The acoustic effects on the body are more complex. Research has concluded that with low frequency sound in the region of 50 — 100Hz at levels of 150dB or more, intolerable sensations in the chest and thoracic region can be produced–even with the ears protected. Other physiological changes that occur include chest wall vibration and some respiratory-rhythm changes in human subjects, together with sensations of hypopharyngeal fullness (gagging). The frequency range between 50-100Hz also produces mild nausea and giddiness at levels of 150-155 dB, at which point subjective tolerance is reached. At 150 to 155 dB (0.63 to 1.1 kPa); respiration-related effects include subcostal discomfort, coughing, severe substernal pressure, choking respiration, and hypopharyngeal discomfort. Vladimir Gavreau, who worked extensively with high SPL low frequency sound at the Centre National De La Recherché Scientific states, after being subjected for five minutes to an acoustic emitter producing 196Hz at levels of 160 dB, "we became aware of a painful resonance within our bodies-everything inside us seemed to vibrate when we spoke or moved." These feelings subsequently disappeared after a period of three hours. At medium to high audio frequencies the pronounced visceral effects that are evident with low frequency vibration are absent. However, disturbance of the equilibrium can be achieved at levels above 140 dB for unprotected ears. At even higher levels, tickling sensations and heating may occur in air-filled cavities such as those of the nose and mouth and gaps between the fingers. A 7kHz acoustic beam at a level of 165 dB produced the sensation of strong heating between the fingers of the subject that were being held close together. This was due to the high degree of friction that was created although the effect vanished when the fingers were subsequently opened apart. High audio frequencies (above 10 kHz) and ultrasound (above 20kHz) have no pronounced effect on the individual unless the level is in the range of 140dB with more prominent effects of heating of air cavities, hair and textiles becoming apparent at levels of 160dB. Various assaultive acoustic devices have been designed to operate within this audio region. Most of these devices utilize a sonic property called heterodyning in which 2 differing frequencies combine to form the sum and difference of the initial frequencies.For example, the frequencies of 16000 Hz and 16002 Hz can be combined in the ear to form 32000Hz and 2Hz. Devices operating in the ultrasonic region in this manner also have the added advantage of operating in a very directional and focussed manner at an imperceptible frequency region.
Posted on: Fri, 27 Sep 2013 03:34:41 +0000
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