Brian C Moore, PhD from Cambridge in the UK recently had a paper published in the International Journal of Audiology (IJA) with the title “A review of the perceptual effects of hearing loss for frequencies above 3kHz”. The IJA is the official journal of the International Society of Audiology and the Society holds biennial international conventions. The most recent one was in Vancouver, Canada and brought together over 1300 participants from 26 countries. The next will be in Capetown, South Africa in 2018.
But, back to the paper. This is an interesting article for several reasons: it’s a nice review of those studies from the last century where speech was low pass filtered below 3000 Hz with a subsequent degradation of speech intelligibility; more recent work showing that speech intelligibility improved with a wider bandwidth extending above 3000 Hz; speech intelligibility continues to improve with bandwidths extending beyond 5000 Hz; and an improved ability to separate speech from noise if they are separated when information is provided in the 6000-10,000 Hz region.
This paper is a reminder that degradation of speech intelligibility is a multi-factorial issue that depends on the nature of the noise (steady state vs. fluctuation), spatial separation of the signal and the noise, and required pure tone threshold bandwidth (amplified or not). And in addition Dr. Moore reminds us of more recent research from Dr. Sharon Kujawa and her colleagues showing that cochlear pure tone sensitivity is only part of the story- neural degradation is also a major factor even in regions of normal cochlear function (at least for pure tone detection such as on the audiogram).
This paper is also interesting since it functions as a review of the relevant research to demonstrate the simplistic and ad hoc nature of compensation for hearing loss caused by occupational noise (industrial or music).
In Canada, and many other jurisdictions, noise exposure can be compensable based on an arithmetic calculation of several pure tone thresholds. Some countries like Canada have this legislation implemented on a provincial (a local level) and other countries have federal mandates such as the United States and England. Depending on where a worker may live in Canada, a hearing loss of 25 dB (with any combination of pure tones ranging from 500 Hz to 3000 Hz) may (or may not be) compensable. Some provinces (e.g. Quebec) require exposure to 90 dBA for at least 5 years, whereas others (e.g. Ontario) require exposure to 85 dBA for 5 years.
Some countries make age adjustments (and even adjustments for gender) according to published tables, and other countries and jurisdictions merely subtract 0.5 dB from the average for every year older than 60.
There is limited science underlying these decisions made for the wide range of compensation approaches as evidenced by the variability around the world. A worker in a mine in England is still subject to the identical exposure as a worker in a mine in Canada, yet compensation calculations are widely divergent.
I don’t necessarily see a simple solution to this problem. Compensation levels and action levels around the world are as much about politics, public policy, and economics as they are about science. But I wonder if advocating for inclusion of pure tone frequencies above 3000 Hz may get a bit closer to the real situation?
Realistically I don’t think that measuring the wave I delay in an ABR or calculating the SP/AP ratio would be clinically feasible. Both of these approaches are useful for determining what the world is starting to call “hidden hearing loss”. Hidden hearing loss is communication breakdown somewhere in the auditory system despite having normal pure tone thresholds on an audiogram.
But perhaps the inclusion of 4000 Hz and 6000 Hz in a compensation calculation may move us one step closer to reality.