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

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

Effects of impulsive sound (e.g., from "blast waves") in animals and humans were reviewed extensively by Altmann33 and are not discussed further in this article, with the exception of one device that was tested at the Air Force Research Laboratory. Another item, the vortex ring generator,59 is not strictly an acoustic device but rather is designed to integrate modalities of concussion, flash, chemicals such as malodors and tear gas, and marker dyes into a single delivery system. Diversionary devices such as "flash-bang grenades"60 are also not exclusively acoustic devices. Therefore, additional details are not provided in this review.

Infrasound and Low-Frequency Sound

Although by common definition low-frequency sound is in the audible range, in this article the nonlethal weapon potential of low-frequency sound and infrasound are Jointly considered. On the basis of animal experimentation, it is known that the attenuation of low-frequency sound in some body organs is much less than that of high-frequency sound.61 Therefore, infrasound and low-frequency sound may be more likely to be absorbed and to have some effect on body function.

Some investigators have suggested that infrasound can directly affect the vestibular system, thereby inducing vertigo and, potentially, motion sickness (see articles by Harris et al.62 and Von Gierke and Parker63 for reviews). The mechanism of these effects could be a change in the activity of the labyrinth and/or otolith organs.

The idea that low-frequency vibrations may be used to make people feel ill may be traced back to the observation that some people feel queasy during earthquakes. Tesla reportedly duplicated the effect with a "vibrating chair."64 However, those observations were based on mechanical vibration in solids. The possible responses to vibration include the degradation of performance and the production of discomfort.85 Although it has been suggested that there are parallels between the effects of low-frequency sound/infrasound and vibration,66,67 mechanical vibration couples to the body much more efficiently than do airborne sound waves. Therefore, intense levels of low-frequency noise would be necessary to achieve the same level of discomfort resulting from low-frequency vibration applied to the body via mechanical contact.68

"Body resonance" could be important in correlating the mechanical amplification of vibration in various parts of the body with physiological responses; that is, different parts of the body are in resonance at varying frequencies.69 For example, Von Gierke and Parker70 reported that human thoracoabdominal viscera exhibit resonance at 4 to 6 Hz. Kjellberg and Wikström71 reported that stomach motility in humans is affected by wholebody vibration (at 3 Hz and 6 Hz), as measured by electrogastrography. Changes in physiological function may be directly attributable to the differential vibratory movement or deformation of particular body structures. Because sound couples to the body less efficiently than does mechanical vibration (as noted above), the possible effect of infrasound on body organs with different resonant frequencies is less clear. It has been hypothesized that body resonances, such as the abdomen at 10 Hz and the chest wall at 60 Hz, could be stimulated by high-intensity infrasound.72 Unlike other investigators who suggested detrimental effects of infrasound, Arabadzhi73 hypothesized that infrasound of moderate intensity at frequencies of 8 to 13 Hz could promote maintenance of a human's state of alertness. This has not been verified.