and distance values, including any other additional adjustments necessary (e.g., adjustment to the airplane reference speed).
(2) 90 Percent Confidence Interval (Clustered Data): The test methods described in part 36 are conducted for a minimum of six repeated (e.g., at identical configuration, target thrust (power), and speed conditions) noise measurements for each of the three noise measurement points (flyover, lateral and approach).
These repeated tests create three clusters of data for correction and evaluation. The 90 percent confidence interval validity evaluation of clusters of data can be easily determined. If n measurements of EPNLr, (L1, L2, . . . .
Ln,) are obtained under approximately the same conditions and it can be assumed that they constitute a random sample from a normal population, then the statistical 90 percent confidence interval value for each clustered group can be derived.
(3) 90 Percent Confidence Interval (NPD Database): Noise-Power-Distance (NPD) database confidence intervals are determined by using a more general formulation than that used for a normal cluster of data points. The confidence interval is to be calculated about each data regression line, whether it is generated for flight test data, a combination of flight test and static test data, or analytical results.
(4) Single Test Values: When more than one noise measurement system is used at any one noise measurement point, the resulting noise level is to be the average of the measured noise levels for each noise measurement point. Some applicants prefer to provide multiple microphones at each noise measurement point.
When multiple microphones are used during the noise testing, the average noise level is to be reported for each noise measurement.
(5) Valid Conditions: All valid noise measurements that are witnessed by an FAA observer or an appropriate designee and determined to be valid are to be included in the confidence interval calculations even when they produce results that are outside the 90 percent confidence limit of ± 1.5 dB. The cause of erratic or possibly invalid noise data may include: testing under different temperature and humidity extremes, anomalous winds aloft, changes in measuring and recording equipment, changes in airplane hardware, background noise, shift in instrument calibrations, not testing in accordance with the approved test plan, etc. The FAA observer is to make a determination, during the course of noise certification testing, as to the validity of all noise measurements.
A noise measurement may not be excluded from the confidence interval calculations at a later date without FAA approval. Noise measurements determined in the field to be invalid for any reason must be repeated.
c. Procedures
(1) Applicant's Responsibility: An applicant is responsible not only for the development of equivalent procedures but also for the development of procedures to quantify a 90 percent confidence interval determination when the approved equivalent procedures are used. The confidence interval procedures that are used in conjunction with approved equivalent procedures may be unique to each of the three measurement conditions (flyover, lateral and approach) and may be suitable for either simplified or integrated methods of evaluating the EPNLr levels.
(2) Methods for Calculating 90 percent Confidence Interval: Calculation methods for determining 90 percent confidence interval values for clustered and some other approved test results are presented in Appendix 1 of the appended ICAO TM. The TM provides confidence interval calculation methods for: clustered measurements, regression mean line, static test-derived NPD curves, and analytically derived NPD curves, and provides working examples (clustered, first-order regression curve, second-order regression curve, and the pooled data set).
(3) Reporting Requirements: EPNLr levels for each of the three noise measurement points (flyover, lateral, and approach) and the associated values of the 90% confidence intervals are to be reported to the FAA in noise certification compliance documentation.
(4) Retest Requirements: The FAA may require an applicant to retest or provide additional test data for any of the three noise measurement points (flyover, lateral or approach) when the reported results indicate:
(i) A required measurement is reported to be invalid, or
(ii) An insufficient number of measurements were conducted by the applicant to determine a suitable data sample, or
(iii) Data scatter indicates that the data are not from a normal population or trend (e.g., a discontinuity due to low power SBV operation), or
(iv) The 90 percent confidence interval for a noise measuring condition exceeds the allowable ±1.5 dB, or
(v) The test was not conducted in accordance with an approved noise certification compliance demonstration plan.
231. Section A36.6 Nomenclature: Symbols and Units
Symbol Unit Meaning Antilog - Antilogarithm to the base 10.
C(k) dB Tone correction factor. The factor to be added to PNL(k) to account for the presence of spectral irregularities such as tones at the k-th increment of time.
D s Duration time. The time interval between the limits of t(1) and t(2) to the nearest 0.5 second.
D dB Duration correction. The factor to be added to PNLTM to account for the duration of the noise.
EPNL EPNdB Effective perceived noise level . The value of PNL adjusted for both spectral irregularities and duration of the noise. (The unit EPNdB is used instead of the unit dB).
EPNLr EPNDB Effective perceived noise level adjusted for reference conditions
f(i) Hz Frequency. The geometrical mean frequency for the i-th onethird octave band.
F(i,k) dB Delta-dB. The difference between the original sound pressure level and the final background sound pressure level in the i-th one-third octave band at the k-th interval of time. In this case, background sound pressure level means the broadband noise level that would be present in the one-third octave band in the absence of the tone.
H dB dB-down. The value to be subtracted from PNLTM that defines the duration of the noise.
H per cent Relative humidity. The ambient atmospheric relative humidity.
I - Frequency band index. The numerical indicator that denotes any one of the 24 one-third octave bands with geometrical mean frequencies from 50 to 10,000 Hz.
K - Time increment index. The numerical indicator that denotes the number of equal time increments that have elapsed from a reference zero.
Log - Logarithm to the base 10.
log n(a) - Noy discontinuity coordinate. The log n value of the intersection point of the straight lines representing the variation of SPL with log n.
M(b), M(c), etc. - Noy inverse slope. The reciprocals of the slopes of straight lines representing the variation of SPL with log n.
N noy The perceived noisiness at any instant of time that occurs in a specified frequency range.
n(i,k) noy The perceived noisiness at the k-th instant of time that occurs in the i-th one-third octave band.
n(k) noy Maximum perceived noisiness. The maximum value of all of the 24 values of n(i) that occurs at the k-th instant of time.
N(k) noy Total perceived noisiness. The total perceived noisiness at the kth instant of time calculated from the 24-instantaneous values of n(i,k).
p(b), p(c), etc. - Noy slope. The slopes of straight lines representing the variation of SPL with log n.
PNL PNdB The perceived noise level at any instant of time. (The unit PNdB is used instead of the unit dB).
PNL(k) PNdB The perceived noise level calculated from the 24 values of SPL (i,k), at the k-th increment of time. (The unit PNdB is used instead of the unit dB).
PNLM PNdB Maximum perceived noise level . The maximum value of PNL(k).
(The unit PNdB is used instead of the unit dB).
PNLT TPNdB Tone-corrected perceived noise level . The value of PNL adjusted for the spectral irregularities that occur at any instant of time.
(The unit TPNdB is used instead of the unit dB).
PNLT(k) TPNdB The tone-corrected perceived noise level that occurs at the k-th increment of time. PNLT(k) is obtained by adjusting the value of PNL(k) for the spectral irregularities that occur at the k-th increment of time. (The unit TPNdB is used instead of the unit dB).
PNLTM TPNdB Maximum tone-corrected perceived noise level . The maximum value of PNLT(k). (The unit TPNdB is used instead of the unit dB).
PNLTr TPNdB Tone-corrected perceived noise level adjusted for reference conditions
s(i,k) dB Slope of sound pressure level. The change in level between adjacent one-third octave band sound pressure levels at the i-th band for the k-th instant of time.
Ds(i,k) dB Change in slope of sound pressure level..
s'(i,k) dB Adjusted slope of sound pressure level. The change in level between adjacent adjusted one-third octave band sound pressure levels at the i-th band for the k-th instant of time.
s (i,k) dB Average slope of sound pressure level.
SPL dB re 20 mPa Sound pressure level. The sound pressure level that occurs in a specified frequency range at any instant of time.
SPL(a) dB re 20 mPa Noy discontinuity coordinate. The SPL value of the intersection point of the straight lines representing the variation of SPL with log n.
SPL(b) SPL(c) dB re 20 mPa Noy intercept. The intercepts on the SPL-axis of the straight lines representing the variation of SPL with log n.
SPL(i,k) dB re 20mPa The sound pressure level at the k-th instant of time that occurs in the i-th one-third octave band.
SPL'(i,k) dB re 20mPa Adjusted sound pressure level. The first approximation to background sound pressure level in the i-th one-third octave band for the k-th instant of time.
SPL(i) dB re 20mPa Maximum sound pressure level. The sound pressure level that occurs in the i-th one-third octave band of the spectrum for PNLTM.
SPL(i)r dB re 20mPa Corrected maximum sound pressure level. The sound pressure level that occurs in the i-th one-third octave band of the spectrum for PNLTM corrected for atmospheric sound absorption.
SPL"(i,k) dB re 20mPa Final background sound pressure level. The second and final approximation to background sound pressure level in the i-th one-third octave band for the k-th instant of time.
I s Elapsed time. The length of time measured from a reference zero.
t(1), t(2) s Time limit. The beginning and end, respectively, of the noise time history defined by h.
Dt s Time increment. The equal increments of time for which PNL(k) and PNLT(k) are calculated.
T s Normalizing time constant. The length of time used as a reference in the integration method for computing duration corrections, where T = 10s.
t(°F)(°C) °F °C Temperature. The ambient air temperature.
a(i) dB/100ft dB/100m Test atmospheric sound attenuation The atmospheric attenuation of sound that occurs in the i-th one-third octave band at the measured air temperature and relative humidity.
a(i)o dB/100ft dB/100m Reference atmospheric sound attenuation The atmospheric attenuation of sound that occurs in the i-th one-third octave band at a reference air temperature and relative humidity.
A1 degrees First constant climb angle (Gear up, speed of at least V2+10 kt (V2 +19 km/h), takeoff thrust)
A2 degrees Second constant climb angle (Gear up, speed of at least V2+10 kt (V2 +19 km/h), after cut-back) d e degrees Thrust cutback angles. The angles defining the points on the takeoff flight path at which thrust reduction is started and ended respectively.
h degrees Approach angle.
hr degrees Reference approach angle.
q degrees Noise angle (relative to flight path) . The angle between the flight path and noise path. It is identical for both measured and corrected flight paths.
ψ degrees Noise angle (relative to ground). The angle between the noise path and the ground. It is identical for both measured and corrected flight paths. .
m Engine noise emission parameter.
D1 EPNdB PNLT correction. The correction to be added to the EPNL calculated from measured data to account for noise level changes due to differences in atmospheric absorption and noise path length between reference and test conditions.
D2 EPNdB Adjustment to duration correction. The adjustment to be made to the EPNL calculated from measured data to account for noise level changes due to the noise duration between reference and test conditions.
D3 EPNdB Source noise adjustment. The adjustment to be made to the EPNL calculated from measured data to account for noise level changes due to differences between reference and test engine operating conditions.
a. Explanation
This section contains standard noise symbols, their associated units, and the meaning of terms that are used during the testing, analysis, and evaluation of airplane noise certification.
b. Supplementary Information
(1) Common Symbols: Some of the airplane noise certification symbols identified in this section may also be common to other aircraft noise certification activities, such as the noise certification of small airplanes and rotorcraft.
c. Procedures
None.
232. Section A36.7 Sound Attenuation in Air
233. Section A36.7.1
The atmospheric sound attenuation of sound must be determined in accordance with the procedure presented in section A36.7.2.
234. Section A36.7.2
The relationship between atmospheric sound attenuation, frequency, temperature, and humidity is expressed by the following equations.
A36.7.2(a) For calculations using the English System of Units:
( ) ( ) 4215.23106833.40log10 45325.14-1033.610000log05.210 −−×+ ×+ −×+ = q dh q a f f i
and
( ) −×+−×− × −×+− = 37100.32510589.9 10 210288074.297274664.1log 1001010 qq q d H f
where
( )h d is listed in Table A36-4 and f0 in Table A36-5;
( )a i is the attenuation coefficient in dB/1000 ft;
q 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):
( ) ( ) a q h d q i f f = + × − + × + × − − 10 2 05 0 1000 11394 1916984 10 0 842994 10 3 2 755624 . log . .
log . .
10-3
and
d q q q = − + × − × − × − + × − 101010 1328924 3179768 10 2 10 2173716 10 4 2 17496 10 6 3 0f Hlog . .
. .
where
( )h d is listed in Table A36-4 and f0 in Table A36-5;
( )a i is the attenuation coefficient in dB/100 m;
q is the temperature in °C; and
H is the relative humidity, expressed as a percentage.
235. Section 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.
Table A36-4. Values of h(d)
d h(d) d h(d)
0.00 0.000 2.50 0.450
0.25 0.315 2.80 0.400
0.50 0.700 3.00 0.370
0.60 0.840 3.30 0.330
0.70 0.930 3.60 0.300
0.80 0.975 4.15 0.260
0.90 0.996 4.45 0.245
1.00 1.000 4.80 0.230
1.10 0.970 5.25 0.220
1.20 0.900 5.70 0.210
1.30 0.840 6.05 0.205
1.50 0.750 6.50 0.200
1.70 0.670 7.00 0.200
2.00 0.570 10.00 0.200
2.30 0.495
Table A36-5. Values of f0
one-third octave center frequency
f0 (Hz)
one-third octave center frequency
f0 (Hz) 50 50 800 800 63 63 1000 1000 80 80 1250 1250 100 100 1600 1600 125 125 2000 2000 160 160 2500 2500 200 200 3150 3150 250 250 4000 4000 315 315 5000 4500 400 400 6300 5600 500 500 8000 7100 630 630 10000 9000
a. Explanation
This section specifies equations for calculating the atmospheric attenuation of sound.
b. Procedures
(1) Data Adjustment Procedures for Atmospheric Sound Attenuation Effects: Procedure (1) under section A36.2.2.3 and Procedure (2) under section A36.9.3.2.1 of this AC provide example procedures (using the equations given in section A36.7.2) to adjust noise certification data from test to reference conditions accounting for differences in atmospheric sound attenuation.
c. Procedures
None
236. Section A36.8. [Reserved]
237. Section A36.9. Adjustment of Airplane Flight Test Results
238. Section 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.
a. Explanation
This section specifies methods for adjustment of noise data when test conditions differ from reference conditions.
b. Supplemental Information
(1) Adjustments to Reference Conditions: Most noise certification tests are conducted during conditions other than the reference conditions. During these tests, the airplane may be at a different altitude (height over the microphone) or deviate laterally from the intended flight path. The engine thrust (power), atmospheric conditions, airplane altitude, and/or gross weight might also differ from reference conditions.
Therefore, measured noise data must be adjusted to reference conditions to determine whether compliance with part 36, Appendix B, certification noise limits may be achieved. Reference 7 requires that adjustment procedures and analysis methods be reviewed and approved by FAA AEE. FAA requires an audit procedure conducted by VNTSC to ensure that data adjustment and analysis methods that are proposed by applicants satisfy requirements of current regulations and approved procedures ( See Appendix 2 of this AC). Any changes, including software revisions, firmware upgrades, or instrumentation changes are subject to VNTSC review before they can be used for noise certification evaluations. Program validation should be planned and the required information submitted to the FAA very early in the certification cycle, since VNTSC evaluation and FAA approval may take longer than 18 months. FAA Order 8110.4B (reference 7) specifies that future amendments to part 36 will be evaluated by VNTSC to determine whether approved data adjustment methods and analysis methods will be subject to re-evaluation. Applicants will be notified if re-evaluation is required. An appropriate Directorate NCS will provide guidelines for such re-evaluations.
(2) Reference Conditions: Reference conditions are specified under section B36.7(a)(5).
(3) Ambient Noise Adjustments: See section A36.3.10 of this AC.
(4) Band-Dropping: FAA policy does not allow the implementation of band-dropping or band-elimination techniques. One-third octave band sound pressure levels are not to be set to zero, except as
might normally occur as the result of calculations during the noise data evaluation process (for example, during frequency extrapolation of masked high-frequency band levels).
(5) Minimum Approach Distance: At the reference approach parameters of 394 feet altitude and –3 degree glide slope, the reference minimum distance equals 394 feet times the cosine of –3, or 393.46 feet between the aircraft ILS antenna and the microphone measuring station.
(6) High Altitude Test Sites: For test sites at or above 1,200 feet (366 meters), data must be adjusted to account for jet noise suppression due to the difference in the engine jet velocity and jet velocity shear effects resulting from the change in air density. This adjustment is described in Appendix 6 of the appended ICAO TM.
c. Procedures
(1) Data Adjustments: The applicant is to adjust all measured data to certification reference conditions by FAA-approved procedures. The applicant must submit all proposed data adjustment procedures to the FAA ACO for review and approval. The FAA may require submittal of extensive detailed information about the data adjustment process and analysis methods for evaluation and acceptance. The Volpe Center Acoustics Facility of the U.S. Department of Transportation’s Research and Special Programs Administration (VNTSC) assists the FAA in the evaluation and approval of applicants' data adjustment and analysis methods.
(2) VNTSC Evaluations: Appendix 2 of this AC provides the information and forms that an applicant may use to submit noise, airplane tracking, and meteorological measurement data to the FAA for evaluation and approval of the applicant’s noise certification data adjustment and analysis methods.
(3) NPD Database: To facilitate data adjustments, many applicants convert measured noise certification test data into a Noise Power Distance (NPD) database as an equivalent procedure. The NPD database should be constructed utilizing data that are representative of reference conditions. For example, the selected NPD airspeed must be representative of the range of airspeeds for the configurations and gross weights which are anticipated to be used for reference conditions. Similar adjustments can be made to test conditions for distance, airplane altitude, engine thrust (power) and atmospheric sound attenuation to create a useful NPD database. Then, when the reference airspeed and other reference conditions are identified and approved, final noise adjustments can easily be made to the NPD database values to obtain reference EPNL values.
(4) Applicant’s Responsibility: The applicant is responsible for:
(i) Making data adjustments in accordance with a FAA approved noise demonstration compliance plan
(ii) Ensuring version control of all noise data analysis software and informing the cognizant FAA ACO of all revisions.
239. 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
a. Explanation
This section specifies the types of differences between test and reference conditions that require adjustment of measured noise values.
b. Supplemental Information
(1) Test Atmospheric Conditions: Noise certification tests are rarely, if ever, conducted during atmospheric conditions that comply with reference conditions (i.e., sea level ambient pressure, 77º Fahrenheit and 70 percent relative humidity).
(2) Sound Propagation: The sound that propagates along the path between an airplane and the noise measurement points is affected by the meteorological conditions of the air through which the sound passes.
Temperature and relative humidity affect the atmospheric sound attenuation of transmitted source noise. Noise test data are to be adjusted to reference conditions using an FAA-approved procedure.
(3) Homogeneous Path: Reference atmospheric conditions (see section B36.7(a)(5)) along the propagation path are considered homogeneous (uniform in nature and composition). The reference conditions are sea level (pressure altitude), 77° Fahrenheit temperature, a constant 70 percent relative humidity and zero wind.
(4) Required Adjustments of Noise Values: Measured noise values must be adjusted to the reference ambient temperature and relative humidity conditions specified in part 36 and to reference airplane position and engine performance conditions. Reference airplane position and engine performance conditions must be derived using FAA-approved airplane performance data at the reference ambient conditions. These adjustments account for the effects of atmospheric sound attenuation, sound propagation distance, airplane speed, and airplane source noise.
(5) Effect of Airspeed on Source Noise: Sections 2.3.4.7 and 2.3.4.9 of the appended ICAO TM provide guidance on the effect that airspeed has on airplane source noise. Applicants must obtain FAA approval for adjustment methods to account for the effect that airspeed has on source noise (e.g., when differences between test and reference airspeeds exceed 15 knots (28 km/h) true airspeed).
(6) Other Adjustments to Noise Values: Part 36 is intended to produce accurate, repeatable, and comparable values of EPNLr. Some adjustments inherent in the FAA approval of equivalent procedures may also apply to the overall adjustment of noise values from test to reference conditions. These adjustments may account for:
(i) Airplane flight path altitude;
(ii) Airplane flight path climb or descent angle;
(iii) Airplane airspeed;
(iv) Extrapolation of adjusted data for an NPD data base;
(v) Airplane configurations (or modifications) that differ from the reference airplane;
(vi) Background noise levels;
(vii) Engine thrust (power) setting;