Man-made infrasound occurs at higher intensity levels than those found in nature. It is therefore conceivable that with the increase in man-made sources, there may exist potential danger to man’s health. Stephens and Bryan have reported complaints of people about infrasound, including disorientation, nausea and general feelings of discomfort . In short, responses generally resemble those seen during whole-body vibration, and are mostly of a non-specific nature, resembling reactions to mild stress or alarm [8,9].
Data obtained in comprehensive experiments by Mohr et al. reveal that exposures to high-intensity infrasonics (100-160 db) for short duration (two minutes) have adverse effects on man . Results of these studies are summarized in Figure 10-1.
Mohr et al.’s data have been confirmed by Nixon . Whether or not symptoms similar to those described in Figure 10-1 would occur for prolonged exposure to low intensities of infrasound still remains an open question. There is, however, a report by Green and Dunn which shows that there exists a correlation (0.5) between infrasound exposure and disturbance of certain activities, such as increase in absenteeism in school children and unskilled workers and a higher rate of automobile accidents during periods of higher infrasonic exposure .
A variety of bizarre sensations in the ear have also been reported during exposure to airborne infrasonic waves. These include fluttering or pulsating sensations .
There is some evidence that intense infrasound (120db Sound Pressure Level or above) can stimulate the vestibular system, as can low-frequency vibration, leading to disequilibrium if the stimulation is intense enough; nevertheless, there is no evidence that the hearing organ may be affected by exposures to infrasonic waves encountered in real-life situations [8,9]. However, Guignard and Coles (1965) have demonstrated that a very high-frequency mechanical vibration may produce a small TTS [temporary threshold shift] involving the lower audiometric frequencies and from this it may be inferred that airborne infrasound could possibly also have an effect on hearing .
Various experiments have attempted to shed light on this problem . Results are presented in Table 10-1, The data contained in Table 10-1 reveals that:
- Only small, if any, TTS can be observed following exposures to moderate and intense infrasonics.
- Recovery to pre-exposure hearing levels is rapid when TTS do occur.
The data available suggests that infrasonics do not pose a serious problem to the hearing mechanism when intensities are below 130 db SPL (which is generally the case); however, where high intensities are present (above 130 dB SPL) there may exist a serious hazard.
7. Stephens, R.W.B. and Bryan, M.E., Annoyance Effects Due to Low Frequency Sound, Proceedings of Fall Meeting of British Acoustical Society, 71-109, Nov. 1971.
8. Mohr, G.C., Cole, J.N., Guild, E.C. and Von Gierke, M.E., Effects of Low Frequency and Infrasonic Noise on Man, Aerospace Medicine, 36, 817-824, 1965.
9. Nixon, W. and Johnson, D.L., Infrasound and Hearing, paper presented at the International Congress on Noise as a Public Health Problem, Dubrovnik, Yugoslavia, 1973.
10. Nixon, C.W., Some Effects of Noise on Man. Proceedings of 1971 Intersociety Energy Conversion Engineering Conference, Boston, Mass., August 1971.
11. Green, J.E. and Dunn, F., Correlation of Naturally Occurring Infrasonics and Selected Human Behavior, JASA, 44, 1456, 1968.
12. Yeowart, N.S., Bryan, M.E. and Tempest, W., The Monaural M.A.P. Threshold of Hearing at Frequencies from 1.5 to 100 c/s. J. Sound and Vibration, 6, 335-342, 1967,
13. Guignard, J.C. and Colas, R.R.A., Fifth International Congress on Acoustics, Liege, Sept. 1965.
Office of Noise Abatement and Control
July 27, 1973
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