Auditory Toughening

From time to time, I am asked about the concept of auditory toughening and what would the implications for musicians be.  It is worthwhile to do a brief review.  Much of the data is 20 years old but all of these experiments were well done and highly controlled for confounding factors.

When one thinks of auditory toughening, it conjures up the imagery of doing pushups on your ears, or lifting heavy weights connected to your earlobes.  But as Yoda might say “Romantic this sounds, but false it is”.

Auditory toughening was first noted by Miller, Watson, and Covell in 1963{{1}}[[1]]Miller, J.D., Watson, C.S., and Covell, W.P. (1963).  Deafening effects of noise on the cat.  Acta Otolaryngologica, Suppl. 176, 1-91.[[1]].  They defined auditory toughening or “the training effect” as the auditory system’s ability to modify its susceptibility to damage from noise, depending on previous exposures.  Specifically, when the auditory system was “toughened” by non-damaging exposure to noise for a number of days, ensuing hearing loss as a result of a damaging level of spectrally similar noise is LESS than that which would occur if there was no previous toughening.

This phenomenon has been observed for permanent threshold shift (PTS) in a wide range of mammals (not humans!) as well as for temporary threshold shift (TTS) for teenagers (Miyakita et al, 1992{{2}}[[2]]Miyakita, T., Hellstrom, P.A., Frimansson, E., and Axelsson, A. (1992).  Effect of low level acoustic stimulation on temporary threshold shift in young humans, Hearing Research, 60, 149-155.[[2]]).

In 1991, Campo, Subramaniam and Henderson{{3}}[[3]]Campo, P., Subramaniam, M., and Henderson, D. (1991).  Effect of conditioning exposures on hearing loss from traumatic exposure”, Hearing Research, 55, 195-200.[[3]] found that after a toughening or training of the ear for 10 days to a 500 Hz band of noise (even after a 5 day period of recovery), a smaller PTS was found in chinchillas that were not toughened (by as much as 15 dB).  It should be noted that in this experiment Campo et al. toughened the chinchillas to a non-damaging level of 500 Hz, and then exposed the chinchillas to a damaging level of 500 Hz noise.  That is the same spectrum was used for the toughening as was used to create a hearing loss.

Subramaniam, Hendersonand Spongr (1991){{4}}[[4]]Subramaniam, M., Henderson, D., and Spongr, V. (1991).  Frequency differences in the development of protection against NIHL by low level “toughening” exposures.  Journal of the Acoustical Society of America 89(4, part 2), 1865-1874.[[4]] then looked at what would happen if chinchillas were toughened with a 500 Hz stimulus, but later were exposed to a high frequency (4000 Hz) stimulus.  In this case, the reverse was found- the toughening actually INCREASED the PTS over that of the non-toughened, or control group.

In summary, it seems that if the non-damaging toughening stimulus is the same frequency as the more traumatic one presented later then there will be some protection from this effect.  However, if the toughening stimulus is low frequency and the traumatic one is high frequency, then the low frequency “toughening” effect may not be toughening at all and may exacerbate the hearing loss.

The physiology of the effect is not well understood.  Possible explanations may be related to the effects of the efferent pathways and the effects of the presence of proteins in the outer hair cells.  More research on this topic is certainly required.

Musicians frequently ask the question- “Should  I wear my hearing protection when I practice as well as when I perform?”  The answer is certainly not clear.  One could argue that if they are practicing the same music as what is performed, then maybe there would be a beneficial toughening effect so perhaps they should not wear their hearing protection when practicing.  BUT this certainly needs more work to be definitive.

At this point, I would suggest that hearing protection should be worn always.  A cellist may practice his or her instrument a lot, but what if they are seated downwind of the percussion or cymbal?  The toughening effect in this case would result in a deleterious effect on their hearing.

Auditory toughening is an interesting effect but at this point my clinical recommendation is to wear the hearing protection as much as possible and not to rely on the potential benefits of auditory toughening.

About Marshall Chasin

Marshall Chasin, AuD, is a clinical and research audiologist who has a special interest in the prevention of hearing loss for musicians, as well as the treatment of those who have hearing loss. I have other special interests such as clarinet and karate, but those may come out in the blog over time.