(3) Temperature and Relative Humidity Limits: Noise data are acceptable only if the air temperature is in the range from 36° to 95° Fahrenheit (F), and the relative humidity is in the range from 20 to 95 percent. Temperature and relative humidity may be measured with a psychrometer, a device that measures wet and dry bulb temperatures of the air. Relative humidity is then computed from these temperatures. Sufficient measurements should be made to determine all adjustments required by the regulations. Persons responsible for performing the test should be alert to changes in the conditions. At a minimum, measurements should be made immediately before the first run in a series and immediately after the last run. This will normally result in an interval of not more than 1 hour, because the test airplane is required to refuel after 1 hour of flight time. In marginal or changing conditions, shorter intervals would be more appropriate.
(4) Use of Airport Facility: The regulations also permit the use of airport facility weather-measuring equipment. In deciding if the equipment is suitable, it is necessary to verify: (a) that the measurements are representative of the conditions near the microphone; (b) that the equipment is providing reliable information; (c), that the equipment has recently been calibrated; and (d), and that the equipment is FAA-approved.
(5) Temperature Inversions: The effects of inversions and anomalous wind conditions are difficult to quantify. When temperature inversions are present (that is when the air temperature increases with height over any portion of the atmosphere between the ground and the aircraft), flight conditions may be unstable, which hampers the ability of the pilot to set up a consistent, stabilized climb within the permitted operational tolerances.
Also, under these conditions, it is possible to have a situation in which the surface temperature and relative humidity meet the permissible test criteria but the conditions aloft are much drier, with consequent high sound absorption characteristics and the possibility of underestimating the noise level. The noise spectrum of propellerdriven airplanes contains relatively less high- frequency noise than that of jet airplanes, so the effects may not be very significant unless there is a severe inversion.
(6) Anomalous Winds: The presence of anomalous wind conditions may be assessed by noting the airspeed variation as the airplane climbs. If the wind is uniform or changes speed or direction slowly with altitude, there is no difficulty in maintaining a constant climb speed. If there are strong variations in the wind (wind shear) or rising and descending air, there will be variations in airspeed that are not easily controllable. Variations of ±5 knots during the overflight relative to the reference velocity (Vy) are permitted by the regulation, and this criterion may be used to evaluate the presence of anomalous wind conditions.
(7) Air Temperature Measurements vs. Altitude: At the beginning of the test and, if considered necessary, at intervals during the test, an observer on the test airplane is to monitor the air temperature during a climb. This climb may be a noise data–recording climb or may be dedicated to temperature measurement. The information must be assessed if a judgment is to be made about the acceptability of the conditions for noise measurements. The presence of anomalous wind conditions can be assessed during the data acquisition.
380. Section G36.101(c)
The flight test procedures, measuring equipment, and noise measurement procedures must be approved by the FAA.
a. Explanation
All aspects of the noise certification tests that will be used to show regulatory compliance with Part 36 are to be conducted in accordance with documentation that has been reviewed and approved by the FAA. This is best accomplished by means of a detailed test plan.
b. Supplemental Information
(1) Equivalent Procedures: Equivalent procedures for determining noise levels may be proposed by the applicant. These procedures may be developed to supplement Part 36 procedures and evaluations.
Equivalent procedures are measurement or analytical methods not identical to those specified in Part 36, but which the FAA approves as yielding equivalent results. In effect, equivalent procedures are those that are judged
to provide the same certification stringency and confidence as would be obtained if the procedures were in complete compliance with Part 36.
Use of equivalent procedures may be requested by applicants for many reasons, such as to:
(i) Make use of previously acquired certification test data for the airplane type
(ii) Permit more reliable demonstration of small noise level differences among derived versions of an airplane type
(iii) Minimize the cost of demonstrating compliance with the requirements of 14 Code of Federal Regulations (CFR) Part 36 by keeping airplane flight time, airfield usage, and equipment and personnel costs to a minimum.
c. Procedures
(1) Test Plan: The applicant must prepare a test plan for review and approval by the FAA. The test plan is to include descriptions of the test airplane and the test site, details of all the equipment that will be used, flight limitations and procedures, and data adjustments. The approved test plan will control the performance of the tests and the analysis of the test results.
(2) Examples: An example of an equivalent procedure is the use of flight path intercepts instead of standing start take-offs (see figure in paragraph 402). Another example of an equivalent procedure is the certification of a derivative model of a parent airplane for which a complete database of noise and performance measurements was obtained during the parent model's noise certification. Under these circumstances, it may be possible to show the derivative model's noise levels by doing comparison flyover tests of the derivative prototype and the parent airplanes and documenting the changes or differences between the two.
(3) FAA Approvals: No general rules can be stated for the acceptability of an equivalent procedure, and each such application is to be reviewed and approved by the FAA on its own merits. The FAA Office of Environment and Energy (AEE) has the final approval authority over all equivalent procedures. Any proposed equivalent procedures are to be documented in the test plan.
381. Section G36.101(d)
Sound pressure level data for noise evaluation purposes must be obtained with acoustical equipment that complies with section G36.103 of this appendix.
382. Section G36.103 Acoustical measurement system
The acoustical measurement system must consist of approved equipment with the following characteristics:
(a) A microphone system with frequency response compatible with measurement and analysis system accuracy as prescribed in section G36.105 of this appendix.
a. Explanation
Microphones with the characteristics specified in section G36.105 are appropriate for measuring the noise of small propeller-driven airplanes.
b. Procedure
The test plan is to include a description of the proposed microphone system in a form suitable for FAA verification of compliance with the provisions of section G36.105.
383. Section G36.103(b) & (c)
(b) Tripods or similar microphone mountings that minimize interference with the sound being measured.
(c) Recording and reproducing equipment characteristics, frequency response, and dynamic range compatible with the response and accuracy requirements of section G36.105 of this appendix.
a. Explanation
The regulations call for the microphone to be mounted in an inverted position as described in section G36.107. All other support instrumentation and materials, including personnel, must be placed at sufficient distance from the microphone to avoid causing contaminating effects, such as distortion of the sound field or increased background noise. Special design for the microphone mounting system is required, to avoid interference effects from the microphone holder legs and the edge effects of the plate.
Modern digital recording equipment should not have dynamic range limitation problems in recording propeller- driven small airplane noise levels under the conditions specified.
b. Procedures
(1) Equipment Availability: The microphone support is to be designed to minimize reflections that could distort the sound field. An example of the microphone mounting is given in paragraph 391. Commercially available recording and reproducing equipment that meets the regulatory requirements of G36.105 is readily available.
384. Section G36.103(d)
Acoustic calibrators using sine wave or broadband noise of known sound pressure level. If broadband noise is used, the signal must be described in terms of its average and maximum rootmean-square (rms) value for non-overload signal level.
a. Explanation
Typical acoustical calibration instruments generate either single-frequency sound or broadband sound. The noise measure and limits that are used to evaluate noise levels are defined in terms of the rootmean-square (rms) sound pressure level. With broadband noise, the level of the peaks may be many times the rms value. It is therefore necessary to verify that the peaks of a broadband signal do not exceed the dynamic range of any of the system components, which could cause erroneous results.
Modern digital recording equipment should not have dynamic range limitation problems in recording propeller- driven small airplane noise levels under the conditions specified.
b. Supplemental Information
(1) Field Calibrations: It is possible to calibrate measurement and recording systems in a laboratory, and in fact this is usually done. However various circumstances, such as differing environmental conditions, may cause minor changes in equipment sensitivity. Unintentional damage may also occur during equipment setup and noise testing. Field calibrations are therefore required.
c. Procedure
(1) Acoustic Calibrations: An acoustic calibrator is to be used to calibrate the measurement and recording system. The root-mean-square value of the calibration signal should be reported, and, if a broadband calibrator is used, the peak level should be reported. Calibration signals are to be recorded on the tape recorder (if used) at the beginning and end of each test series and at FAA - approved intervals throughout the test when there may be any delay in the performance of the test. The system should be allowed to reach a stable operating condition in accordance with the manufacturer’s recommendations prior to the initial calibration.
385. Section G36.105(a)
The noise produced by the airplane must be recorded. A magnetic tape recorder, graphic level recorder, or sound level meter is acceptable when approved by the regional certificating authority.
a. Explanation
A permanent record of each noise event is required so that the results are not solely dependent on readings taken in the field as the events occur. The means of making this permanent record are subject to approval by the FAA Local Aircraft Certification Office (ACO) certifying authority.
b. Supplemental Information
(1) Tape Recorders: A tape recorder can be used to preserve a complete acoustical record of the events. If there are questions about the data observed during the tests, the recorded data can be replayed, multiple times if necessary, to verify the results. A more detailed analysis of the airplane noise signal may also be useful to the applicant for research and development purposes.
(2) Graphic Level Recorders: A graphic level recorder can be used to provide a permanent record of the noise levels, but no replay or reproduction of the acoustical signal is possible.
(3) Sound Level Meters: The record that results from the use of a sound level meter depends on the design features of the instrument. The least complex instrument uses an electromechanical metering mechanism, requiring the operator to observe the highest level indicated by the moving needle in the meter display during each event. Other, more complex instruments can be set to hold the maximum noise level reached during each event and show this level on a digital display. Some currently available digital units are capable of storing entire time-histories of noise levels for multiple runs. These histories can be recalled to the instrument’s display, transmitted to a printer, or downloaded to a computer.
c. Procedures
(1) Recommendation: The recommended procedure is to record each noise event on a tape recorder. This recorded data can be played back and analyzed as much as necessary to verify that consistent results have been obtained.
(2) Other Methods: Other methods are acceptable, provided that appropriate measures are taken to ensure the validity of the data, and are subject to FAA approval. Previously-approved methods include:
(i) Obtaining a graphic level record;
(ii) Reading a sound level meter in the field as the event occurs and keeping a handwritten log;
(iii) Printing or transferring to a personal computer, the entire time - history after the test has been completed.
386. Section G36.105(b)&(c)
(b) The characteristics of the complete system must comply with the requirements in International Electrotechnical Commission (IEC) Publications No. 651, entitled "Sound Level Meters" and No. 561, entitled "Electro-acoustical Measuring Equipment for Aircraft Noise Certification" as incorporated by reference under § 36.6 of this part. Sound level meters must comply with the requirements for Type 1 sound level meters as specified in IEC Publication No. 651.
(c) The response of the complete system to a sensibly plane progressive sinusoidal wave of constant amplitude must be within the tolerance limits specified in IEC Publication No. 651, over the frequency range 45 to 11,200 Hz.
a. Explanation
There are many types of microphones and recording equipment. Not all such instruments posses the performance characteristics necessary to ensure consistent and valid data measurement. Specifications in the referenced documents are applicable to equipment used for sensing and recording small-airplane noise measurements.
b. Supplemental Information
At the time of preparation of this Advisory Circular (AC), International Electrotechnical Commision (IEC) 651 Publication has been replaced by IEC 61672-1, and IEC 561 has been replaced by IEC 61265.
c. Procedure
(1) Approval of Equipment: FAA approval of all sensing, recording, and reproducing equipment is required. The equipment to be used for the noise certification tests is to be described in the test plan in sufficient detail for the FAA to determine whether it meets the required standards.
387. Section G36.105(d)
If equipment dynamic range limitations make it necessary, high frequency pre-emphasis must be added to the recording channel with the converse de-emphasis on playback. The pre-emphasis must be applied such that the instantaneous recorded sound pressure level of the noise signal between 800 and 11,200 Hz does not vary more than 20 dB between the maximum and minimum onethird octave bands.
a. Explanation
There may be circumstances in which the characteristics of the noise and the recording equipment are such that the noise signal can not be adequately recorded without additional signal conditioning. Electronic shaping of the noise signal may be used to correct this deficiency. This requirement applies only when the noise signal is recorded on a tape recorder.
Modern digital recording equipment should not have dynamic range limitation problems in recording propeller- driven small-airplane noise levels under the specified conditions. Since the use of older analog equipment is permitted under Appendix G, the process of applying pre emphasis and de-emphasis to the recorded acoustical data is allowed.
The process of applying pre-emphasis and de-emphasis to the recorded acoustical data is not allowed in International Civil Aviation Organization (ICAO) Annex 16 Chapter 10.
b. Supplemental Information
(1) Low-Frequency Noise: The noise spectra of propeller-driven airplanes consist primarily of lowfrequency noise generated by the propeller. Harmonics of the propeller noise are also present and contribute to the A-weighted sound level. Most of the acoustical energy is typically below 1,000 Hz, but measured noise levels can be noticeably affected by the presence of higher frequency energy due in part, to the effects of the A- weighting filter.
(2) Dynamic Range: The dynamic range available with analog recording equipmentis generally more limited than that of digital equipment., and therefore the levels of the airplane noise signal at higher frequencies may be lower than the internal noise levels of analog equipment. This effect can be overcome in some cases, by selectively increasing the levels in the higher frequencies before recording by applying pre-
emphasis, and then applying complementary de-emphasis on playback. Typically, digital tape recorders have a dynamic range that greatly exceeds that of analog recorders, and therefore pre-emphasis is not usually required.
(3) PreEmphasis Systems: Use of preemphasis 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 de-emphasis corrections (either numerically to analyzed data or on a band-by-band basis using separate gain stages for each one-third-octave filter) will not be accepted by FAA. In addition, use of a preemphasis / de-emphasis system will require testing and documentation of the filters and gain stages involved to ensure that any errors are quantified and minimized and that the system performs predictably and reliably.
c. Procedure
(1) The need for preemphasis of the noise signal should be assessed, and the appropriate equipment should be included in the recording/playback system if necessary.
388. Section G36.105(e)
The output noise signal must be read through an "A" filter with dynamic characteristics designated "slow" as defined in IEC Publication No. 651. A graphic level recorder, sound level meter, or digital equivalent may be used.
a. Explanation
This section is limited to providing basic guidance on the requirements of 14 CFR Part 36 Appendix G.
b. Supplemental Information
(1) Filtered Noise Level: The noise level from each flyover test is to be measured in terms of the maximum A-weighted sound level, in decibel (dB (A)) units, using an A-weighting filter with dynamic characteristics (or meter response characteristics) designated as "slow," as defined in IEC 61672-1, "Sound Level Meters." The A-weighting curve is used to account for the combined effects of hearing acuity and human perception of loudness. In effect, human perception of sound is generally less sensitive at lower frequencies, increases with frequency, up to about 4,000 Hz, and decreases for frequencies greater than 4,000 Hz.
(2) Basis of Measurement: The A-weighting correction curve has been precisely defined by national and international standards (e.g., IEC 81872-1) for the measurement of sound (such as environmental noise) and is a standard feature in sound level meters and other sound analysis equipment used for noise assessments. When used for this purpose, a sound level meter will provide a "thermometer-type" rating of the audible sound. This rating (or scale) is known as the A-weighted sound level and has been scientifically proven to be a good indicator of the perceived loudness of sounds. The measurement of an A-weighted sound level requires at least three separate and precise calculations, each of which must be built into sound level meters complying with national and international standards. The sound field measured by the microphone is comprised of fluctuations in sound pressure, which are first converted to an equivalently fluctuating electrical signal by the microphone. This signal passes through the A-weighting (frequency) filter and is subsequently time-averaged to provide a more stable electrical signal that fluctuates much more slowly. The level of this time-averaged signal is converted to a logarithmic scale, using units known as decibels (abbreviated as “dB”). The A-weighted sound level is displayed in units of dB (A), denoting A-weighted sound pressure level, referenced to a sound pressure of 20 micropascals. This sound pressure, equal to a level of zero dB, represents the standardized threshold of human audibility for a tonal sound at a frequency of 1,000 Hz.
(3) Meter Response Speed: A sound level meter will (typically) have two options for time averaging
- namely, "fast" and "slow" meter responses. These options govern the rate of fluctuation of the meter's needle or indicator when the sound is rapidly varying. In effect, the fast response is extremely difficult to read on a meter when a noise level is varying. The slow response is easier to read. An effective 2-second averaging period (1-
second time constant) results in the slow response, which should be used in the 14 CFR Part 36 Appendix G noise tests.
(4) Maximum Sound Level: As would be expected, the measured or indicated A-weighted sound level will increase as the airplane approaches the measurement site and will decrease after the airplane passes over the site. The highest value of the A-weighted sound level that occurs during the overflight is called the maximum A-weighted sound level. This is the value that must be measured during each test.
Note carefully that this maximum value may not occur at the exact moment when the airplane is directly over the microphone. It usually occurs slightly before or after the airplane reaches the overhead position due to the directivity characteristics of propeller, engine, and exhaust noise emissions.
c. Procedure
(1) Equipment Requirements: The sound level recording equipment to be used for the tests must be described in the test plan in sufficient detail for the FAA to determine whether it meets the required standards.
In using a graphic level recorder, it is important to set the writing characteristics to correspond to the slow sound level meter response, as required by the regulation.
389. Section G36.105(f)
The equipment must be acoustically calibrated using facilities for acoustic free-field calibration and if the Administrator requests analysis of the tape recording, the analysis equipment shall be electronically calibrated by a method approved by the FAA. Calibrations shall be performed, as appropriate, in accordance with paragraph A36.3(e) of Appendix A of this part.
a. Explanation
All noise levels measured in the field or from tape-recorded data are to be determined with reference to a known sound pressure level. The most reliable way to accomplish this is to calibrate equipment with a noise source of known magnitude (i.e., a calibrator) and to compare the levels of airplane noise signals with this known source.
b. Supplemental Information
(1) Noise Level Variability: There can be variability in the noise levels indicated by the test equipment, primarily due to environmental factors and the internal warm-up that is required by most types of equipment. Occasionally, there may be other changes due to cable problems or even equipment damage.
Proper use of acoustic calibration devices can help identify such occurrences.
c. Procedure
(1) Equipment Calibration: A suitable acoustic calibrator is to be used to provide a reference sound level. This is usually accomplished by placing the calibrator on the microphone and adjusting the gain of the measuring system so that the reading corresponds to the known sound level of the calibrator. Initial, final, and periodic calibrations should be used to verify that any changes in sensitivity are within the limit specified in section A36.3.9.8. It is important that the manufacturer’s recommended system warm-up time be observed in the field prior to equipment calibration. Calibration equipment should be identified in the test plan and is to be FAA approved.
390. Section G36.105(g)
(g) A windscreen must be employed with the microphone during all measurements of aircraft noise when the wind speed is in excess of 5 knots (9 km/hr).
a. Explanation
Under windy conditions, turbulence can cause the microphone to respond in a way that is indistinguishable from its response to airplane noise. Use of a suitable windscreen over the microphone can reduce the effects of turbulence.
Windscreen corrections must be applied to the measured sound pressure levels if a windscreen is used during the measurements. The applicant must furnish data substantiating the correction.
ICAO Annex 16 Chapter 10 does not require the use of windscreens for inverted microphones.
b. Supplemental Information
(1) Turbulence: Microphones are designed to respond to pressures, primarily the fluctuating pressures of acoustic signals. A fluctuating pressure can also occur as a result of turbulence from air flowing over the sensing surface of the microphone, or from the physical presence of the microphone body in the air flow.
These effects can significantly influence the measured noise level. The windscreen is to be of a designthat minimizes interference with the acoustic signal.
c. Procedure
(1) Use of Windscreens: The use of a windscreen is mandated when the wind speed exceeds 5 knots. It is recommended that a windscreen be used at all times, not only because of the added protection it provides for the microphone, but also to enable uninterrupted measurements when the wind speed is fluctuating above and below 5 knots.
391. Section G36.107 Noise measurement
(a) The microphone must be a pressure type, 12.7 mm in diameter, with a protective grid, mounted in an inverted position such that the microphone diaphragm is 7 mm above and parallel to a white-painted metal circular plate. This white-painted metal plate shall be 40 cm in diameter and at least 2.5 mm thick. The plate shall be placed horizontally and flush with the surrounding ground surface with no cavities below the plate. The microphone must be located three-quarters of the distance from the center to the back edge of the plate along a radius normal to the line of flight of the test airplane.
a. Explanation
The Part 36 sections for large transports (Appendix C) and helicopters (Appendices H and J) require a microphone setup with the sensing element mounted 4 feet. (1.2m) above ground level. Appendix G is the only section of Part 36 that requires a microphone-mounting arrangement other than a 4 foot pole microphone. For Appendix G tests, the microphone is mounted in an inverted position so that the diaphragm is 7 millimeters (mm) above and parallel to a white-painted metal circular plate. The plate must be 40 centimeter (cm) in diameter and at least 2.5 mm thick. The plate is placed horizontally and flush with the surrounding ground surface with no cavities below the plate. The microphone is located three-quarters of the distance from the center to the edge of the plate along a radius normal to the line of flight of the test airplane. Figure 13 shows a typical inverted microphone installation.
The inverted microphone setup in the above figure should be taken as an example of the design and construction of the microphone holder and the ground plate. The legs of the microphone holder must be firmly attached to the plate so that the microphone holder does not vibrate during the test. The plate must be painted white to reflect the sun's rays; such reflection will reduce the thermal effects on the microphone -sensing element.
A metal spacer in the shape of a coin is a practical tool to use in setting the space between the microphone diaphragm and the ground plate. The spacer thickness should be 7 mm minus the space between the microphone protective grid and the microphone diaphragm.
Microphone sensitivity changes with frequency and the angle at which the sound reaches the microphone. To obtain consistent results, it is necessary to set up the microphone so that these changes do not influence the measured noise level.
The specified ground plane microphone configuration greatly minimizes the interference effects of reflected sound waves inherent in pole-mounted microphone installations. For a 4 foot microphone, such effects typically occur in the frequency region that is most significant for propeller-driven aircraft noise.
b. Supplemental Information
(1) Microphone Sensitivity: The specified ground plane configuration places the microphone diaphragm into an effective sound pressure field for the frequency range of interest. Microphones designed for uniform pressure response are appropriate for use in such installations.
(2) Microphone Placement: The spacing of the microphone diaphragm relative to the plate is critical, since it must be inserted completely within the effective sound pressure field, and the depth of this field varies with frequency and sensor size. For frequencies of interest, the 7 mm spacing has been determined to provide the best compromise of associated technical considerations.
(3) Alternate Configurations: The specifications for plate material, dimensions, and installation in the local ground surface represent the best practical method for obtaining an effective sound pressure field for the frequencies of interest. Experiments have been performed with many alternative configurations; however, only the configuration specified in the regulation will be accepted for Appendix G noise certification purposes.
c. Procedures
(1) Plate Installation in Local Ground Surface: Care must be taken during installation to ensure that the ground surface beneath the plate is level and contains no voids or gaps. One way to achieve this is by pressing the plate into the ground surface at the desired location, applying slight pressure, then removing the plate to determine if any areas under the plate are recessed. These recesses can then be filled - in with loose material, such as sand or soil, to obtain a level, uniform underlying surface. Care must also be taken to ensure that the edges of the plate are flush with the surrounding ground surface. This is especially important for plates that are thicker than the specified minimum of 2.5 mm. In some cases it may be appropriate to moisten the soil with water immediately before installation, to allow the surface to mold itself around the plate. In such cases, acoustical measurements should not be performed until the ground has dried.
(2) Design and Construction of Microphone Support: The support should be designed so that it minimizes any potential interference with sound waves from the aircraft arriving in the vicinity of the microphone.
If a spider-like structure such as that in the illustration of Figure 13 is used, the number of legs should be limited to three or four. As specified, the legs must be no larger than 2 mm in diameter. Ideally the support collar should be as small as possible, and it should also implement some sort of tightening device, such as a set screw, to facilitate adjustment of the microphone diaphragm height above the plate. The support must be stable and must orient the microphone in such a way that the diaphragm is parallel to the plate.
(3) Cable Support: In some cases, it may be desirable to provide additional support to the microphone cable as it leads away from the plate. A metal rod or similar sort of support may be used for this purpose. Any such support should be as small as possible and located as far away from the plate as is practical.
The microphone cable should lead directly away from the plate without crossing above any more of the plate's surface than is necessary.
(4) Microphone Height Determination: A practical method for ensuring that the microphone diaphragm is located at the specified height above the plate is to use a temporary metal spacer. This temporary metal spacer should be constructed so that when inserted between the microphone’s protective grid and the plate’s surface, the diaphragm is located at the 7 mm height. For most microphones of the specified type, the spacing between the protective grid and the diaphragm is about 1 to 1.5 mm. Once the microphone support mechanism has been secured, the spacer is removed. The height of the spacer must account for the distance between the protective grid and the diaphragm.
(5) Windscreens: The use of windscreens is required when wind speed exceeds 5 knots. The applicant must provide data to substantiate data adjustments for windscreen insertion-loss.
392. Section G36.107(b)
Immediately prior to and after each test, a recorded acoustic calibration of the system must be made in the field with an acoustic calibrator for the purposes of checking system sensitivity and providing an acoustic reference level for the analysis of the sound level data. If a tape recorder or graphic level recorder is used, the frequency response of the electrical system must be determined at a level within 10 dB of the full-scale reading used during the test, utilizing pink or pseudorandom noise.
a. Explanation
Acoustic calibration is required at the beginning and end of each day of testing to verify that the noise sensing, measurement, and recording equipment has maintained its quality and sensitivity.
b. Supplemental Information
(1) Component Check: The sensitivity of various components of the recording system can change during a test. These changes may result from changes in the environment (primarily temperature) or for other causes. Usually, if the equipment is turned on and allowed to stabilize before the first calibration, any changes during the test period will be small. Any remaining substantial changes in sensitivity may be symptoms of equipment problems.
c. Procedures
(1) Calibrations: An acoustic calibrator is to be used at the start of the tests, and the system’s sound level indication should be adjusted to the known sound output level of the calibrator. The readout, indicator, or display of the system, with calibrator applied, should then be read at various intervals and at the end of the tests. Any change in sensitivity should be within manufacturer's tolerances. If a tape recorder or graphic level recorder is used, the frequency response can be evaluated by the application of an appropriate input signal (such as pink noise, or swept-sine) and subsequent analysis of the recorded signal. The output level should be uniform as a function of frequency, within the specifications provided by the manufacturer. This procedure can be carried out in the laboratory before the start of the tests.
(2) Excessive Drift: When the measured level of the calibration at the end of the day’s testing is within ±0.1 dB of the calibration value at the start of the day's testing, it can be judged that the system sensitivity did not excessively drift and that the test results are acceptable as-is. When a comparison between calibration levels measured at the start and the end of the day’s testing indicates a drift in excess of ±0.1 dB, but within ±1.0 dB, a calibration drift adjustment is to be applied to all measured data as follows: Subtract the calibration signal level measured at the end of the test from the calibration signal level measured at the start of the test.
Divide this difference by 2, then add the resulting calibration drift adjustment to all measured levels. If the difference in calibration levels measured at the start and the end of the day’s testing exceeds ±1.0 dB, the measured data are not valid for certification use.
393. Section G36.107(c)
The ambient noise, including both acoustic background and electrical systems noise, must be recorded and determined in the test area with the system gain set at levels which will be used for aircraft noise measurements. If aircraft sound pressure levels do not exceed the background sound pressure levels by at least 10 dB (A), a takeoff measurement point nearer to the start of the takeoff roll must be used and the results must be adjusted to the reference measurement point by an approved method.
a. Explanation
Under some circumstances, the ambient noise from other sound sources in the vicinity of the test site can have an effect on the measured airplane noise levels. Measurement options (such as site selection, instrumentation configuration, etc.) should be explored in order o ensure that the measured airplane noise level is at least 10 dB (A) greater than the combined background noise.
One of the reasons that this provision is included in the regulations is to address the specifics of measuring the noise levels of aerobatic airplanes. Such airplanes, with their high climb rates, may fly high enough over the measuring location that the measured airplane noise levels do not exceed the ambient noise levels by 10 dB (A). In these circumstances, the method described in this section and shown in Figure 14 may be
used. For airplanes other than aerobatic airplanes, the method described in this section can only be used after AEE approval has been obtained.
H T H R SOR test location 8200 ft reference location
Figure 14 - Adjusted Measurement Location (method is intended for highperformance airplanes)
Annex 16 does not have a corresponding provision, since the ICAO Annex 16 standard exempts aerobatic airplanes from noise certification.
b. Supplemental Information
(1) Background Noise: Typically, with instrumentation properly configured, the electrical background noise will be substantially lower than the level of the airplane noise signal, and achieving a 10 dB (A) difference will not be difficult. If the ambient noise from other sources (e.g., highway, fixed power equipment) is within 10 dB (A) of the airplane noise, the applicant has two options. A quieter site can be selected (this is the best option) or the flight procedure can be changed so that the noise level of the airplane is higher. The latter case is equivalent to measuring the noise at a point closer to the start of take-off roll.
c. Procedures
(1) Increased Airplane Noise: If a site with lower noise levels cannot be used, it will be necessary to fly the airplane so that the target height over the microphone is less than it would be at the reference microphone station (8,200 feet from the start of the take-off roll) as shown in Figure 14. In this case, the airplane height at the microphone location is likely to be outside the ±20 percent tolerance specified in the regulation.
Adjustment of data to reference conditions should be performed in an approved manner.
(2) Reporting Requirements: Background noise levels are to be reported to the FAA in the compliance documentation.
394. Section G36.109 Data Recording, Reporting and Approval
(a) Data representing physical measurements and adjustments to measured data must be recorded in permanent form and appended to the record, except that corrections to measurements for normal equipment response deviations need not be reported. All other adjustments must be approved.
Estimates must be made of the individual errors inherent in each of the operations employed in obtaining the final data.
a. Explanation
Complete documentation of the measurement program should be provided as part of the permanent record of the noise tests. The regulation provides procedures for adjusting the measured data to reference conditions. The operational and weather data and the associated adjustments are to be included in the report.