Community Response to High-Energy Impulsive Sounds: An Assessment of the Field Since 1981 (1996)

Prior estimates of the exposure of survey respondents to high-energy impulsive sounds were made in the course of the present analyses to facilitate comparison in common terms and to reflect additional information. This appendix describes adjustments made to impulsive noise exposure estimates for present purposes and tabulates this information for the benefit of those interested in conducting further analyses.

The impulsive noise exposures of respondents in attitudinal surveys around Ft. Bragg, North Carolina (Schomer, 1982), and Ft. Lewis, Washington (Schomer, 1985), were estimated by prediction and modified (in the case of Ft. Bragg) by site measurements. Although the basic operational data that went into the prediction of CDNL are no longer available, they may be usefully approximated for present purposes.

CDNL values in each study were estimated from computer-based noise exposure prediction methods (supplemented by some on-site measurements at Ft. Bragg). The CDNL at each study site was recalculated for purposes of this report by considering all the blasts to have come from just the nearest firing point. For each site, the number of rounds was adjusted such that the newly predicted CDNL closely approximates the original CDNL. Because this new analysis was based on prediction, only the original predicted levels were used for the target comparison levels. CSEL data were converted to approximate pressure-doubled

levels by adding 4.5 dB so that these calculated results could be compared directly with measurements of (pressure-doubled) sonic boom levels.

In CHABA (1981), the mean peak level of the sonic booms (Borsky, 1965) was used to directly estimate the mean CSEL. However, the mean peak level does not correspond directly to the mean CSEL level. Rather, if the mean CSEL level comes from a Gaussian distribution, then the peak levels come from a lognormal distribution. Since Pierce and Maglieri (1972) have shown that sonic boom levels are normally distributed, Sutherland et al. (1990: Figure 3, Figure 4, and Figure 5) can be used to estimate standard deviations for sonic boom levels in the Oklahoma City study. These references show that the boom pressures follow a lognormal distribution since the levels are normally distributed. Aitchison and Brown (1969) give the equations to find the mean and standard deviation of the decibel levels given the mean and standard deviation of the pressures for a lognormal distribution. Including a factor of 47.85 Pa/(lb/ft²) to convert the reference sound pressure from pascals to the unit of pounds per square foot used to report the peak sound pressures of the sonic booms yielded an approximate constant of 127.6 dB instead of 94 dB for −10 log (p₀²). A scaling factor equal to 10 log (e²) decibels per neper was also needed to convert from base-e to base-10 logarithms and to introduce the decibel as the unit for the standard deviation and the mean peak sound pressure level. The results are given by the following equations:

σ² = (10 log e²)² 1 n (1 + α²/η²) (25)

and

L_m = 127.6 + (10 log e²)α − 0.5 σ² (26) (26)

where σ is the standard deviation of the sound levels, L_m is the mean of the sound levels, α is the mean of the peak sound pressures, and η is the standard deviation of the peak sound pressures. (Because σ was estimated from Fig. 3-5 in Sutherland et al., 1990, Eq. 25 was not used herein, but it is included for completeness.) As in CHABA (1981), a factor of about 25 dB is used to convert from the sonic boom peak sound pressure level to CSEL.

The data presented in Figure 2 are tabulated below in TABLE 6 for the convenience of those who may wish to undertake further analyses of them.