In audiology it is almost a “Mom and apple pie” statement that hearing loss as a result of noise exposure (noise induced hearing loss or NIHL), and also hearing loss associated with aging (presbycusis), are symmetrical- the same in both ears. Therefore, an audiogram that shows significant differences between the right and left ear is usually a warning sign that there may be something more sinister going on. These clients are typically investigated by means of MRI or some form of evoked potentials (such as ABRs) to ensure that there isn’t something growing on the VIIIth auditory nerve.
Yet, in the clinic, we see differences between the right ear and the left ear all the time. If the difference is significant (or recent) and is not attributable to ear wax or to a middle ear problem, an MRI or other appropriate test is scheduled. Even in such cases where an MRI is called for, tumors of the VIIIth auditory nerve are rarely found. Some people just have hearing that is worse on one side. It may be related to a virus, or perhaps a disruption of blood flow to the affected ear that has starved the nerve cells of the oxygen that is so important for the cell’s normal metabolism.
More often than not, the case history will provide clues. “When I was 12 my best friend let off a firecracker beside my right ear” or “I am a right-handed hunter”. In the case of rifles and shot guns, the blast emanates from the end of the barrel, so a right-handed shooter will have the left ear nearer the blast, and a left-sided audiometric asymmetry may be observed. (I am taking this last point from the literature and not personal experience- we don’t use guns in Canada- we just throw rose petals at our enemies in hopes they will change their ways. Capital punishment in Canada is forcing someone to watch a Toronto Maple Leafs hockey game.)
Robert Dobie wrote an article in the September 2014 issue of Ear and Hearing . In it Dr. Dobie reported the results of a review of 2044 industrial workers in a data base (Lempert and Henderson, 1973) from 40 years ago, that has only recently become available. He found that occupational noise exposure does not usually cause or exacerbate audiometric asymmetry. And, barring some exceptions, I suspect that this finding is true, at least for indoor noise exposure.
Of course, there are any number of non-occupational sources of noise exposure such as portable music, shooting, and sporting events (though not Maple Leafs games, where the loudest cheers were measured around 32 dBA).
Two acoustic principles explain why indoor industrial noise exposure is symmetrical- wavelength phenomena and room reverberation. In an outdoor workplace, there are no walls for sound to reflect off, so “room reverberation” is not an issue. These two phenomena will be expanded on in parts 2 and 3 of this blog series.
Music as a source of hearing loss is a slightly different animal from most occupational noise exposure. There are many similarities between industrial noise and music- both are vibrations in the air, both can easily exceed 85 dBA, and both are broadband, meaning that there can be music and noise energy across the piano keyboard from the very low frequency sounds to the very high, above the top end of the piano keyboard. There are also some differences- the intense energy of music seems to be concentrated more on the right side of the piano keyboard, whereas that of industrial noise exposure is in the left side. The biggest difference, however, is that music is played and listened to in a relatively non-reverberant, non-echoey venue, whereas industrial noise is found in a highly echoey, reverberant location.
With industrial noise, the sound levels reaching both of the ears of the worker may be similar despite the offending machine being off to one side. In contrast, musical noise exposure is like Las Vegas- what happens in Vegas stays in Vegas! Music that is generated on the left side of the head stays there and is reduced in sound level by the time it reaches the right side of the head; exposures can therefore be asymmetrical.
