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FAA Federal Aviation Regulations (FARS, 14 CFR)

FARs   >   Part 36   >   Appendix H3 - Part C -- Noise Evaluation and Calculation Under 36.803

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PART C -- NOISE EVALUATION AND CALCULATION UNDER §36.803

 

Section H36.201      Noise Evaluation in EPNdB.

(a) Effective Perceived Noise Level (EPNL), in units of effective perceived noise decibels (EPNdB), shall be used for evaluating noise level values under §36.803 of this part. Except as provided in paragraph (b) of this section, the procedures in appendix A of Part 36 must be used for computing EPNL. appendix B includes requirements governing determination of noise values, including calculations of:

(1) Instantaneous perceived noise levels;

(2) Corrections for spectral irregularities;

(3) Tone corrections;

(4) Duration corrections;

(5) Effective perceived noise levels; and

(6) Mathematical formulation of noy tables.

(b) Notwithstanding the provisions of section A36.4.3.1(a), for helicopter noise certification, corrections for spectral irregularities shall start with the corrected sound pressure level in the 50 Hz one-third octave band.

 

Section H36.203      Calculation of noise levels.

(a) To demonstrate compliance with the noise level limits of section H36.305, the noise values measured simultaneously at the three noise measuring points must be arithmetically averaged to obtain a single EPNdB value for each flight.

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(b) The calculated noise level for each noise test series, i.e., takeoff, flyover, or approach must be the numerical average of at least six separate flight EPNdB values. The 90 percent confidence limit for all valid test runs under section H36.111(d) of this appendix applies separately to the EPNdB values for each noise test series.

 

Section H36.205      Detailed data correction procedures

(a) General. If the test conditions do not conform to those prescribed as noise certification reference conditions under section H36.305 of this appendix, the following correction procedure shall apply:

(1) If a positive value results from any difference between reference and test conditions, an appropriate positive correction must be made to the EPNL calculated from the measured data. Conditions which can result in a positive value include:

(i) Atmospheric absorption of sound under test conditions which is greater than the reference;

(ii) Test flight path at an altitude which is higher than the reference; or

(iii) Test weight which is less than maximum certification weight.

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(2) If a negative value results from any difference between reference and test conditions, no correction may be made to the EPNL calculated from the measured data, unless the difference results from:

(i) An atmospheric absorption of sound under test conditions which is less than the reference; or

(ii) A test flight path at an altitude which is lower than the reference.

(3) The following correction procedures may produce one or more possible correction values which must be added algebraically to the calculated EPNL to bring it to reference conditions:

(i) The flight profiles must be determined for both reference and test conditions. The procedures require noise and flight path recording with a synchronized time signal from which the test profile can be delineated, including the aircraft position for which PNLTM is observed at the noise measuring station. For takeoff, the flight profile corrected to reference conditions may be derived from FAA approved manufacturer's data.

(ii) The sound propagation paths to the microphone from the aircraft position corresponding to PNLTM are determined for both the test and reference profiles. The SPL values in the spectrum of PNLTM must then be corrected for the effects of --

(A) Change in atmospheric sound absorption;

(B) Atmospheric sound absorption on the linear difference between the two sound path lengths; and

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(C) Inverse square law on the difference in sound propagation path length. The corrected values of SPL are then converted to PNLTM from which PNLTM must be subtracted. The resulting difference represents the correction which must be added algebraically to the EPNL calculated from the measured data.

(iii) The minimum distances from both the test and reference profiles to the noise measuring station must be calculated and used to determine a noise duration correction due to any change in the altitude of aircraft flyover. The duration correction must be added algebraically to the EPNL calculated from the measured data.

(iv) From FAA approved data in the form of curves or tables giving the variation of EPNL with rotor rpm and test speed, corrections are determined and must be added to the EPNL, which is calculated from the measured data to account for noise level changes due to differences between test conditions and reference conditions.

(v) From FAA approved data in the form of curves or tables giving the variation of EPNL with approach angle, corrections are determined and must be added algebraically to the EPNL, which is calculated from measured data, to account for noise level changes due to differences between the 6 degree and the test approach angle.

(b) Takeoff profiles. (1) Figure H1 illustrates a typical takeoff profile, including reference conditions.

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(i) The reference takeoff flight path is described in section H36.3(c).

(ii) The test parameters are functions of the helicopter's performance and weight and the atmospheric conditions of temperature, pressure, wind velocity and direction.

(2) For the actual takeoff, the helicopter approaches position C in level flight at 65 feet (20 meters) above ground level at the flight track noise measuring station and at either Vy5 knots (9 km/hr) or the maximum speed of the curve tangential at the ordinate of the height-speed envelope plus 3.0 knots (5 knots), whichever speed is greater. Rotor speed is stabilized at the normal operating RPM (1 percent), specified in the flight manual. The helicopter is stabilized in level flight at the speed for best rate of climb using minimum engine specifications (power or torque and rpm) along a path starting from a point located 1640 feet (500 meters) forward of the flight-track noise measuring station and 65 feet (20 meters) above the ground. Starting at point B, the helicopter climbs through point C to the end of the noise certification takeoff flight path represented by position I. The position of point C may vary within limits allowed by the FAA. The position of the helicopter shall be recorded for a distance (CI) sufficient to ensure recording of the entire interval during which the measured helicopter noise level is within 10 dB of PNLTM, as required by this rule. Station A is the flight-track noise measuring station. The relationships between the measured and corrected takeoff flight profiles can be used to determine the corrections which must be applied to the EPNL calculated from the measured data.

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(3) Figure H1 also illustrates the significant geometrical relationships influencing sound propagation. Position L represents the helicopter location on the measured takeoff flight path from which PNLTM is observed at station A, and Lr is the A and Nρ corresponding position on the reference sound propagation path. AL and ALr both form the angle Φ with their respective flight paths. Position T represents the point on the measured takeoff flight path nearest station A, and Tr is the corresponding position on the reference flight path. The minimum distance to the measured and reference flight paths are indicated by the lines AT and ATr, respectively, which are normal to their flight paths.

(c) Level flyover profiles. (1) The noise type certification level flyover profile is shown in Figure H2. Airspeed must be stabilized within 5 knots of the reference airspeed given in section H36.3(d). For each run, the difference between airspeed and ground speed shall not exceed 10 knots between the 10 dB down points. Rotor speed must be stabilized at the maximum continuous RPM within one percent, throughout the 10 dB down time period. If the test requirements are otherwise met, flight direction may be reversed for each subsequent flyover, to obtain three test runs in each direction.

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(2) Figure H2 illustrates comparative flyover profiles when test conditions do not conform to prescribed reference conditions. The position of the helicopter shall be recorded for a distance (DJ) sufficient to ensure recording of the entire interval during which the measured helicopter noise level is within 10 dB of PNLTM, as required. The flyover profile is defined by the height AG which is a function of the operating conditions controlled by the pilot. Position M represents the helicopter location on the measured flyover flight path for which PNLTM is observed at station A, and Mr is the corresponding position on the reference flight path.

(d) Approach profiles. (1) Figure H3 illustrates a typical approach profile, including reference conditions.

(2) The helicopter approaches position H along a 6 (0.5) average approach slope throughout the 10 dB down period. The approach procedure shall be acceptable to the FAA and shall be included in the Flight Manual.

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(3) Figure H3 illustrates portions of the measured and reference approach flight paths including the significant geometrical relationships influencing sound propagation. EK represents the measured approach path with approach angle η, and Er and Kr represent the reference approach angle of 6. Position N represents the helicopter location on the measured approach flight path for which PNLTM is observed at station A, and Nr is the corresponding position on the reference approach flight path. The measured and corrected noise propagation paths are AN and ANr, respectively, both of which form the same angle with their flight paths. Position S represents the point on the measured approach flight path nearest station A, and Sr is the corresponding point on the reference approach flight path. The minimum distance to the measured and reference flight paths are indicated by the lines AS and ASr, respectively, which are normal to their flight paths.

 

(e) Correction of noise at source during level flyover. (1) For level overflight, if any combination of the following three factors, 1) airspeed deviation from reference, 2) rotor speed deviation from reference, and 3) temperature deviation from reference, results in an advancing blade tip Mach number which deviates from the reference Mach value, then source noise adjustments shall be determined. This adjustment shall be determined from the manufacturer supplied data approved by the FAA.

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(2) Off-reference tip Mach number adjustments shall be based upon a sensitivity curve of PNLTM versus advancing blade tip Mach number, deduced from overflights carried out at different airspeeds around the reference airspeed. If the test aircraft is unable to attain the reference value, then an extrapolation of the sensitivity curve is permitted if data cover at least a range of 0.3 Mach units. The advancing blade tip Mach number shall be computed using true airspeed, onboard outside air temperature, and rotor speed. A separate PNLTM versus advancing blade tip Mach number function shall be derived for each of the three certification microphone locations, i.e., centerline, sideline left, and sideline right. Sideline left and right are defined relative to the direction of the flight on each run. PNLTM adjustments are to be applied to each microphone datum using the appropriate PNLTM function.

(f) PNLT corrections. If the ambient atmospheric conditions of temperature and relative humidity are not those prescribed as reference conditions under this appendix (77 degrees F and 70 percent, respectively), corrections to the EPNL values must be calculated from the measured data under paragraph (a) of this section as follows:

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(1) Takeoff flight path. For the takeoff flight path shown in Figure H1, the spectrum of PNLTM observed at station A for the aircraft at position Lr is decomposed into its individual SPLi values.

(i) Step 1. A set of corrected values are then computed as follows:

            SPLic=SPLi+(α i−α io)AL

+(α io)AL−ALr)

+20 log(AL/ALr)

Where SPLi and SPLic are the measured and corrected sound pressure levels, respectively, in the i-th one-third octave band. The first correction term accounts for the effects of change in atmospheric sound absorption where ai and aio are the sound absorption coefficients for the test and reference atmospheric conditions, respectively, for the -ith one-third octave band and Lr A is the measured takeoff sound propagation path. The second correction term accounts for the effects of atmospheric sound absorption on the change in the sound propagation path length where Lr A is the corrected takeoff sound propagation path. The third correction term accounts for the effects of the inverse square law on the change in the sound propagation path length.

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(ii) Step 2. The corrected values of the SPLic are then converted to PNLT and a correction term calculated as follows:

Δ1=PNLT−PNLTM

Which represents the correction to be added algebraically to the EPNL calculated from the measured data.

(2) Approach flight path. (i) The procedure described in paragraph (f)(1) of this section for takeoff flight paths is also used for the approach flight path, except that the value for SPLic relate to the approach sound propagation paths shown in Figure H3 as follows:

SPLic=SPLi+(α-α io) AM+

α(AM−AMr)+20 log(AM/AMr)

Where the lines NS and Nr Sr are the measured and referenced approach sound propagation paths, respectively.

(ii) The remainder of the procedure is the same as that prescribed in paragraph (d)(1)(ii) of this section, regarding takeoff flight path.

(3) Sideline microphones. The procedure prescribed in paragraph (f)(1) of this section for takeoff flight paths is also used for the propagation to the sideline microphones, except that the values of SPLic relate only in the measured sideline sound propagation path as follows:

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SPLic-SPLi + (α io−α+io)KX

+α io(KX−KXr)+20 log(KX/KXr)

K is the sideline measuring station where

X=L and Xr=Ln for takeoff

X=M and Xr=Mn for approach

X=N and Xr=Nr for flyover

(4) Level flyover flight path. The procedure prescribed in paragraph (f)(1) of this section for takeoff flight paths is also used for the level flyover flight path, except that the values of SPLic relate only to the flyover sound propagation paths as follows:

SPLic=SPLi+(α-α io) AN + α io (AN−ANr)+20 log (AN/ANr)

(g) Duration corrections. (1) If the measured takeoff and approach flight paths do not conform to those prescribed as the corrected and reference flight paths, respectively, under section A36.5(d)(2) it will be necessary to apply duration corrections to the EPNL values calculated from the measured data. Such corrections must be calculated as follows:

(i) Takeoff flight path. For the takeoff flight path shown in Figure H1, the correction term is calculated using the formula --

Δ2=−10 log (AT/ATr) + 10 log (V/Vr)

which represents the correction which must be added algebraically to the EPNL calculated from the measured data. The lengths AT and ATr are the measured and corrected takeoff minimum distances from the noise measuring station A to the measured and the corrected flight paths, respectively. A negative sign indicates that, for the particular case of a duration correction, the EPNL calculated from the measured data must be reduced if the measured flight path is at greater altitude than the corrected flight path.

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(ii) Approach flight path. For the approach flight path shown in Figure H3, the correction term is calculated using the formula --

Δ2=−10 log (AS/ASr) + 10 log (V/Vr)

where AS is the measured approach minimum distance from the noise measuring station A to the measured flight path and 394 feet is the minimum distance from station A to the reference flight path.

(iii) Sideline microphones. For the sideline flight path, the correction term is calculated using the formula --

Δ2=−10 log (KX/KXr)+10 log (V/Vr)

K is the sideline measuring station

where X=T and Xr=Tr for takeoff

where X=S and Xr=Sr for approach

where X=G and Xr=Gr for flyover

(iv) Level flyover flight paths. For the level flyover flight path, the correction term is calculated using the formula --

Δ2=−10 log (AG/AGr)+10 log (V/Vr)

where AG is the measured flyover altitude over the noise measuring station A.

(2) The adjustment procedure described in this section shall apply to the sideline microphones in the take-off, overflight, and approach cases. Although the noise emission is strongly dependent on the directivity pattern, variable from one helicopter type to another, the propagation angle &thetas; shall be the same for test and reference flight paths. The elevation angle ψ shall not be constrained but must be determined and reported. The certification authority shall specify the acceptable limitations on ψ. Corrections to data obtained when these limits are exceeded shall be applied using FAA approved procedures.

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