iPod devices
In recent months the risk to damage of hearing from iPod devices has received increasing media attention e.g. the recent court case against APPLE for not supplying sufficient warnings as to the damage potential of the devices. The information as to the risk of iPods and “Walkman” devices has been available, if obscured by disinterest from none other than hearing rehabilitation professionals. The hallmark research was carried out in Australia and published in the Medical Journal of Australia, in 1998 under the title of “Latent cochlear damage in personal stereo users”.
A previous study showing equivocal evidence had been published by Meyer-Bisch (Audiology, 1996). However, the Australian study by us was not conducted using conventional hearing tests, but using the otoacoustic emission approach which indicates overall cochlear damage which was significantly higher in personal-stereo users relative to non-users. Moreover the risk was shown to increase with increasing exposure; that due to 6 hours per week use equivalent in effect to the effect upon cochlear damage in workers in noisy industry.
There is no reason to suppose that the later iPod devices are any less damaging in their potential to the earlier Walkman devices. Both use similar button earphones which can deliver high impact music with almost no transmission loss down the ear canal, compared with free-field music from loudspeakers.
The LePage and Murray study was particularly significant because it also showed that the otoacoustic emission approach to assessing the damage can assess the level of damage long before there is any clinical hearing loss. Whereas the particular individual sueing for iPod-produced hearing loss might well claim their was insufficient public knowledge, it is not just the manufacturers (according to the BBC report), but the hearing industry itself must share some of the responsibility for this state of public indifference. In general audiological professionals working in rehabilitation are so busy with the huge market posed by hearing loss, that it really is left to others solelyinterested in prevention of hearing loss to get the message across. Alas, the prevention process generally is not commercially viable of itself, and depends on governments to ensure viability of the research programs for the public good.
The power of otoacoustic emissions
Click-evoked otoacoustic emissions (CEOAEs) have provided a totally different approach to quantifying not just the prevalence of hearing loss, but evidently some ability to predict the future prevalence. The predictive value was demonstrated in a trio of papers presented at the Better Hearing Australia conference, Adelaide August 7th – 11th, 1994. The first of these articles shows a phenomenon first detected in cross-sectional population data in 1991, suggesting that the emission strengths of young adults was actually lower than the emission strengths of ages either side of this group. The second article shows that the prevalence estimates on the basis of CEOAEs were not too far different from those actual prevalence of hearing levels in the British population (Adrian Davis’s study). The third presentation concerns a computer model which makes a number of assumptions about how low values of emission strengths in the Australian population might translate into numbers of people with a hearing loss, purely on the basis of the emission picture.
Audience discussion following the presentation of the three papers
We continued to collect click-evoked data and further analysis four years later revealed a similar picture to those shown in the above articles. This stimulated a good deal of discussion and the following article was invited by the Journal of the Australian Acoustical Society. It explores the possibility of using declining emission strength as early warning for hearing loss. LePage, E. L. (1998). Occupational Noise-Induced Hearing Loss: Origin, Characterisation and Prevention. Acoust.Aust., 26(2), 57-61.
Over the past decade these articles have led to some extended discussion and it has resurfaced again in July 2004 questioning whether the predictions had any value (and suggesting that they did). This link introduces the topic and presents the LePage response to the discussion paper.
Inner Ear Mechanisms 