Jon K. Francine
Brent S. Stewart
Hubbs--Sea World Res. Inst., 2595 Ingraham St., San Diego, CA 92109
The sonic boom generated by a Titan IV space launch vehicle launched on 7 November 1991 was recorded at two locations on San Miguel Island, California. A Bruel & Kjaer microphone carrier system recorded the overpressure near a haul-out site for about 40 000 pinnipeds (seals and sea lions); a boom event analysis recorder (BEAR) was placed under the predicted focal region. The boom reached the pinniped site 4 min and 38 s after launch. It was neither an N nor a U wave at either site; rather, it had several distinct positive peaks, which indicates that the focal region was uprange of the recording sites. The first and fourth positive peaks were 853 ms apart. At the pinniped site, most of the boom's energy was at low frequencies (ca<500 Hz), as indicated by the 46-dB difference between the 124-dB unweighted sound exposure level (SEL) and the 78 dB A-weighted SEL. The peak overpressure recorded near the pinnipeds was 64.1 Pa (1.34 psf; 130 dB peak SPL). The BEAR system recorded a peak overpressure of 90.5 Pa (1.89 psf; 133 dB peak SPL). Empirical tests of predictive models for focal regions and descriptions of the frequency characteristics of focused booms are still needed to evaluate the potential risks to wildlife from these booms.