FAR 36, APPENDIX A4: Section A36.7 -- Sound Attenuation in Air -- FAA FARS, 14 CFR

Section A36.7 Sound Attenuation in Air

A36.7.1 The atmospheric attenuation of sound must be determined in accordance with the procedure presented in section A36.7.2.

A36.7.2 The relationship between sound attenuation, frequency, temperature, and humidity is expressed by the following equations.

A36.7.2(a) For calculations using the English System of Units:

and

where

η(δ) is listed in Table A36-4 and f₀ in Table A36-5;

α(i) is the attenuation coefficient in dB/1000 ft;

&thetas; is the temperature in °F; and

H is the relative humidity, expressed as a percentage.

A36.7.2(b) For calculations using the International System of Units (SI):

and

where

η(δ) is listed in Table A36-4 and f₀ in Table A36-5;

α(i) is the attenuation coefficient in dB/100 m;

&thetas; is the temperature in °C; and

H is the relative humidity, expressed as a percentage.

A36.7.3 The values listed in table A36-4 are to be used when calculating the equations listed in section A36.7.2. A term of quadratic interpolation is to be used where necessary.

Section A36.8 [RESERVED]

Section A36.9 Adjustment of Airplane Flight Test Results.

A36.9.1 When certification test conditions are not identical to reference conditions, appropriate adjustments must be made to the measured noise data using the methods described in this section.

A36.9.1.1 Adjustments to the measured noise values must be made using one of the methods described in sections A36.9.3 and A36.9.4 for differences in the following:

(a) Attenuation of the noise along its path as affected by "inverse square" and atmospheric attenuation

(b) Duration of the noise as affected by the distance and the speed of the airplane relative to the measuring point

(c) Source noise emitted by the engine as affected by the differences between test and reference engine operating conditions

(d) Airplane/engine source noise as affected by differences between test and reference airspeeds. In addition to the effect on duration, the effects of airspeed on component noise sources must be accounted for as follows: for conventional airplane configurations, when differences between test and reference airspeeds exceed 15 knots (28 km/h) true airspeed, test data and/or analysis approved by the FAA must be used to quantify the effects of the airspeed adjustment on resulting certification noise levels.

A36.9.1.2 The "integrated" method of adjustment, described in section A36.9.4, must be used on takeoff or approach under the following conditions:

(a) When the amount of the adjustment (using the "simplified" method) is greater than 8 dB on flyover, or 4 dB on approach; or

(b) When the resulting final EPNL value on flyover or approach (using the simplified method) is within 1 dB of the limiting noise levels as prescribed in section B36.5 of this part.

A36.9.2 Flight profiles.

As described below, flight profiles for both test and reference conditions are defined by their geometry relative to the ground, together with the associated airplane speed relative to the ground, and the associated engine control parameter(s) used for determining the noise emission of the airplane.

A36.9.2.1 Takeoff Profile.

Note: Figure A36-4 illustrates a typical takeoff profile.

(a) The airplane begins the takeoff roll at point A, lifts off at point B and begins its first climb at a constant angle at point C. Where thrust or power (as appropriate) cut-back is used, it is started at point D and completed at point E. From here, the airplane begins a second climb at a constant angle up to point F, the end of the noise certification takeoff flight path.

(b) Position K₁ is the takeoff noise measuring station and AK₁ is the distance from start of roll to the flyover measuring point. Position K₂ is the lateral noise measuring station, which is located on a line parallel to, and the specified distance from, the runway center line where the noise level during takeoff is greatest.

(c) The distance AF is the distance over which the airplane position is measured and synchronized with the noise measurements, as required by section A36.2.3.2 of this part.

A36.9.2.2 Approach Profile.

Note: Figure A36-5 illustrates a typical approach profile.

(a) The airplane begins its noise certification approach flight path at point G and touches down on the runway at point J, at a distance OJ from the runway threshold.

(b) Position K₃ is the approach noise measuring station and K_3O is the distance from the approach noise measurement point to the runway threshold.

(c) The distance GI is the distance over which the airplane position is measured and synchronized with the noise measurements, as required by section A36.2.3.2 of this part.

The airplane reference point for approach measurements is the instrument landing system (ILS) antenna. If no ILS antenna is installed an alternative reference point must be approved by the FAA.

A36.9.3 Simplified method of adjustment.

A36.9.3.1 General. As described below, the simplified adjustment method consists of applying adjustments (to the EPNL, which is calculated from the measured data) for the differences between measured and reference conditions at the moment of PNLTM.

A36.9.3.2 Adjustments to PNL and PNLT.

(a) The portions of the test flight path and the reference flight path described below, and illustrated in Figure A36-6, include the noise time history that is relevant to the calculation of flyover and approach EPNL. In figure A36-6:

(1) XY represents the portion of the measured flight path that includes the noise time history relevant to the calculation of flyover and approach EPNL; X_rY_r represents the corresponding portion of the reference flight path.

(2) Q represents the airplane's position on the measured flight path at which the noise was emitted and observed as PNLTM at the noise measuring station K. Q_r is the corresponding position on the reference flight path, and K_r the reference measuring station. QK and Q_rK_r are, respectively, the measured

and reference noise propagation paths, Q_r being determined from the assumption that QK and Q_rK_r form the same angle &thetas; with their respective flight paths.

(b) The portions of the test flight path and the reference flight path described in paragraph (b)(1) and (2), and illustrated in Figure A36-7(a) and (b), include the noise time history that is relevant to the calculation of lateral EPNL.

(1) In figure A36-7(a), XY represents the portion of the measured flight path that includes the noise time history that is relevant to the calculation of lateral EPNL; in figure A36-7(b), X_rY_r represents the corresponding portion of the reference flight path.

(2) Q represents the airplane position on the measured flight path at which the noise was emitted and observed as PNLTM at the noise measuring station K. Q_r is the corresponding position on the reference flight path, and K_r the reference measuring station. QK and Q_rK_r are, respectively, the measured and reference noise propagation paths. In this case K_r is only specified as being on a particular Lateral line; K_r and Q_r are therefore determined from the assumptions that QK and Q_rK_r:

(i) Form the same angle &thetas; with their respective flight paths; and

(ii) Form the same angle ψ with the ground.

Note: For the lateral noise measurement, sound propagation is affected not only by inverse square and atmospheric attenuation, but also by ground absorption and reflection effects which depend mainly on the angle ψ.

A36.9.3.2.1 The one-third octave band levels SPL(i) comprising PNL (the PNL at the moment of PNLTM observed at K) must be adjusted to reference levels SPL(i)_r as follows:

A36.9.3.2.1(a) For calculations using the English System of Units:

SPL(i)_r=SPL(i)+0.001[α(i)−α(i)₀]QK

+0.001α(i)₀(QK−Q_rK_r)

+20log(QK/Q_rK_r)

In this expression,

(1) The term 0.001[α(i)−α(i)₀]QK is the adjustment for the effect of the change in sound attenuation coefficient, and α(i) and α(i)₀ are the coefficients for the test and reference atmospheric conditions respectively, determined under section A36.7 of this appendix;

(2) The term 0.001α(i)₀(QK − Q_rK_r) is the adjustment for the effect of the change in the noise path length on the sound attenuation;

(3) The term 20 log(QK/Q_rK_r) is the adjustment for the effect of the change in the noise path length due to the "inverse square" law;

(4) QK and Q_rK_r are measured in feet and α(i) and α(i)₀ are expressed in dB/1000 ft.

A36.9.3.2.1(b) For calculations using the International System of Units:

SPL(i)_r=SPL(i)+0.01[α(i)−α(i)₀]QK

+0.01α(i)₀ (QK − Q_rK_r)

+20 log(QK/Q_rK_r)

In this expression,

(1) The term 0.01[α(i) − α(i)₀]QK is the adjustment for the effect of the change in sound attenuation coefficient, and α(i) and α(i)₀ are the coefficients for the test and reference atmospheric conditions respectively, determined under section A36.7 of this appendix;

(2) The term 0.01α(i)₀(QK − Q_rK_r) is the adjustment for the effect of the change in the noise path length on the sound attenuation;

(3) The term 20 log(QK/Q_rK_r) is the adjustment for the effect of the change in the noise path length due to the inverse square law;

(4) QK and Q_rK_r are measured in meters and α(i) and α(i)₀ are expressed in dB/100 m.

A36.9.3.2.1.1 PNLT Correction.

(a) Convert the corrected values, SPL(i)_r, to PNLT_r;

(b) Calculate the correction term Δ₁ using the following equation:

Δ₁=PNLT_r − PNLTM

A36.9.3.2.1.2 Add Δ₁ arithmetically to the EPNL calculated from the measured data.

A36.9.3.2.2 If, during a test flight, several peak values of PNLT that are within 2 dB of PNLTM are observed, the procedure defined in section A36.9.3.2.1 must be applied at each peak, and the adjustment term, calculated according to section A36.9.3.2.1, must be added to each peak to give corresponding adjusted peak values of PNLT. If these peak values exceed the value at the moment of PNLTM, the maximum value of such exceedance must be added as a further adjustment to the EPNL calculated from the measured data.

A36.9.3.3 Adjustments to duration correction.

A36.9.3.3.1 Whenever the measured flight paths and/or the ground velocities of the test conditions differ from the reference flight paths and/or the ground velocities of the reference conditions, duration adjustments must be applied to the EPNL values calculated from the measured data. The adjustments must be calculated as described below.

A36.9.3.3.2 For the flight path shown in Figure A36-6, the adjustment term is calculated as follows:

Δ₂=−7.5 log(QK/Q_rK_r)+10 log(V/V_r)

(a) Add Δ₂ arithmetically to the EPNL calculated from the measured data.

A36.9.3.4 Source noise adjustments.

A36.9.3.4.1 To account for differences between the parameters affecting engine noise as measured in the certification flight tests, and those calculated or specified in the reference conditions, the source noise adjustment must be calculated and applied. The adjustment is determined from the manufacturer's data approved by the FAA. Typical data used for this adjustment are illustrated in Figure A36-8 that shows a curve of EPNL versus the engine control parameter μ, with the EPNL data being corrected to all the other relevant reference conditions (airplane mass, speed and altitude, air temperature) and for the difference in noise between the test engine and the average engine (as defined in section B36.7(b)(7)). A sufficient number of data points over a range of values of μ_r is required to calculate the source noise adjustments for lateral, flyover and approach noise measurements.

A36.9.3.4.2 Calculate adjustment term Δ₃ by subtracting the EPNL value corresponding to the parameter μ from the EPNL value corresponding to the parameter μ_r. Add Δ₃ arithmetically to the EPNL value calculated from the measured data.

A36.9.3.5 Symmetry adjustments.

A36.9.3.5.1 A symmetry adjustment to each lateral noise value (determined at the section B36.4(b) measurement points), is to be made as follows:

(a) If the symmetrical measurement point is opposite the point where the highest noise level is obtained on the main lateral measurement line, the certification noise level is the arithmetic mean of the noise levels measured at these two points (see Figure A36-9(a));

(b) If the condition described in paragraph (a) of this section is not met, then it is assumed that the variation of noise with the altitude of the airplane is the same on both sides; there is a constant difference between the lines of noise versus altitude on both sides (see figure A36-9(b)). The certification noise level is the maximum value of the mean between these lines.

A36.9.4 Integrated method of adjustment

A36.9.4.1 General. As described in this section, the integrated adjustment method consists of recomputing under reference conditions points on the PNLT time history corresponding to measured points obtained during the tests, and computing EPNL directly for the new time history obtained in this way. The main principles are described in sections A36.9.4.2 through A36.9.4.4.1.

A36.9.4.2 PNLT computations.

(a) The portions of the test flight path and the reference flight path described in paragraph (a)(1) and (2), and illustrated in Figure A36-10, include the noise time history that is relevant to the calculation of flyover and approach EPNL. In figure A36-10:

(2) The points Q₀, Q₁, Q_n represent airplane positions on the measured flight path at time t₀, t₁ and t_n respectively. Point Q₁ is the point at which the noise was emitted and observed as one-third octave values SPL(i)₁ at the noise measuring station K at time t₁. Point Q_r1 represents the corresponding position on the reference flight path for noise observed as SPL(i)_r1 at the reference measuring station K_r at time t_r1. Q_1K and Q_r1K_r are respectively the measured and reference noise propagation paths, which in each case form the angle &thetas;₁ with their respective flight paths. Q_r0 and Q_rn are similarly the points on the reference flight path corresponding to Q₀ and Q_n on the measured flight path. Q₀ and Q_n are chosen so that between Q_r0 and Q_rn all values of PNLT_r (computed as described in paragraphs A36.9.4.2.2 and A36.9.4.2.3) within 10 dB of the peak value are included.

(b) The portions of the test flight path and the reference flight path described in paragraph (b)(1) and (2), and illustrated in Figure A36-11(a) and (b), include the noise time history that is relevant to the calculation of lateral EPNL.

(1) In figure A36-11(a) XY represents the portion of the measured flight path that includes the noise time history that is relevant to the calculation of lateral EPNL; in figure A36-11(b), X_rY_r represents the corresponding portion of the reference flight path.

(2) The points Q₀, Q₁ and Q_n represent airplane positions on the measured flight path at time t₀, t₁ and t_n respectively. Point Q₁ is the point at which the noise was emitted and observed as one-third octave values SPL(i)₁ at the noise measuring station K at time t₁. The point Q_r1 represents the corresponding position on the reference flight path for noise observed as SPL(i)_r1 at the measuring station K_r at time t_r1. Q_1K and Q_r1K_r are respectively the measured and reference noise propagation paths. Q_r0 and Q_rn are similarly the points on the reference flight path corresponding to Q₀ and Q_n on the measured flight path.

Q₀ and Q_n are chosen to that between Q_ro and Q_rn all values of PNLT_r (computed as described in paragraphs A36.9.4.2.2 and A36.9.4.2.3) within 10 dB of the peak value are included. In this case K_r is only specified as being on a particular lateral line. The position of K_r and Q_r1 are determined from the following requirements.

(i) Q_1K and Q_r1K_r form the same angle &thetas;₁ with their respective flight paths; and

(ii) The differences between the angles ₁ and _r1 must be minimized using a method, approved by the FAA. The differences between the angles are minimized since, for geometrical reasons, it is generally not possible to choose K_r so that the condition described in paragraph A36.9.4.2(b)(2)(i) is met while at the same time keeping ₁ and _r1 equal.

Note: For the lateral noise measurement, sound propagation is affected not only by "inverse square" and atmospheric attenuation, but also by ground absorption and reflection effects which depend mainly on the angle.

A36.9.4.2.1 In paragraphs A36.9.4.2(a)(2) and (b)(2) the time t_r1 is later (for Q_r1K_r > Q_1K) than t₁ by two separate amounts:

(1) The time taken for the airplane to travel the distance Q_r1Q_r0 at a speed V_r less the time taken for it to travel Q_1Q₀ at V;

(2) The time taken for sound to travel the distance Q_r1K_r-Q_1K.

Note: For the flight paths described in paragraphs A36.9.4.2(a) and (b), the use of thrust or power cut-back will result in test and reference flight paths at full thrust or power and at cut-back thrust or power. Where the transient region between these thrust or power levels affects the final result, an interpolation must be made between them by an approved method such as that given in the current advisory circular for this part.

A36.9.4.2.2 The measured values of SPL(i)₁ must be adjusted to the reference values SPL(i)_r1 to account for the differences between measured and reference noise path lengths and between measured and reference atmospheric conditions, using the methods of section A36.9.3.2.1 of this appendix. A corresponding value of PNL_r1 must be computed according to the method in section A36.4.2. Values of PNL_r must be computed for times t₀ through t_n.

A36.9.4.2.3 For each value of PNL_r1, a tone correction factor C₁ must be determined by analyzing the reference values SPL(i)_r using the methods of section A36.4.3 of this appendix, and added to PNL_r1 to yield PNLT_r1. Using the process described in this paragraph, values of PNLT_r must be computed for times t₀ through t_n.

A36.9.4.3 Duration correction.

A36.9.4.3.1 The values of PNLT_r corresponding to those of PNLT at each one-half second interval must be plotted against time (PNLT_r1 at time t_r1). The duration correction must then be determined using the method of section A36.4.5.1 of this appendix, to yield EPNL_r.

A36.9.4.4 Source Noise Adjustment.

A36.9.4.4.1 A source noise adjustment, Δ₃, must be determined using the methods of section A36.9.3.4 of this appendix.

              A36.9.5 Flight Path Identification Positions
------------------------------------------------------------------------
Position                            Description
------------------------------------------------------------------------
A.................................  Start of Takeoff roll.
B.................................  Lift-off.
C.................................  Start of first constant climb.
D.................................  Start of thrust reduction.
E.................................  Start of second constant climb.
F.................................  End of noise certification Takeoff
flight path.
G.................................  Start of noise certification
Approach flight path.
H.................................  Position on Approach path directly
above noise measuring station.
I.................................  Start of level-off.
J.................................  Touchdown.
K.................................  Noise measurement point.
K[INF]r[/INF].....................  Reference measurement point.
K[INF]1[/INF].....................  Flyover noise measurement point.
K[INF]2[/INF].....................  Lateral noise measurement point.
K[INF]3[/INF].....................  Approach noise measurement point.
M.................................  End of noise certification Takeoff
flight track.
O.................................  Threshold of Approach end of runway.
P.................................  Start of noise certification
Approach flight track.
Q.................................  Position on measured Takeoff flight
path corresponding to apparent
PNLTM at station K See section
A36.9.3.2.
Q[INF]r[/INF].....................  Position on corrected Takeoff flight
path corresponding to PNLTM at
station K. See section A36.9.3.2.
V.................................  Airplane test speed.
V[INF]r[/INF].....................  Airplane reference speed.
------------------------------------------------------------------------

                      A36.9.6 Flight Path Distances
------------------------------------------------------------------------
Distance                 Unit                   Meaning
------------------------------------------------------------------------
AB.....................  Feet (meters).......  Length of takeoff roll.
The distance along the
runway between the start
of takeoff roll and lift
off.
AK.....................  Feet (meters).......  Takeoff measurement
distance. The distance
from the start of roll
to the takeoff noise
measurement station
along the extended
center line of the
runway.
AM.....................  Feet (meters).......  Takeoff flight track
distance. The distance
from the start of roll
to the takeoff flight
track position along the
extended center line of
the runway after which
the position of the
airplane need no longer
be recorded.
QK.....................  Feet (meters).......  Measured noise path. The
distance from the
measured airplane
position Q to station K.
Q[INF]r[/INF]K[INF]r[/   Feet (meters).......  Reference noise path. The
INF].                                          distance from the
reference airplane
position Q[INF]r[/INF]
to station K[INF]r[/
INF].
K[INF]3[/INF]H.........  Feet (meters).......  Airplane approach height.
The height of the
airplane above the
approach measuring
station.
OK[INF]3[/INF].........  Feet (meters).......  Approach measurement
distance. The distance
from the runway
threshold to the
approach measurement
station along the
extended center line of
the runway.
OP.....................  Feet (meters).......  Approach flight track
distance. The distance
from the runway
threshold to the
approach flight track
position along the
extended center line of
the runway after which
the position of the
airplane need no longer
be recorded.
------------------------------------------------------------------------

[Amdt. 36-54, 67 FR 45212, July 8, 2002; Amdt. 36-24, 67 FR 63195, 63196, Oct. 10, 2002]

FAA Federal Aviation Regulations (FARS, 14 CFR)

FARs > Part 36 > Appendix A4 - Section A36.7 -- Sound Attenuation in Air

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