Analysis and Ranking of the Acoustic Disturbance Potential of ...

Report No. 6945
BBN Systems and Technologies Corporation
Lr = Lref - 20 Log R R e f - a. R + a(SD) Rref dB re 1 uPa (13)
where: Lref = Reference source spectrum at 300 m for standard
day conditions
a(SD) = Atmospheric absorption spectrum for standard day conditions.
The procedure for measuring Lre utilizes microphones near the ground so the
ground reflection effect is inc f uded in the measured level and is usually
corrected for in published data. Equation ( 13) is to be applied successively
to each spectrum band in calculation of the Lr spectrum; i.e., the 50 Hz band
level of the Lr spectrum would be used with the 50 Hz band levels of the
absorption spectra to determine the 50 Hz band level of Lr, etc. Since the
spreading loss term is not frequency dependent, it is calculated once and used
repeatedly.
Atmospheric absorption at low frequencies below 30 kHz is produced by
molecular absorption by oxygen and nitrogen molecules. The amount of
absorption is dependent on frequency, temperature, relative humidity, and to a
small degree on atmospheric pressure. The physical relationship between these
parameters is not easily expressed in mathematical relationships, but an
empirical computer algorithm has been developed for closed-form calculation of
absorption coefficients from input of the four atmospheric parameters (ANSI
S1.26-1978).
In a recent study, the transmission loss relationship given in Eq. (13)
was used together with calculated absorption values tabulated in the ANSI
standard to obtain estimates of aircraft noise in pinniped haulout areas in
the Bering Sea (Johnson et al. 1988). The following example from that study
is presented to illustrate the modeling procedure for airborne sound.
Examination of the climatic atlas data showing temperature and humidity
values for the Bering Sea region of interest during the pinniped haulout
season disclosed that the expected range of variation was not large. A table
of absorption coefficients was prepared using excerpts from the ANSI Standard.
The results are shown in Table 4.1 which presents atmospheric absorption
coefficients estimated for spring and summer conditions. Values are presented
showing attenuation per 100 m. Attenuation values over 150 m (500 ft) are
also given to facilitate correction of reference spectra to 150 m and 450 m
altitudes. For flyovers at 300 m the corrections to the standard day conditions
can be used to estimate aircraft noise spectra at the Bering Sea sites.
The correction values shown in Table 4.1 for the 5 deg C, 80% RH
condition in the Bering Sea were used with Eq. (13) to estimate direct path TL
characteristics. Transmission loss spectra were calculated for estimating
received-levels near the ground from level overflights at 150 m, 300 m, and
450 m. Slant ranges of 1 km and 2 km were also considered in the estimations
to represent offset passes. The resulting TL predictions are shown in Fig.
4.1. The aircraft radiated noise spectra shown in Fig. 3.12 and Fig. 3.13 can