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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
Table 4.1. Atmospheric Attenuation for Representative Southern Bering Sea Conditions (Estimated Using ANSI S1.26-1978, Method for the Calculation of the Absorption of Sound by the Atmosphere). tnbrd Day* 15 0eg.C a, &/tool 0.01 0.01 0.01 0.02 0.03 0.05 0.07 0.10 0.14 0.19 0.24 0.30 0.37 0.44 0.53 0.66 O.M 1.19 1.69 2.51 3.71 5.66 8.77 13.27 7m I.Y. 1- (a) 0.02 0.02 0.02 0.03 0.05 0.W 0.11 0.15 0.21 0.29 0.36 0.45 0.56 0.66 0.00 1.02 1.32 l.W 2.54 3.77 5.57 8.46 13.16 19.91 t: I W Corrections for Bering Sea Conditions Add to dmta roportod for u S t Day* ~ cmditlons 0 Deg. C, c, dB/lool 0.00 0.00 -0.01 -0.01 -0.01 0.00 0.01 0.02 0.04 0.07 0.09 0.10 0.10 0.06 -0.01 -0.15 -0.36 -0.66 -1.18 -1.92 -2.87 -4.1 -5.33 -5.99 00% R.Y. c a 1- (dB) .00 .00 -0.02 -0.02 -0.02 -00 0.01 0.03 0.06 0.10 0.13 0.15 0.14 0.1 -0.03 -0.24 -0.56 -1.06 -1.83 -2.97 -4.U -6.31 -8.d -9.37
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BBN Svstems and Technologies Cor~or
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Report No. 6945 BBN Systems and Tec
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Page 132 and 133: FIG. 3.14 REPRESENTATIVE SHIPS AND
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FIG. D. 1 HUMAN VOCALIZATION AND AU
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FIG. D. 3 ESTIMATED HEARING CHARACT
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