What is normal hearing? The role of the audiogram

Hearing difficulty is personal.  It isn’t very helpful to tell someone who has difficulty hearing that they have normal hearing.  So, I have been thinking about “What is normal hearing”.

Search through audiology texts, or an internet search, and you will find numerous references to it being 0dB HL, and many more giving a range, such that hearing better than, say 15, 20 or 25 dB constitutes “normal”.  In fact, rather oddly, some texts describe normal hearing as better than 20 dB HL and then draw a “speech banana” that has points on the graph at levels softer than 20 dB HL.  Are these authors saying that the speech sounds are not useful, or that it’s normal not to hear them?  Lets’ be clear – are we talking “statistical normal” or “no difficulty?  Do you, or your audiologist understand?  A nice review of this topic was published in 2000 by Frederick Martin and Craig Champlin.

I know this isn’t all that recent, but the world of standards moves slowly.  Over the years the sound pressure value, corresponding to 0 dB HL has changed, presumably depending on the sample used in the study.  Interestingly, a Chinese study (Jucai et al, 1980) showed a difference in population samples from different cities, even though the participant selection criteria were identical.

The upper limit of normal was set at 15 dB HL.  The ANSI standard of 1996 uses 15 dB HL as the upper bound of normal, but Martin and Champlin suspected that a greater number of clinicians advise their patients that a 25 dB PTA suggest normal hearing.

Martin and Champlin didn’t just speculate.  They worked with a major hearing aid manufacturer and found that over half a million hearing-aid purchasers whose pure tone thresholds were better than 25 dB HL had sought help in dealing with their hearing impairments.

I believe that the definition of hearing loss has been set conservatively because of public funding considerations.  This is not in itself wrong, but we should not extrapolate from this to say that people with milder loss have no difficulty.  If nothing else, it’s insulting.   This is why we developed a test that immediately gives feedback to the user on the nature of their difficulties (i.e. which speech sounds they have difficulty with).

Thanks to:

Jucai, Zhang, Deng Yuancheng, Wen Guangxin. “The Measurement of Audiometric Zero Level of Air Conduction” Scientia Sinica, XXIII No. 6. (June 1980)

Martin, Frederick, and Champlin, Craig. “Reconsidering the Limits of Normal Hearing” Journal of the American Academy of Audiology (11) 64-66. (2000)

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