods.
Masking criteria. The spectrum of one-third octave band levels below which measured aircraft sound pressure levels are considered to be masked or obscured by background noise. Masking criteria levels are defined as the greater of:
a) pre-detection noise + 3 dB; or
b) post-detection noise + 1 dB.
Post-detection noise. The minimum levels below which measured noise levels are not considered valid. Usually determined by the baseline of an analysis “window” or by the amplitude non-linearity characteristics of components in the measurement and analysis system. Post-detection noise levels are non-additive (i.e., they do not contribute energy to measured aircraft noise levels).
Pre-detection noise. Any noise which can contribute energy to the measured levels of sound produced by the aircraft, including ambient noise present at the microphone site and active instrumentation noise present in the measurement, record/playback and analysis systems.
Reconstructed level. A level, calculated by frequency extrapolation, time extrapolation, or by other means, which replaces the measured value for a masked band.
Sound attenuation coefficient. The reduction in level of sound within a one-third octave band, in dB per 100 meters, due to the effects of atmospheric absorption of sound.
Time extrapolation. A method for reconstruction of high frequency masked data, based on unmasked data in the same one-third octave band, from a different spectrum in the time history.
Valid or unmasked band. Within a single spectrum, any one-third octave band containing a valid level.
Valid or unmasked level. Any one-third octave band level which exceeds the masking criterion for that band.
306(c) Background Noise Adjustment Procedure [ETM 3.6.3] 306(c)(1) Assumptions [ETM 3.6.3.1]
a) A typical aircraft spectru m measured on the ground contains one-third octave band levels which decrease in amplitude with increasing frequency. This characteristic high frequency roll-off is due primarily to the effects of atmospheric absorption.
b) A typical electronic instrumentation floor spectrum contains one-third octave band levels which increase in amplitude with increasing frequency.
c) Due to the assumptions cited in a) and b), as the observed frequency is increased within a one-third octave band aircraft spectrum and once a band becomes masked, all subsequent higher frequency bands will also be masked. This allows the implementation of a Last Good Band (LGB) label to identify the frequency band above which the bands in a spectrum are masked.
d) If, on occasion, a valid level occurs in a band with higher center frequency than the LGB, its presence will most likely be due to small variations in the pre-detection levels and/or due to levels of the measured aircraft one-third octave band spectrum being close to the levels of the background noise in general, so its energy contribution will not be significant. Note that this assumption is valid only in the absence of significant aircraft-generated tones in the region of masking. Th erefore, the possibility of a level being valid in a band with higher center frequency than the LGB may be ignored. Applicants who prefer to implement algorithms for identifying and handling such situations may do so, but no procedure may be used without prior approval by the FAA.
306(c)(2) Step-by step description [ETM 3.6.3.2] 306(c)(2)(A) Determination of pre-detection noise [ETM 3.6.3.2.1] A time-averaged one-third octave band spectrum of pre-detection noise levels for each test run, or group of runs occurring during a short time period, should be obtained by recording and analyzing ambient noise over a representative period of time (30 seconds or more). Care should be taken to ensure that this “ambient” noise sample reasonably represents that which is present during measured aircraft runs. In recording ambient noise, all gain stages and attenuators should be set as they would be during the aircraft runs in order to ensure that the instrumentation noise is also representative. If multiple gain settings are required for aircraft noise measurements, a separate ambient sample should be recorded at each of the settings used.
306(c)(2)(B) Determination of post-detection noise [ETM 3.6.3.2.2] A one-third octave band spectrum of post-detection noise levels should be determined as a result of testing, or from manufacturer’s specifications, for each measurement/analysis configuration used, including different gain and/or sensitivity settings. These minimum valid levels may be determined on the basis of display limitations (e.g., blanking of the displayed indication when levels fall below a cert ain value), amplitude non-linearity or other non-additive limitations. In cases where more than one component or stage of the measurement/analysis system imposes a set of minimum valid levels, the most restrictive in each one-third octave band should be used.
306(c)(2)(C) Testing of pre-detectio n noise versus post-detection noise [ETM 3.5.3.2.3] The validity of pre-detection noise levels must be established before these levels can be used to adjust valid aircraft noise levels. Any pre-detection noise level that is equal to or less than the post-detection noise level in a particular one-third octave band should be identified as invalid and therefore should not be used in the adjustment procedure.
306(c)(2)(D) Determinati on of masking criteria [ETM 3.6.3.2.4] Once the pre-detection noise and post-detection noise spectra are established, the masking criteria can be identified.
For each one-third octave band, compare the valid pre-detection noise level + 3 dB with the post-detection noise level + 1 dB. The highest of these levels is used as the masking criterion for that band. If there is no valid pre-detection noise level for a particular one-third octave band, then th e post-detection noise level + 1 dB is used as the masking criterion for that band. The 3 dB window above pre-detection levels allows for the doubling of energy which could Page 100
occur if an aircraft noise level were equal to the pre-detection level. The 1 dB window above the post-detection levels allows for a reasonable amount of error in the determination of those levels.
306(c)(2)(E) Identification of masked levels [ETM 3.6.3.2.5] Each spectrum in the aircraft noise time history can be evaluated for masking by comparing the one-third octave band levels against the masking criteria levels. Whenever the aircraft level in a particular band is less than or equal to the associated masking criterion, that aircraft level is considered masked. A record must be kept of which bands in each spectrum are masked.
306(c)(2)(F) Determinati on of Last Good Band [ETM 3.6.3.2.6] For each half-second spectral record, determine the highest frequency unmasked one-third octave band (“Last Good Band ” or “LGB”) by starting at the 630 Hz band and incrementing the band number (i.e., increasing frequency) until a masked band is found. At that point, set LGB for that spectral record equal to the band below the masked band. The lowest frequency band that can be identified as LGB is the 630 Hz band. In other words, if both the 630 Hz band and the 800 Hz band are masked, no reconstruction of masked levels may be performed for that spectrum, and the thirteen bands between 630 Hz and 10 kHz inclusive should be left as is and identified as masked. According to the masking limits specified in 306(d)(2), such a spectrum is not valid for calculation of EPNL.
306(c)(2)(G) Adjustment of valid levels for background noise [ETM 3.6.3.2.7] In each half-second spectrum, for each valid band up to and including LGB, perform an energy-subtraction of the valid pre-detection level from the valid measured level in the aircraft noise time history using:
10݈݃ଵሾ10ሺ ಲೝೝೌሻ െ1 0ሺುೝሻଵ ଵ ሿ Energy-subtraction should be performed on all valid one-third octave band noise levels. For any one-third octave band where there is no valid pre-detection noise level, no energy-subtraction may be performed (i.e., this adjustment cannot be applied when either the measured aircraft noise time-history level or the pre-detection noise level is masked).
306(c)(2)(H) Adjustment of valid levels for measurement conditions [ETM 3.6.3.2.8] Before any reconstruction can be done for masked levels, valid levels which have been adjusted for the presence of pre-detection noise must also then be adjusted for frequency-dependent adjustments such as system frequency response, microphone pressure response and free-field response, and windscreen incidence-dependent insertion loss.
These adjustments cannot be applied to masked levels.
306(c)(2)(I) Reconstruction of low frequency masked bands [ETM 36.3.2.9] In cases where a single masked low frequency one-third octave band occurs between two adjacent valid bands, the masked level can be retained, or the arithmetic average of the adjusted levels of the adjacent valid bands may be used in place of the masked level. If the average is used, the le vel should be categorized as reconstructed. However, if masked low frequency bands are found adjacent to other masked low frequency bands, these masked levels should be retained and remain categorized as ma sked. The procedure presented in this 306(c) does not provide for any other form of reconstruction for masked low frequency bands.
306(c)(2)(J) Reconstruction of leve ls for masked high frequency bands [ETM 3.6.3.2.10] Frequency extrapolation and time extrapolation are the methods used to reconstruct masked one-third octave band levels for bands at frequencies higher than LGB for each spectral record. One-third octa ve band sound attenuation coefficients (either in dB per 100 m or in dB per 1000 ft) must be determined before such reconstruction of masked band levels can be performed. Note that sound emission co ordinates must also be calcu lated for each record before reconstruction is performed since the procedure is dependent on propagation distance.
306(c)(2)(J)(i) Frequency extrapolation method [ETM 3.6.3.2.10.1] For a spectrum where the LGB is located at or above the 2 kHz one-third octave band, the frequency extrapolation method is used. This method reconstructs masked high frequency bands starting with the level associated with LGB in the same spectrum. The levels for all bands at higher frequencies than LGB must be reconstructed using this method.
Any frequency-extrapolated levels should be categorized as reconstructed. Reconstruct the level for the masked bands using the following equation:
ܴܵ 60 ܴܵ 60 ܴܵ ܴܵܮ, ൌܮ , ߙ 100 െߙ ோாி 100 20݈݃ଵ 60 ߙ ோாி 100 െߙ 100 20݈݃ଵ 60 which can be reduced to:
ௌோೖ ݔܮ, ൌܮ , ሾ ߙ െߙ ሿ ଵ ሾ ߙோாி െߙ ோாிሿ ଵ, where:
— i is the masked band to be extrapolated;
— k is the record of interest;
— j is the LGB in record k;
— Lxi,k is the frequency-extrapolated level in dB for masked band i and spectral record k;
— Lj,k is the level for LGB in record k after all test-day adjustments have been applied, including predetection noise energy-subtraction, system and microphone adjustments, etc.;
— αj is the test-day sound attenuation coefficient (dB per 100 m) for LGB;
αi is the test-day sound attenuation coefficient (dB per 100 m) for band i;
αjREF is the reference 77°F (25°C), 70 percent relative humidity (RH) sound attenuation coefficient (dB per 100 m) for LGB;
αiREF is the reference 77°F (25°C), 70 percent RH) sound attenuation coefficient (dB per 100 m) for masked band i;
and SRk is the slant range or acoustic propagation distance in meters at the time of sound emission for spectral record k, between the aircraft and the microphone.
This procedure is based on the assumption that the aircraft spectrum is “flat” (i.e., all high frequency band levels are equal) at a distance of 197 ft (60 m) under reference conditions 77°F (25°C), 70 percent RH). The process can be conceptualized by means of the following steps:
a) the level for band j, the highest frequency unmasked band in spectral record k, which has already been adjusted for measurement conditions, is adjusted for test-day propagation effects to obtain the source level and then adjusted using reference propagation effects to the197 ft (60 m) distance from the source;
b) this level is then assigned as the level for all high frequency masked bands (i.e., band i, band i + 1, etc.)
at a distance of 197 ft (60 m);
c) a new source level is determined for each masked high frequency band by removing the associated reference-day propagation effects; and
d) the extrapolated level that would have been measured on the ground, in the absence of background noise, is determined for each masked high frequency band by adding the test-day propagation effects to each of the source levels determined in c) above.
306(c)(2)(J)(ii) Time extrapolation method [ETM 3.6.3.2.10.2] For a spectrum where LGB occurs at or between the 630 Hz one-third octave band and the 1.6 kHz band, use the time extrapolation method. This method reconstructs a masked band in a spectrum from the closest spectral record (i.e., closest in time) for which that band is valid. The levels fo r all one-third octave bands with frequencies greater than that of LGB must be reconstructed using this time extr apolation method. Any time-extrapolated levels should be categorized as reconstructed. Reconstruct the levels for the masked bands by using the following equation:
ሾܴܵܮ, ൌܮ , ߙ 100ሿ 20݈݃ଵሾܴܵሿ100 െ ܴܵ ܴܵ where:
— L xi,k is the time-extrapolated level in dB for masked band i and spectral record k;
— Li,m is the adjusted level in dB for band i in spectral record m, which is the nearest record in time to record k in which band i contains a valid level;
— SRm is the slant range or acoustic propagation distance in meters at the time of sound emission for spectral record m, between the aircraft and the microphone;
— SRk is the slant range or acoustic propagation distance in meters at the time of sound emission for spectral record k, between the aircraft and the microphone; and — αiis the test-day sound attenuation coefficient (dB per 100 m) for band i.
This procedure is based on the assumption that the aircraft spectrum is omnidirectional during the aircraft pass-by.
306(c)(2)(K) Handling of spectra af ter reconstruction of masked bands [ETM 3.6.3.2.11] After reconstruction of masked data has been performed, the background noise adjustment procedure is complete. The adjusted as-measured data set, comprised of adjusted levels, reconstructed levels, and possibly some masked levels, is next used to obtain the test-day PNLT time history desc ribed in A36.4.3 of part 36. The identification of masked data should be kept accessible for use during the tone co rrection procedure, since any tone correction which results from the adjustment for background noise may be eliminated from the process of identifying the maximum tone within a spectrum. When this background noise adjustment procedure is used, the band identified as LGB should be treated Page 103
as the last band of the tone correction calculation in the manner prescribed for the 10 kHz band in A36.4.3.1 of part 36, including the calculation of a new slope for band LGB + 1 that equals the slope at LGB (i.e., s ′(LGB + 1, k) = s′(LGB,k)) in Step 5 of the tone correction procedure.
306(d) General Considerations [ETM 3.6.4] 306(d)(1) Limitations and requirements for an y background noise adjustment procedure [ETM 3.6.4.1] Any method of adjusting for the effects of background noise must be approved by the FAA before it is used. The adjustment procedure presented in 306(c)(2) includes applicable limitations and requirements. Those limitations and requirements which apply to all methodologies are described as follows.
The applicant must be able to demonstrate by means of narrow-band analysis or other methods that no significant aircraft-generated tones occur in the masked one-third octave bands during the EPNL duration.
Neither frequency-dependent adjustments nor energy-subtraction of pre-detection levels can be applied to masked data.
When consecutive one-third octave bands in the range of 2.5 kHz to 10 kHz inclusiv e are masked, and when no consecutive bands are masked in the region of 800 Hz to 2 kHz inclusive, frequency extrapolation, as described in 306(c)(2)(J)(i), must be performed on all consecutive masked bands with nominal frequencies greater than 2 kHz.
When consecutive one-third octave bands in the range of 800 Hz to 2 kHz inclusive are masked, time extrapolation, as described in 306(c)(2)(J)(ii), must be performed on all consecutive masked bands with nominal frequencies greater than 630 Hz.
In cases where a single masked one-third octave band occurs between two adjacent valid bands, the levels of the adjacent adjusted bands may be arithmetically averaged and the averaged level used in place of the masked level. If the masked level is retained it must be included when counting the masked levels in the procedure described in 306(d)(2) 306(d)(2) Rejection of spectra due to masking [ETM 3.6.4.2] A spectrum becomes invalid if the following conditions prevail:
a) if, after any reconstruction of masked bands, more than four one-third octave bands retain masked values;
b) for records within one second of the record associated with the PNLTmax spectrum (i.e., five half-second data records) if:
1) more than four high frequency bands require reconstruction; or
2) the LGB is located at or below the 3150 Hz one-third octave band when the example background noise adjustment procedure presented in 306(c)(2) is used.
Note.— If an invalid spectrum occurs within the 10 dB-down period, the aircraft test run is invalid and cannot be used for aircraft noise certification purposes.
305(d)(3) Special tone correction considerations due to masking [ETM 3.6.4.3] Page 104
307 When the maximum tone correction for a one-third octave band spectrum occurs at a masked or reconstructed band, the tone correction for that spectrum cannot simply be set to zero. The maximum tone correction for the spectrum must be computed, taking masked or reconstructed levels into consideration. Any tone correction resulting from the adjustment for background noise may be eliminated by either one of the following two methods, as appropriate:
a) when the example background noise adjustment proce dure presented in 306(c)(2) is used or, specifically, when all of the high frequency bands in a spectrum are masked for frequencies beyond a certain band (i.e., “LGB”) the band labeled as LGB should be treated as the last band of the tone correction calculation, in the manner prescribed for the 10 kHz band (band number 24) in A36.4.3.1 of part 36, including calculation of a new slope for the band above LGB that equals the slope of the band at LGB (i.e., s′(LGB + 1,k) = s′(LGB,k)) in Step 5 of the tone correction procedure; or
b) for tone corrections that occur at one-third octave bands that are masked or reconstructed, set F equal to zero in Step 9 of the tone correction procedure, and recalculate the maximum tone correction for that spectrum.
Note.— All band levels within a spectru m, whether adjusted, reconstructed or masked, must be included in the computation of the PNL value for that spectrum.
306(d)(4) Handling of masked data in reference conditions data sets [ETM 3.6.4.4] For any one-third octave band spectrum adjusted to reference conditions, all bands, including those containing masked levels or reconstructed levels, includin g values less than 0 dB, must be adju sted for differences between test and reference conditions (i.e., atmospheric absorption and spherical spreading). The special tone correction considerations listed in 306(c)(2) apply both to test and reference data sets.
Noise Reduction Systems [ETM 3.7] An aircraft can employ noise reduction systems that chan ge its configuration or operating condition to reduce noise, or implement devices or subsystems that directly reduce or counteract noise emissions. Two categories, variable noise reduction systems (VNRS) and selectable noise reduction systems (SNRS), have been defined to address differences in activation/actuation for these systems. General guidan ce on noise certification of ai rcraft equipped with these systems is provided below.
307(a) Variable Noise Reduction Systems [ETM 3.7.1] A VNRS is an integral design feature, or subsystem, of an aircraft that automatically changes the configuration or operating condition of the aircraft to reduce noise.
Note 1.— If pilot action is necessary to activate, i.e., select the use of, an automatically controlled noise reduction system or if a pilot can deactivate (deselect) an automatically controlled noise reduction system, such a system is not considered a VNRS.
Note 2.— Aircraft can incorporate variable systems, primarily intended to improve performance, reduce engine emissions and/or increase safety, that may also affect noise. Such aircraft can be noise certificated using the guidance provided for aircraft with VNRS. For such changes to existing type designs, the guidelines provided in 101(d) for “no acoustical change” are applicable.
For a VNRS-equipped aircraft, the VNRS characteristics may prevent flight from being conducted in accordance with the associated reference procedure(s) in part 36. In such cases, the reference procedures for noise certification of an aircraft with a VNRS should depart from those specified in part 36 only to the extent required by those design Page 105
characteristics that cause the departure and should be approved by the FAA after the rulemaking process for an deviation from part 36 is completed (see 201(a)(10) paragraph 2 of this AC).
The impact of a VNRS on nois e certification of an aircraft can extend beyo nd deviations from the part 36 reference procedures. A plan for noise certificatio n of a VNRS-equipped aircraft should take into consideration three key elements, namely:
a) the necessity, if any, to depart from the part 36 procedures;
b) the adaptation/modification of test proced ures to ensure compliance with part 36; and
c) the applicability of existing procedures in part 36 for adjusting the measured data to reference conditions.
Experience to date has shown that one or more of these elements may be interrelated, requiring detailed consideration of all three elements in devising an acceptable plan for noise certification.
307(a)(1) Reference procedures [ETM 3.7.1.1] The part 36 reference procedures typically utilize consta nt flight path and operational parameters. A VNRS can, however, result in non-constant reference flight paths and/or non-constant operational parameters such as non-constant rates of climb and/or non-constant engine/propeller/rotor speeds, respectively, that compel departure from the reference procedures. In addition to reducing noise emissions, a VNRS may, and typically does, impact aircraft performance during a noise certification reference procedure. In some cases, this impact can be indirect via another affected performance parameter. Both direct and indirect impacts on aircraft performance should be addressed in defining any departure necessary from the reference procedures in part 36 to accommodate a VNRS.
Actuation of a VNRS can be a function of one or more ope rational conditions such as airspeed, ground speed, height above ground level, density altitude, pressure altitude and ambient temperature. Beginning and end points on the reference flight path for any transition triggered by a VNRS should be determined using the reference test and meteorological conditions.
307(a)(2) Test conditions and procedures [ETM 3.7.1.2] When a VNRS results in a non-constant reference flight path for the aircraft, the flight path tolerances (height and lateral deviation limits) specified in part 36 for the corresponding constant reference procedure should be applied, subject to approval by the FAA. Similarly, when a VNRS results in a non-constant operational parameter for the aircraft, a reference schedule for the affected operating parameter should be defined along the reference flight path, and the test tolerances permitted by part 36 for that parameter should be applied to the reference schedule, subject to approval by the FAA.
307(a)(3) Adjustments to measured noise data [ETM 3.7.1.3] Adjustments to measured data in part 36 are based on constant reference procedures. A VNRS can, however, result in a non-constant reference procedure(s) that in turn impacts the adjustments to measured data that account for test deviations from reference flight profiles and test conditions. The adjustments to measured data specified in part 36 should be modified only as necessary to account for any departures from the reference procedures in part 36. In many cases, only minor changes to data-processing software that do not affect the adjustment procedures will be needed.
Any modifications, including software revisions, of the adjustments to measured data specified in part 36 are subject to approval by the FAA.
307(a)(4) Guidance for specific VNRS [ETM 3.7.1.4] Specific guidance for VNRS technologies will typically be developed as these technologies are developed and implemented in aircraft designs. Cross references to the appropriate sections of this Advisory Circular for the VNRS technologies for which specific guidance has been generally accepted by FAA are provided in Table 8.
Table 8. Cross references to spec ific guidelines in this ACfor VNRS Variable noise reduction Specific guidelines provided in system (VNRS) this AC Variable rotor speed Chapter 4, 401(h), H36.3:
helicopters H36.103(b) [Takeoff flight test procedures], paragraph 8) 307(b) Selectable Noise Reduction Systems [ETM 3.7.2] (Reserved) Note.— The guidance provided in 307 addresses VNRS only and, by inference, defines as selectable all noise reduction systems that do not satisfy the requirements for classification as VNRS. Guidance specific to SNRS, including a definition specific to SNRS, is not yet provided.
308 CALCULATION OF THE SPEED OF SOUND [ETM 3.8] For the purposes of noise certification the value of the speed of sound, c, is calculated from the equation taken from ISO 9613-1:1993(E):
c = 1125.9 (T/T0)½ ft/s, or c = 343.2 (T/T0)½ m/s where T0 = 293.15 K and T is the absolute ambient air temperature in kelvins.
Note.— At the noise certification reference temperature of 25°C, T = 298.15 K, and c therefore equals 1135.5 ft/s
(346.1 m/s).
309 REFERENCE TABLES USED IN THE MANUAL CALCULATION OF
OF EFFECTIVE PERCEIVED NOISE LEVEL [ETM 3.9] Table 9 and Table 10 and Figure 7 contain information useful for the manual calculation of EPNL. Such manual calculations are often used to verify the accuracy of computer programs used for calculating certification noise levels.
5 10 15 20 25 30 35 40 45 50 55 10/12/2017 AC 36-4D Table 9. Perceived noisiness (noys) as a function of sound pressure level (One-third octave band center frequencies (Hz)) SPL 50 63 80 100 125 160 200 250 315 400 500 630 800 1000 1250 1600 2000 2500 3150 4000 5000 6300 8000 10000 4 0.10
0.10 0.11 0.10 6 0.11 0.12 0.11 0.10 7 0.12 0.14 0.13 0.11 8 0.14 0.16 0.14 0.13 9 0.10 0.16 0.17 0.16 0.14
0.11 0.17 0.19 0.18 0.16 0.10 11 0.13 0.19 0.22 0.21 0.18 0.12 12 0.10 0.14 0.22 0.24 0.24 0.21 0.14 13 0.11 0.16 0.24 0.27 0.27 0.24 0.16 14 0.13 0.18 0.27 0.30 0.30 0.27 0.19
0.10 0.14 0.21 0.30 0.33 0.33 0.30 0.22 16 0.10 0.10 0.10 0.10 0.10 0.11 0.16 0.24 0.33 0.35 0.35 0.33 0.26 17 0.11 0.11 0.11 0.11 0.11 0.13 0.18 0.27 0.35 0.38 0.38 0.35 0.30 0.10 18 0.10 0.13 0.13 0.13 0.13 0.13 0.15 0.21 0.30 0.38 0.41 0.41 0.38 0.33 0.12 19 0.11 0.14 0.14 0.14 0.14 0.14 0.17 0.24 0.33 0.41 0.45 0.45 0.41 0.36 0.14
0.13 0.16 0.16 0.16 0.16 0.16 0.20 0.27 0.36 0.45 0.49 0.49 0.45 0.39 0.17 21 0.10 0.14 0.18 0.18 0.18 0.18 0.18 0.23 0.30 0.39 0.49 0.53 0.53 0.49 0.42 0.21 0.10 22 0.11 0.16 0.21 0.21 0.21 0.21 0.21 0.26 0.33 0.42 0.53 0.57 0.57 0.53 0.46 0.25 0.11 23 0.13 0.18 0.24 0.24 0.24 0.24 0.24 0.30 0.36 0.46 0.57 0.62 0.62 0.57 0.50 0.30 0.13 24 0.10 0.14 0.21 0.27 0.27 0.27 0.27 0.27 0.33 0.40 0.50 0.62 0.67 0.67 0.62 0.55 0.33 0.15
0.11 0.16 0.24 0.30 0.30 0.30 0.30 0.30 0.35 0.43 0.55 0.67 0.73 0.73 0.67 0.60 0.36 0.17 26 0.13 0.18 0.27 0.33 0.33 0.33 0.33 0.33 0.38 0.48 0.60 0.73 0.79 0.79 0.73 0.65 0.39 0.20 27 0.10 0.14 0.21 0.30 0.35 0.35 0.35 0.35 0.35 0.41 0.52 0.65 0.79 0.85 0.85 0.79 0.71 0.42 0.23 28 0.11 0.16 0.24 0.33 0.38 0.38 0.38 0.38 0.38 0.45 0.57 0.71 0.85 0.92 0.92 0.85 0.77 0.46 0.26 29 0.13 0.18 0.27 0.35 0.41 0.41 0.41 0.41 0.41 0.49 0.63 0.77 0.92 1.00 1.00 0.92 0.84 0.50 0.30
0.10 0.14 0.21 0.30 0.38 0.45 0.45 0.45 0.45 0.45 0.53 0.69 0.84 1.00 1.07 1.07 1.00 0.92 0.55 0.33 31 0.11 0.16 0.24 0.33 0.41 0.49 0.49 0.49 0.49 0.49 0.57 0.76 0.93 1.07 1.15 1.15 1.07 1.00 0.60 0.37 32 0.13 0.18 0.27 0.36 0.45 0.53 0.53 0.53 0.53 0.53 0.62 0.83 1.00 1.15 1.23 1.23 1.15 1.07 0.65 0.41 33 0.14 0.21 0.30 0.39 0.49 0.57 0.57 0.57 0.57 0.57 0.67 0.91 1.07 1.23 1.32 1.32 1.23 1.15 0.71 0.45 34 0.10 0.16 0.24 0.33 0.42 0.53 0.62 0.62 0.62 0.62 0.62 0.73 1.00 1.15 1.32 1.41 1.41 1.32 1.23 0.77 0.50
0.11 0.18 0.27 0.36 0.46 0.57 0.67 0.67 0.67 0.67 0.67 0.79 1.07 1.23 1.41 1.51 1.51 1.41 1.32 0.84 0.55 36 0.13 0.21 0.30 0.40 0.50 0.62 0.73 0.73 0.73 0.73 0.73 0.85 1.15 1.32 1.51 1.62 1.62 1.51 1.41 0.92 0.61 37 0.15 0.24 0.33 0.43 0.55 0.67 0.79 0.79 0.79 0.79 0.79 0.92 1.23 1.41 1.62 1.74 1.74 1.62 1.51 1.00 0.67 38 0.17 0.27 0.37 0.48 0.60 0.73 0.85 0.85 0.85 0.85 0.85 1.00 1.32 1.51 1.74 1.86 1.86 1.74 1.62 1.10 0.74 39 0.10 0.20 0.30 0.41 0.52 0.65 0.79 0.92 0.92 0.92 0.92 0.92 1.07 1.41 1.62 1.86 1.99 1.99 1.86 1.74 1.21 0.82
0.12 0.23 0.33 0.45 0.57 0.71 0.85 1.00 1.00 1.00 1.00 1.00 1.15 1.51 1.74 1.99 2.14 2.14 1.99 1.86 1.34 0.90 41 0.14 0.26 0.37 0.50 0.63 0.77 0.92 1.07 1.07 1.07 1.07 1.07 1.23 1.62 1.86 2.14 2.29 2.29 2.14 1.99 1.48 1.00 42 0.16 0.30 0.41 0.55 0.69 0.84 1.00 1.15 1.15 1.15 1.15 1.15 1.32 1.74 1.99 2.29 2.45 2.45 2.29 2.14 1.63 1.10 43 0.19 0.33 0.45 0.61 0.76 0.92 1.07 1.23 1.23 1.23 1.23 1.23 1.41 1.86 2.14 2.45 2.63 2.63 2.45 2.29 1.79 1.21 44 0.10 0.22 0.37 0.50 0.67 0.83 1.00 1.15 1.32 1.32 1.32 1.32 1.32 1.52 1.99 2.29 2.63 2.81 2.81 2.63 2.45 1.99 1.34
0.12 0.26 0.42 0.55 0.74 0.91 1.08 1.24 1.41 1.41 1.41 1.41 1.41 1.62 2.14 2.45 2.81 3.02 3.02 2.81 2.63 2.14 1.48 46 0.14 0.30 0.46 0.61 0.82 1.00 1.16 1.33 1.52 1.52 1.52 1.52 1.52 1.74 2.29 2.63 3.02 3.23 3.23 3.02 2.81 2.29 1.63 47 0.16 0.34 0.52 0.67 0.90 1.08 1.25 1.42 1.62 1.62 1.62 1.62 1.62 1.87 2.45 2.81 3.23 3.46 3.46 3.23 3.02 2.45 1.79 48 0.19 0.38 0.58 0.74 1.00 1.17 1.34 1.53 1.74 1.74 1.74 1.74 1.74 2.00 2.63 3.02 3.46 3.71 3.71 3.46 3.23 2.63 1.98 49 0.10 0.22 0.43 0.65 0.82 1.08 1.26 1.45 1.64 1.87 1.87 1.87 1.87 1.87 2.14 2.81 3.23 3.71 3.97 3.97 3.71 3.46 2.81 2.18
0.12 0.26 0.49 0.72 0.90 1.17 1.36 1.56 1.76 2.00 2.00 2.00 2.00 2.00 2.30 3.02 3.46 3.97 4.26 4.26 3.97 3.71 3.02 2.40 51 0.14 0.30 0.55 0.80 1.00 1.26 1.47 1.68 1.89 2.14 2.14 2.14 2.14 2.14 2.46 3.23 3.71 4.26 4.56 4.56 4.26 3.97 3.23 2.63 52 0.17 0.34 0.62 0.90 1.08 1.36 1.58 1.80 2.03 2.30 2.30 2.30 2.30 2.30 2.64 3.46 3.97 4.56 4.89 4.89 4.56 4.26 3.46 2.81 53 0.21 0.39 0.70 1.00 1.18 1.47 1.71 1.94 2.17 2.46 2.46 2.46 2.46 2.46 2.83 3.71 4.26 4.89 5.24 5.24 4.89 4.56 3.71 3.02 54 0.25 0.45 0.79 1.09 1.28 1.58 1.85 2.09 2.33 2.64 2.64 2.64 2.64 2.64 3.03 3.97 4.56 5.24 5.61 5.61 5.24 4.89 3.97 3.23
0.30 0.51 0.89 1.15 1.35 1.71 2.00 2.25 2.50 2.83 2.83 2.83 2.83 2.83 3.25 4.26 4.89 5.61 6.01 6.01 5.61 5.24 4.26 3.46 56 0.34 0.59 1.00 1.29 1.50 1.85 2.15 2.42 2.69 3.03 3.03 3.03 3.03 3.03 3.48 4.56 5.24 6.01 6.44 6.44 6.01 5.61 4.56 3.71 57 0.39 0.67 1.09 1.40 1.63 2.00 2.33 2.61 2.88 3.25 3.25 3.25 3.25 3.25 3.73 4.89 5.61 6.44 6.90 6.90 6.44 6.01 4.89 3.97 58 0.45 0.77 1.18 1.53 1.77 2.15 2.51 2.81 3.10 3.48 3.48 3.48 3.48 3.48 4.00 5.24 6.01 6.90 7.39 7.39 6.90 6.44 5.24 4.26 59 0.51 0.87 1.29 1.66 1.92 2.33 2.71 3.03 3.32 3.73 3.73 3.73 3.73 3.73 4.29 5.61 6.44 7.39 7.92 7.92 7.39 6.90 5.61 4.56 Page 108