Note: Microphone extension cables as configured in the field need not be included for the linear operating range determination.
141. Section A36.3.1.16
Windscreen insertion loss means, in decibels, at a stated nominal one-third octave midband frequency, and for a stated sound incidence angle on the inserted microphone, the indicated sound pressure level without the windscreen installed around the microphone minus the sound pressure level with the windscreen installed.
a. Explanation
Sections A36.3.1.1 through A36.3.1.16 provide technical definitions of a total noise measurement system, its microphone system, and terms that relate to the acoustical performance characteristics of the measurement system and/or its components.
b. Supplemental Information
(1) Applicability: These definitions may be applicable to measurement systems used in noise certification of other types of aircraft (e.g., small airplanes and rotorcraft).
c. Procedures
(1) Applicant’s Responsibility: An applicant’s noise compliance demonstration plan must take these definitions into account in proposing measurement systems for use in noise certification.
142. Section A36.3.2 Reference environmental conditions
143. Section A36.3.2.1
The reference environmental conditions for specifying the performance of a measurement system are:
a) air temperature 73.4°F (23°C);
b) static air pressure 101.325 kPa; and
c) relative humidity 50 %.
a. Explanation
This section specifies the required environmental conditions that serve as reference for defining the performance of the measurement systems used for noise certification.
b. Supplemental Information
(1) These environmental specifications correspond to the recommended requirements of the International Electrotechnical Commission (IEC).
c. Procedures
None
144. Section A36.3.3 General
Note: Measurements of aircraft noise that are made using instruments that conform to the specifications of this section will yield one-third octave band sound pressure levels as a function of
time. These one-third octave band levels are to be used for the calculation of effective perceived noise level as described in section A36.4.
145. Section A36.3.3.1
The measurement system must consist of equipment approved by the FAA and equivalent to the following:
a) a windscreen (see A36.3.4);
b) a microphone system (see A36.3.5);
c) a recording and reproducing system to store the measured aircraft noise signals for subsequent analysis (see A36.3.6);
d) a one-third octave band analysis system (see A36.3.7); and
(e) calibration systems to maintain the acoustical sensitivity of the above systems within specified tolerance limits (see A36.3.8).
a. Explanation
This section specifies the types of components required for a noise certification measurement system.
b. Supplemental Information
(1) Measurement System Criteria: The specifications for a measurement system allow flexibility in applicant procurement of measurement system components. 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 reanalysis or auditing of acoustic data. A recording also facilitates later off-line 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 acoustic data, for elements such as 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. Proper implementation of such adjustments in the field would be extremely difficult.
(2) Approval of Measurement System: FAA approval must 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 use have already been approved, but implementations of new technology and variants or upgrades of existing components may require evaluation by VNTSC before FAA approval. 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.
c. Procedures
(1) Validation of Measurement System Configuration: FAA Order 8110.4B (reference 7) provides an FAA policy and procedure to promote uniform implementation of the noise certification requirements of part 36.
Each applicant must submit information to VNTSC about the measurement, recording, and analysis instrumentation and software used. VNTSC will determine whether any listed components require evaluation for approval by FAA.
(2) Changes in Measurement System Configuration: If an applicant makes changes to the approved instrumentation, VNTSC must be notified before aircraft noise certification testing, to determine whether additional evaluation and FAA approval are required.
146. Section A36.3.3.2
For any component of the measurement system that converts an analog signal to digital form, such conversion must be performed so that the levels of any possible aliases or artifacts of the digitization process will be less than the upper boundary of the linear operating range by at least 50 dB at any frequency less than 12.5 kHz. The sampling rate must be at least 28 kHz. An anti-aliasing filter must be included before the digitization process.
147. Section A36.3.4 Windscreen
148. Section A36.3.4.1
In the absence of wind and for sinusoidal sounds at grazing incidence, the insertion loss caused by the windscreen of a stated type installed around the microphone must not exceed –1.5 dB at nominal one-third octave midband frequencies from 50 Hz to 10 kHz inclusive.
a. Explanation
This section specifies the methodology for determining windscreen insertion loss and insertion loss tolerances for purposes of noise certification.
b. Supplemental Information
(1) Determination of Data Adjustments for Windscreen Insertion Loss: The physical condition of a windscreen can significantly affect its performance, and manufacturer-provided data for windscreen insertion loss are valid only for new or clean, dry windscreens. Insertion loss data adjustments for windscreens may be obtained by free-field calibration in an anechoic chamber.
c. Procedures
(1) Applicant's Responsibility: The applicant must specify proposed windscreen types and their insertion loss characteristics in a noise compliance demonstration plan.
149. Section A36.3.5 Microphone system
150. Section A36.3.5.1
The microphone system must meet the specifications in sections A36.3.5.2 to A36.3.5.4.
Various microphone systems may be approved by the FAA on the basis of demonstrated equivalent overall electro-acoustical performance. Where two or more microphone systems of the same type are used, demonstration that at least one system conforms to the specifications in full is sufficient to demonstrate conformance.
Note: An applicant must still calibrate and check each system as required in section A36.3.9.
151. Section A36.3.5.2
The microphone must be mounted with the sensing element 4 ft (1.2 m) above the local ground surface and must be oriented for grazing incidence, i.e., with the sensing element substantially in the plane defined by the predicted reference flight path of the aircraft and the measuring station. The microphone mounting arrangement must minimize the interference of the supports with the sound to be measured. Figure A36-1 illustrates sound incidence angles on a microphone.
Grazing incidence 90°
60° 120°
O° 180°
Sensing element
Figure A36-1: Illustration of Sound Incidence Angles on a Microphone
152. Section A36.3.5.3
The free-field sensitivity level of the microphone and preamplifier in the reference direction, at frequencies over at least the range of one-third-octave nominal midband frequencies from 50 Hz to 5 kHz inclusive, must be within ±1.0 dB of that at the calibration check frequency, and within ±2.0 dB for nominal midband frequencies of 6.3 kHz, 8 kHz and 10 kHz.
153. Section A36.3.5.4
For sinusoidal sound waves at each one-third octave nominal midband frequency over the range from 50 Hz to 10 kHz inclusive, the free-field sensitivity levels of the microphone system at sound incidence angles of 30°, 60°, 90°, 120° and 150°, must not differ from the free-field sensitivity level at a sound incidence angle of 0° (“normal incidence”) by more than the values shown in Table A36-1. The free-field sensitivity level differences at sound incidence angles between any two adjacent sound incidence angles in Table A36-1 must not exceed the tolerance limit for the greater angle.
Table A36-1 Microphone Directional Response Requirements
a. Explanation
This section specifies the required performance characteristics of microphone systems that may be used during noise certification testing.
b. Supplemental Information
(1) Grazing Incidence: The purpose of the grazing incidence specification is to minimize the effects of variations in the microphone response during measurement of airplane noise. By orienting the microphone so that the airplane is substantially within the plane of the microphone diaphragm (grazing incidence) during the 10 dB-down period, all sound from the airplane impinges on the microphone at approximately the same angle relative to its axis.
Normal Incidence (0 º) Grazing Incidence (90 º )
Nominal midband frequency kHz
Maximum difference between the free-field sensitivity level of a microphone system at normal incidence and the free-field sensitivity level at specified sound incidence angles dB _______________________________________________________________________
Sound Incidence angle degrees
0.05 to 1.6
2.0
2.5
3.15
4.0
5.0
6.3
8.0
10.0
0.5
0.5
0.5
0.5
0.5
0.5
1.0
1.5
2.0
0.5
0.5
0.5
1.0
1.0
1.5
2.0
2.5
3.5
1.0
1.0
1.0
1.5
2.0
2.5
3.0
4.0
5.5
1.0
1.0
1.5
2.0
2.5
3.0
4.0
5.5
6.5
1.0
1.0
1.5
2.0
2.5
3.0
4.0
5.5
7.5
(2) Microphone Characteristics: These specifications are based on the performance characteristics of typical one-half-inch condenser microphones designed for nearly uniform frequency response at grazing incidence. Other microphones may be used, provided they meet the specified performance requirements. For example, pre-polarized (electret condenser) free-field microphones exist that 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-field applications, they can be used in airplane noise certification testing if their performance at grazing incidence meets the requirements of section A36.3.5.
(3) Microphone Specifications: 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.
c. Procedures
(1) Microphone Orientation: For microphones located directly under the flight path, an orientation angle of 90 degrees from vertical is appropriate regardless of target altitude. For lateral noise measurements, applicants may wish to reorient the microphones for grazing incidence for each target altitude 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 ± 30 degrees of grazing incidence.
(2) Applicant's Responsibility: The applicant must specify proposed microphone types and orientation for noise certification testing in a noise certification compliance demonstration plan.
154. Section A36.3.6 Recording and reproducing systems
155. Section A36.3.6.1
A recording and reproducing system, such as a digital or analog magnetic tape recorder, a computer-based system or other permanent data storage device, must be used to store sound pressure signals for subsequent analysis. The sound produced by the aircraft must be recorded in such a way that a record of the complete acoustical signal is retained. The recording and reproducing systems must meet the specifications in sections A36.3.6.2 to A36.3.6.9 at the recording speeds and/or data sampling rates used for the noise certification tests. Conformance must be demonstrated for the frequency bandwidths and recording channels selected for the tests.
156. Section A36.3.6.2
The recording and reproducing systems must be calibrated as described in section A36.3.9.
a) For aircraft noise signals for which the high frequency spectral levels decrease rapidly with increasing frequency, appropriate pre-emphasis and complementary de-emphasis networks may be included in the measurement system. If pre-emphasis is included, over the range of nominal one-third octave midband frequencies from 800 Hz to 10 kHz inclusive, the electrical gain provided by the preemphasis network must not exceed 20 dB relative to the gain at 800 Hz.
157. Section A36.3.6.3
For steady sinusoidal electrical signals applied to the input of the entire measurement system including all parts of the microphone system except the microphone at a selected signal level within 5 dB of that corresponding to the calibration sound pressure level on the reference level range, the
time-average signal level indicated by the readout device at any one-third octave nominal midband frequency from 50 Hz to10 kHz inclusive must be within ±1.5 dB of that at the calibration check frequency. The frequency response of a measurement system, which includes components that convert analog signals to digital form, must be within ±0.3 dB of the response at 10 kHz over the frequency range from 10 kHz to 11.2 kHz.
Note: Microphone extension cables as configured in the field need not be included for the frequency response determination. This allowance does not eliminate the requirement of including microphone extension cables when performing the pink noise recording in section A36.3.9.5.
158. Section A36.3.6.4
For analog tape recordings, the amplitude fluctuations of a 1 kHz sinusoidal signal recorded within 5 dB of the level corresponding to the calibration sound pressure level must not vary by more than ±0.5 dB throughout any reel of the type of magnetic tape used. Conformance to this requirement must be demonstrated using a device that has time-averaging properties equivalent to those of the spectrum analyzer.
159. Section A36.3.6.5
For all appropriate level ranges and for steady sinusoidal electrical signals applied to the input of the measurement system, including all parts of the microphone system except the microphone, at one-third-octave nominal midband frequencies of 50 Hz, 1 kHz and 10 kHz, and the calibration check frequency, if it is not one of these frequencies, the level non-linearity must not exceed ±0.5 dB for a linear operating range of at least 50 dB below the upper boundary of the level range.
Note 1. - Level linearity of measurement system components may be tested according to the methods described in IEC 61265 as amended.
Note 2. - Microphone extension cables configured in the field need not be included for the level linearity determination.
160. Section A36.3.6.6
On the reference level range, the level corresponding to the calibration sound pressure level must be at least 5 dB, but no more than 30 dB less than the upper boundary of the level range.
161. Section A36.3.6.7
The linear operating ranges on adjacent level ranges must overlap by at least 50 dB minus the change in attenuation introduced by a change in the level range controls.
Note: It is possible for a measurement system to have level range controls that permit attenuation changes of either 10 dB or 1 dB, for example. With 10 dB steps, the minimum overlap required would be 40 dB, and with 1 dB steps the minimum overlap would be 49 dB.
162. Section A36.3.6.8
An overload indicator must be included in the recording and reproducing systems so that an overload indication will occur during an overload condition on any relevant level range.
163. Section A36.3.6.9
Attenuators included in the measurement system to permit range changes must operate in known intervals of decibel steps.
a. Explanation
Sections A36.3.6.2 through A36.3.9 specify the performance characteristics required for a recording and reproducing system used to store airplane noise signals recorded during noise certification testing for subsequent analysis.
b. Supplemental Information
(1) Recorder Types: An applicant has a choice of recorder types that will satisfy the FAA requirement for recording “the complete acoustic signal” during certification testing. In addition to a magnetic tape recorder, other means of obtaining a “true” acoustic recording include digital audiotape (DAT), recordable 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.
c. Procedures
(1) Frequency Range for Recordings: The time-varying waveform produced by the microphone response to noise signals during certification tests must 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, must 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 antialiasing filter intrudes within the 10 kHz one-third-octave passband. In such cases, the recorder must 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 bandwidth of the uppermost and lowermost bands is included, the center frequencies of the one-third-octave bands immediately outside this range are specified.
(2) Digital Recording Levels: The overload characteristic of a digital system is determined primarily by the limits of the analog-to-digital-conversion. Since such an overload condition is characterized by an abrupt, catastrophic type of distortion, the level range should be set so that the anticipated maximum signal level is at least 10 dB, and preferably 20 dB, below the upper boundary of the linear operating range.
(3) Dynamic Range Limits-Digital Recorders: The lower limit of a digital recording system’s usable dynamic range is more often determined by amplitude non-linearity (due primarily to “quantization error”) than by the presence of a noise floor. Digital devices (such as recorders or analyzers) that are to be used for aircraft noise certification purposes must be tested to determine the extent of such non-linearity.
(4) Consider a 16-Bit Quantization System: The theoretical dynamic range of such a system is usually assumed to be near 96 dB (i.e., 20*log10 (2^16)). At the lower limit of this range, there is a potential for 6 dB error in the digitized signal versus the analog input signal that it represents. IEC 61265 imposes a ±2.0 dB limit on acceptable linearity error outside of an instrument’s linear operating range. 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 dynamic range is further decreased. Significant improvement of amplitude linearity can be obtained by system designers via implementation of techniques such as over-sampling and dithering.
Therefore, testing must be performed to determine the actual limits for each digital recording system.
Assumptions based on experience with analog systems do not always apply.
(5) Pre-emphasis Systems: Use of pre-emphasis will only be allowed if the system also employs complementary de-emphasis. Attempts to compensate for the effects of a pre-emphasis filter by applying onethird-octave-band de-emphasis adjustments (either numerically to analyzed data via a pink noise correction 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/de-emphasis 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.
(6) Attenuator Specification: This requirement allows for the use of switchable voltage input range settings (now commonplace on DAT recorders) as controllable attenuation steps for gain-setting purposes. In all cases, attenuators must have fixed repeatable steps. Any devices in the measurement system that use vernier or continuously–adjustable gain controls must also have some demonstrable means of being fixed, or locked at a specific setting to eliminate non-traceable gain errors.
164. Section A36.3.7 Analysis systems
165. Section A36.3.7.1
The analysis system must conform to the specifications in sections A36.3.7.2 to A36.3.7.7 for the frequency bandwidths, channel configurations and gain settings used for analysis.
166. Section A36.3.7.2
The output of the analysis system must consist of one-third octave band sound pressure levels as a function of time, obtained by processing the noise signals (preferably recorded) through an analysis system with the following characteristics:
a) A set of 24 one-third octave band filters, or their equivalent, having nominal midband frequencies from 50 Hz to 10 kHz inclusive;
b) Response and averaging properties in which, in principle, the output from any one-third octave filter band is squared, averaged and displayed or stored as time-averaged sound pressure levels;
c) The interval between successive sound pressure level samples must be 500 ms ±5 milliseconds (ms) for spectral analysis with or without slow time-weighting, as defined in section A36.3.7.4;
d) For those analysis systems that do not process the sound pressure signals during the period of time required for readout and/or resetting of the analyzer, the loss of data must not exceed a duration of 5 ms; and
e) The analysis system must operate in real time from 50 Hz through at least 12 kHz inclusive.
This requirement applies to all operating channels of a multi-channel spectral analysis system.
167. Section A36.3.7.3
The minimum standard for the one-third octave band analysis system is the class 2 electrical performance requirements of IEC 61260 as amended, over the range of one-third octave nominal midband frequencies from 50 Hz through 10 kHz inclusive.
Note: IEC 61260 specifies procedures for testing of one-third octave band analysis systems for relative attenuation, anti-aliasing filters, real time operation, level linearity, and filter integrated response (effective bandwidth).
168. Section A36.3.7.4
When slow time averaging is performed in the analyzer, the response of the one-third octave band analysis system to a sudden onset or interruption of a constant sinusoidal signal at the respective one-third octave nominal midband frequency, must be measured at sampling instants 0.5, 1, 1.5 and 2 seconds(s) after the onset and 0.5 and 1s after interruption. The rising response must be - 4 ± 1 dB at 0.5s, -1.75 ± 0.75 dB at 1s, -1 ± 0.5 dB at 1.5s and -0.5 ± 0.5 dB at 2s relative to the steady-
state level. The falling response must be such that the sum of the output signal levels, relative to the initial steady-state level, and the corresponding rising response reading is -6.5 ± 1 dB, at both 0.5 and 1s. At subsequent times the sum of the rising and falling responses must be -7.5 dB or less. This equates to an exponential averaging process (slow time-weighting) with a nominal 1s time constant
(i.e., 2s averaging time).
169. Section A36.3.7.5
When the one-third octave band sound pressure levels are determined from the output of the analyzer without slow time-weighting, slow time-weighting must be simulated in the subsequent processing. Simulated slow time-weighted sound pressure levels can be obtained using a continuous exponential averaging process by the following equation:
Ls(i,k) = 10 log [(0.60653)10 0.1Ls[i,(k-1)] + (0.39347) 100.1 L(i,k) ]
where Ls(i,k) is the simulated slow time-weighted sound pressure level and L(i,k) is the asmeasured 0.5 s time average sound pressure level determined from the output of the analyzer for the k-th instant of time and the i-th one-third octave band. For k =1, the slow time-weighted sound pressure Ls[i,(k-1=0)] on the right hand side should be set to 0 dB. An approximation of the continuous exponential averaging is represented by the following equation for a four sample averaging process for k ‡ 4:
Ls(i,k) = 10 log [(0.13)10 0.1L[i,(k -3)] + (0.21) 10 0.1L[i,(k -2)] +(0.27) 10 0.1L[i,(k -1)] + (0.39)10 0.1L[i,k] ]
where Ls(i,k) is the simulated slow time-weighted sound pressure level and L(i,k) is the as measured 0.5s time average sound pressure level determined from the output of the analyzer for the kth instant of time and the i-th one-third octave band.
The sum of the weighting factors is 1.0 in the two equations. Sound pressure levels calculated by means of either equation are valid for the sixth and subsequent 0.5s data samples, or for times greater than 2.5 s after initiation of data analysis.
Note: The coefficients in the two equations were calculated for use in determining equivalent slow time-weighted sound pressure levels from samples of 0.5 s time average sound pressure levels.
The equations do not work with data samples where the averaging time differs from 0.5 s.
170. Section A36.3.7.6
The instant in time by which a slow time-weighted sound pressure level is characterized must be 0.75s earlier than the actual readout time.
Note: The definition of this instant in time is needed to correlate the recorded noise with the aircraft position when the noise was emitted and takes into account the averaging period of the slow time-weighting. For each 0.5 second data record this instant in time may also be identified as 1.25 seconds after the start of the associated 2 second averaging period.
171. Section A36.3.7.7
The resolution of the sound pressure levels, both displayed and stored, must be 0.1 dB or finer.
a. Explanation
This section specifies the required performance characteristics of an analysis system for use in airplane noise certification.
b. Supplemental Information
(1) Analyzer Specifications: IEC 61260 specifies the Class 2 electrical performance requirements of one-third-octave-band filters, including tolerances for the attenuation in the transition bands (“skirts”) adjacent to the one-third-octave pass-bands. Most digital one-third-octave-band analysis systems offer only hardwired filtering algorithms that emulate the response of a traditional third-order analysis filter having a maximally-flat pass-band. However, some analysis systems allow the selection of other filtering algorithms which might not provide equivalent performance. FAA policy requires an applicant to demonstrate the effects that alternate filter design response characteristics might have on noise certification EPNL values.
(2) Determination of Bandwidth Error Corrections: The manufacturer can establish the geometric center frequencies of one-third-octave-band filters using either Base 2 or Base 10 systems. While the use of either method results in frequencies close to the nominal center frequencies referred to in part 36, 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.
(3) Externally Controlled Linear-Integrating Analyzers: In cases where a computer or other external device is used to control and/or communicate with an analyzer performing linear integration, extra care should be taken to ensure that the integration period requirements are met. Some analyzers from major manufacturers have required factory modification in order to provide an integration time within 5 milliseconds of the specified 500-millisecond integration period.
c. Procedures
None
172. Section A36.3.8 Calibration systems
173. Section A36.3.8.1
The acoustical sensitivity of the measurement system must be determined using a sound calibrator generating a known sound pressure level at a known frequency. The minimum standard for the sound calibrator is the class 1L requirements of IEC 60942 as amended.
174. Section A36.3.9 Calibration and checking of system
175. Section A36.3.9.1
Calibration and checking of the measurement system and its constituent components must be carried out to the satisfaction of the FAA by the methods specified in sections A36.3.9.2 through A36.3.9.10. The calibration adjustments, including those for environmental effects on sound calibrator output level, must be reported to the FAA and applied to the measured one-third-octave sound pressure levels determined from the output of the analyzer. Data collected during an overload indication are invalid and may not be used. If the overload condition occurred during recording, the associated test data are invalid, whereas if the overload occurred during analysis, the analysis must be repeated with reduced sensitivity to eliminate the overload.
176. Section A36.3.9.2
The free-field frequency response of the microphone system may be determined by use of an electrostatic actuator in combination with manufacturer’s data or by tests in an anechoic free-field facility. The correction for frequency response must be determined within 90 days of each test series.
The correction for non-uniform frequency response of the microphone system must be reported to the
FAA and applied to the measured one-third octave band sound pressure levels determined from the output of the analyzer.
177. Section A36.3.9.3
When the angles of incidence of sound emitted from the aircraft are within –30° of grazing incidence at the microphone (see Figure A36-1), a single set of free-field corrections based on grazing incidence is considered sufficient for correction of directional response effects. For other cases, the angle of incidence for each 0.5 second sample must be determined and applied for the correction of incidence effects.
178. Section A36.3.9.4
For analog magnetic tape recorders, each reel of magnetic tape must carry at least 30 seconds of pink random or pseudo-random noise at its beginning and end. Data obtained from analog taperecorded signals will be accepted as reliable only if level differences in the 10 kHz one-third-octaveband are not more than 0.75 dB for the signals recorded at the beginning and end.
179. Section A36.3.9.5
The frequency response of the entire measurement system while deployed in the field during the test series, exclusive of the microphone, must be determined at a level within 5 dB of the level corresponding to the calibration sound pressure level on the level range used during the tests for each one-third octave nominal midband frequency from 50 Hz to 10 kHz inclusive, utilizing pink random or pseudo-random noise. Within six months of each test series the output of the noise generator must be determined by a method traceable to the U.S. National Institute of Standards and Technology or to an equivalent national standards laboratory as determined by the FAA. Changes in the relative output from the previous calibration at each one-third octave band may not exceed 0.2 dB. The correction for frequency response must be reported to the FAA and applied to the measured one-third octave sound pressure levels determined from the output of the analyzer.
180. Section A36.3.9.6
The performance of switched attenuators in the equipment used during noise certification measurements and calibration must be checked within six months of each test series to ensure that the maximum error does not exceed 0.1 dB.
181. Section A36.3.9.7
The sound pressure level produced in the cavity of the coupler of the sound calibrator must be calculated for the test environmental conditions using the manufacturer’s supplied information on the influence of atmospheric air pressure and temperature. This sound pressure level is used to establish the acoustical sensitivity of the measurement system. Within six months of each test series the output of the sound calibrator must be determined by a method traceable to the U.S. National Institute of Standards and Technology or to an equivalent national standards laboratory as determined by the FAA. Changes in output from the previous calibration must not exceed 0.2 dB.
182. Section A36.3.9.8
Sufficient sound pressure level calibrations must be made during each test day to ensure that the acoustical sensitivity of the measurement system is known at the prevailing environmental conditions corresponding with each test series. The difference between the acoustical sensitivity levels recorded immediately before and immediately after each test series on each day may not exceed
0.5 dB. The 0.5 dB limit applies after any atmospheric pressure corrections have been determined for the calibrator output level. The arithmetic mean of the before and after measurements must be used to