High-Intensity Acoustics for Military Nonlethal Applications: A Lack of Useful Systems

Military Medicine, Feb 2007 by Jauchem, James R, Cook, Michael C

Investigations with rodents have supplemented the human studies. For example, Yamamura and Kishi,113 using a test to measure equilibrium, found that endurance time of rats was reduced by exposure to 16 Hz at 105 dB after 10 minutes. Additional studies of rodents are not covered in this review.

In one series of experiments,114 tracking behavior of five adult male rhesus monkeys (Macaca mulatta) was substantially disrupted by exposure to 10-Hz infrasound at 160 dB. There was no evidence of any postexposure alterations in tympanometry, distortion product otoacoustic emissions, or auditory brainstem evoked potentials.

In another series,115 three micropigs (Sus scrofa) were exposed to acoustic energy (40 and 80 Hz at 164 and 167 dB, respectively) generated in a progressive wave tube by two Mark VI acoustic generators (Team Corp., Burlington, Washington). Large changes in air pressure were necessary to obtain high sound levels (with obviously high volumes of air flowing directly across the animals). No gross physiological effects were evident after 1 to 2 minutes of exposure. Immediately after release from the test chamber, the subjects showed normal ambulation (i.e., no vertigo) and readily consumed both food and water. There was no evidence of incapacitation induced by the acoustic energy per se.

Exposure to infrasound of 10 to 20 Hz, in a reverberant resonant chamber,95 had minimal impact on consummatory and escape behavior. Rhesus monkeys performing a continuous, compensatory-tracking task were also not substantially affected.

Assessment of the Practicality of Using Acoustic Weapons

Infrasound and other acoustic generators represent a completely new mode of weapons based on novel physical principles (compared with existing nonlethal weapons). This novel approach may be part of the attraction for some. In this case, however, a lack of understanding of the physical principles could lead to the premature development of "prototype weapons" before testing or even reasonable consideration of such principles has occurred. Studies mentioned above (e.g., Ref. 79) have shown that the weapon capabilities of audible sound generators have been grossly overstated.

Vogel116 reviewed potential psychological effects of high-intensity acoustic energy. He suggested that sound pressure levels necessary to create annoyance (i.e., 115 dB) could be produced out to ranges of 1000 feet but such effects would be of negligible use as a nonlethal weapon.

One may consider useful indoor applications to be more likely, because energy can be concentrated more easily. On the basis of the previous work by Von Békésy117 and Von Gierke and Parker,63 who reported a pain threshold in humans at a level of 140 dB at frequencies of 15 and 20 Hz, respectively, some of the exposures in our indoor experiments (including those with a sound pressure of 145 dB at 20 Hz) might have been expected to cause noticeable effects. Such effects, however, were not seen in experiments with either pigs or monkeys exposed to this high sound level.95 This lack of effect is consistent with some of the earlier work by Mueller and Mayes,118 in which squirrel monkeys exposed to 2 Hz at 140 dB showed no evidence of discomfort. Some of those exposures lasted for 6 hours.