Temporary Threshold Shifts

Loud concerts can cause temporary shifts in the threshold of hearing in the mid frequency region. In one study, a group of 20 young adult females were exposed to sound at 110 dB for 60 minutes, 3 minutes on and 1 minute off to simulate a concert. The results were:

Emphasizing the wide variability of such tests, 2 of the subjects showed greater than 40 dB threshold shift at 4000 Hz while 9 showed less than 20 dB. None showed a loss (shift) above the guideline at 1000Hz or 8000 Hz, so the shifts are concentrated in the maximum sensitivity range of human hearing. There is no firmly established correlation between temporary threshold shifts and permanent threshold shifts. However, it is prudent to point out that if permanent hearing damage followed the pattern of the temporary threshold shifts, it would be the worst kind of damage because it would diminish hearing acuity in the frequency range that is most important for the understanding of human speech.

Audiometry of Rock Musicians

Since controlled studies of human subjects which might do harm cannot be ethically carried out, one alternative is to find persons who voluntarily subject themselves to very loud sounds and measure their hearing. Such an effort was made in Britain with a group of 42 musicians who agreed to a hearing screen every three years.

The study is not very meaningful because of the small numbers, but it is consistent with the study of temporary threshold shifts in that the damage was in the mid-range frequencies.

Animal Studies

Some animal studies have attempted to assess hearing damage from loud sounds. One British study subjected a group of 6 chinchillas to rock music for 2.5 hours at 1 meter from a loudspeaker (average 107 dBA) and then did histological studies of their inner ears. There was a large variation in the the amount of damage done to their inner ears. Perhaps those with a small amount of damage were in the corner of their cage with their paws over their ears. The only significant outcome seems to be the variability - if there is wide variability in the damage to these animal's ears, it would not be surprising if the susceptibility of human's ears also show wide variability in the vulnerability to damage. The problem is that you would not know whether you are particularly susceptible to damage until you had sustained damage.

Can you limit sound levels?

In 1973 the Leeds City Council in Britain limited their discoteques to a sound level of 96 dBA peak by a license restriction. In the face of legal action a year later, the restriction was removed. The Association of Ballrooms mounted the successful legal challenge by showing that the the equivalent continuous sound level could not be assessed visually with a sound level meter and that the peak intensity of concerts exceeded the average by about 10 dBA. The controversy spawned numerous studies of concert sound levels. Typical results:

OSHA Workplace Standards

The Occupational Safety and Health Administration (OSHA) established a national workplace standard for noise exposure called the OSHA Hearing Conservation Standard 29 CFR 1910.95. It limits the time-weighted-average noise exposure for unprotected workers according to the following table.

Maximum impact noise 140 dBA