822 

14 CFR Ch. I (1–1–19 Edition) 

Pt. 36, App. H 

(3) To comply with this appendix, a min-

imum of six takeoffs, six approaches, and six 
level flyovers is required. To be counted to-
ward this requirement, each flight event 
must be validly recorded at all three noise 
measuring stations. 

(4) The approved values of V

H

and V

y

used 

in calculating test and reference conditions 
and flight profiles must be reported along 
with measured and corrected sound pressure 
levels. 

Section H36.113 Atmospheric attenuation of 

sound. 

(a) The values of the one-third octave band 

spectra measured during helicopter noise 
certification tests under this appendix must 
conform, or be corrected, to the reference 
conditions prescribed in section H36.3(a). 
Each correction must account for any dif-
ferences in the atmospheric attenuation of 
sound between the test-day conditions and 
the reference-day conditions along the sound 
propagation path between the aircraft and 
the microphone. Unless the meteorological 
conditions are within the test window pre-
scribed in this appendix, the test data are 
not acceptable. 

(b) 

Attenuation rates. 

The procedure for de-

termining the atmospheric attenuation rates 
of sound with distance for each one-third oc-
tave bands must be determined in accord-
ance with SAE ARP 866A (Incorporated by 
reference, see § 36.6). The atmospheric at-
tenuation equations are provided in both the 
International and English systems of units 
in section A36.7 of appendix A to this part. 

(c) 

Correction for atmospheric attenuation. 

(1) 

EPNL values calculated for measured data 
must be corrected whenever— 

(i) The ambient atmospheric conditions of 

temperature and relative humidity do not 
conform to the reference conditions, 77 

°

F 

and 70%, respectively, or 

(ii) The measured flight paths do not con-

form to the reference flight paths. 

(iii) The temperature and relative humid-

ity measured at 33 feet (10 meters) above the 
ground must be used to adjust for propaga-
tion path absorption. 

(2) The mean attenuation rate over the 

complete sound propagation path from the 
aircraft to the microphone must be com-
puted for each one-third octave band from 50 
Hz to 10,000 Hz. These rates must be used in 
computing the corrections required in sec-
tion H36.111(d) of this appendix. 

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 evalu-
ating 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 A includes requirements governing 
determination of noise values, including cal-
culations of: 

(1) 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 ta-

bles. 

(b) Notwithstanding the provisions of sec-

tion A36.4.3.1(a), for helicopter noise certifi-
cation, 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. 

(b) The calculated noise level for each 

noise test series, 

i.e.

, takeoff, flyover, or ap-

proach 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 certifi-
cation reference conditions under section 
H36.305 of this appendix, the following cor-
rection procedure shall apply: 

(1) If there is any difference between meas-

ured test and reference conditions, an appro-
priate correction must be made to the EPNL 
calculated from the measured noise data. 
Conditions that can result in a different 
value include: 

(i) Atmospheric absorption of sound under 

measured test conditions that are different 
from the reference test conditions; or 

(ii) Measured flight path that is different 

from the reference flight path. 

(2) 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 re-
cording with a synchronized time signal 
from which the test profile can be delin-
eated, including the aircraft position for 
which PNLTM is observed at the noise meas-
uring station. For takeoff, the flight profile 

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