Nonlinear propagation of high-frequency energy from blast waves as it pertains to bat hearing

Open Access
- Author:
- Loubeau, Alexandra
- Graduate Program:
- Acoustics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 17, 2006
- Committee Members:
- Victor Ward Sparrow, Committee Chair/Co-Chair
Thomas B Gabrielson, Committee Member
John Stephen Lamancusa, Committee Member
Anthony A Atchley, Committee Member - Keywords:
- nonlinear acoustics
blast wave
explosion
outdoor sound propagation
high frequency
bat - Abstract:
- Close exposure to blast noise from military weapons training can adversely affect the hearing of both humans and wildlife. One concern is the effect of high-frequency noise from Army weapons training on the hearing of endangered bats. Blast wave propagation measurements were conducted to investigate nonlinear effects on the development of blast waveforms as they propagate from the source. Measurements were made at ranges of 25, 50, and 100 m from the blast. Particular emphasis was placed on observation of rise time variation with distance. Resolving the fine shock structure of blast waves requires robust transducers with high-frequency capability beyond 100 kHz, hence the limitations of traditional microphones and the effect of microphone orientation were investigated. Measurements were made with a wide-bandwidth capacitor microphone for comparison with conventional 3.175-mm (1/8-in.) microphones with and without baffles. The 3.175-mm microphone oriented at 90 degrees to the propagation direction did not have sufficient high-frequency response to capture the actual rise times at a range of 50 m. Microphone baffles eliminate diffraction artifacts on the rise portion of the measured waveform and therefore allow for a more accurate measurement of the blast rise time. The wide-band microphone has an extended high-frequency response and can resolve shorter rise times than conventional microphones. For a source of 0.57 kg (1.25 lb) of C-4 plastic explosive, it was observed that nonlinear effects steepened the waveform, thereby decreasing the shock rise time, from 25 to 50 m. At 100 m, the rise times had increased slightly. For comparison to the measured blast waveforms, several models of nonlinear propagation are applied to the problem of finite-amplitude blast wave propagation. Shock front models, such as the Johnson and Hammerton model, and full-waveform marching algorithms, such as the Anderson model, are investigated and compared to experimental results. The models successfully predict blast wave rise times at medium distances in a homogeneous atmosphere, although rise time predictions are shorter than what was measured in an inhomogeneous atmosphere. Atmospheric turbulence, absent in the models, may be the primary cause of this difference in rise times at longer distances. Results from the measurements and models indicate that bats located within approximately 200 m of the detonation of 0.57 kg of C-4 will be exposed to audible levels of high-frequency energy, but whether those levels could be damaging to bat hearing cannot be established at this time.