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AUDIOLOGY FEATURE

Music is

BY MARSHALL CHASIN

Marshall Chasin recaps what we know acoustically about music and noise, and discusses the potentially damaging levels of music, how temporary threshold shift (TTS) is not necessarily temporary and gives us some considerations for protective devices for musicians.

ost of what is known about the example, can we relate an exposure of 85 for decades beginning with the 1966 CHABA effects of music on loss dBA for 20 hours a week to the potential report that defined the on/off fraction rule derives from six studies from risk for someone who is exposed to 90 dBA which essentially weighted the exposure by Mthe five year period between for 18 hours a week? Such a relationship the time varying levels of the noise source – 1968 and 1973. These were large scale is called an “exchange rate” or “trading the quieter periods are balanced against the studies performed on industrial workers relationship”. louder periods. This is one reason why music and these were later used to develop The 3 dB exchange rate is based on exposure for any given gross measure, such the policies for a number of national and the “equal energy hypothesis” that the as dBA or Leq, tends to result in less hearing international regulations, such as that effects of noise (or music) exposure that is loss for musicians and those who listen to of the Occupational Safety and Health summed over time adds up to a well-defined music, than their industrial colleagues. Administration (OSHA), the National exposure energy that is independent of The audiometric configuration of long- Institute for Occupational Safety and being steady state or intermittent. In term noise exposure is even similar to that Health (NIOSH) and later contributed to this scenario, an exposure to 90 dBA for of long-term music exposure. It is frequently the ISO 1990 Standard R-1999 model. The 40 hours a week is identical to 93 dBA difficult to differentiate a noise induced ISO model is considered accurate enough for 20 hours a week, and so on. This “3 dB hearing loss from a music induced hearing exchange rate” is the policy of NIOSH in the loss purely on audiometric data. A thorough for use by regulators and administrators to United States and most other jurisdictions case history is required as the differentiating make policy suggestions for workers but around the world [1]. The 5 dB exchange element. It is therefore not surprising that group results should not be used when an rate is predicated on the assumption that many of the research results using noise as individual is considered [1]. Table 1 shows equal amounts of temporary threshold shift a can apply (or have been applied) the predicted permanent hearing loss (TTS) are equally damaging. In this scenario, to music in many national and international for several levels of noise exposure for a an exposure of 90 dBA for 40 hours a week regulations and policies. number of studies. is identical to 95 dBA for 20 hours a week, and so on. There is very little theoretical Temporary threshold shift (TTS) Exchange rates research to support this view since PTS As the name suggests, TTS is a temporary In 1966 the Committee on Hearing and is not correlated to TTS. Subsequently loss in sensitivity to certain for a Bioacoustics (CHABA) attempted to develop this “5 dB exchange rate” is not found period following an exposure to noise or a model that would relate exposure level commonly in policies around the world, music. The pathophysiology is not well to duration of exposure in an attempt to but the Occupational Safety and Health understood but appears to be in part develop damage risk contours (DRC). For Administration in the United States and a related to the temporary disarticulation few jurisdictions in Canada do subscribe to between the outer hair cells and the this view. tectorial membrane in the cochlea (which re-establishes itself after 16-18 hours) and Music is noise glutamate levels that become ototoxic, “Acoustically, music has a Acoustically, music has a similar structure where the levels return to a normal (lower) as noise. Both music and noise have level after 16-18 hours. TTS is considered to similar structure as noise. significant low fundamental be a cochlear sensory (rather than neural) Both music and noise have energy which musicians call the tonic (or dysfunction. TTS has to be discussed in two note name), and higher frequency harmonic time periods. significant low frequency or broadband energy. Music, like noise, can Prior to the year 2000 TTS was fundamental energy which have levels in the 60-70 dBA region considered to be a benign feature of and also sound levels in excess of 110 dBA. exposure to noise or music resolving in 16-18 musicians call the tonic (or One difference is that many sources of hours – a temporary cochlear phenomenon. industrial noise exposure are steady state, in Because TTS was “temporary” it was note name), and higher that the sound levels have minimal variation a paradigm commonly used to assess frequency harmonic or over time, whereas music is characterised whether a person was subjected to an by highly fluctuating levels. This overly high level of noise or music. Much broadband energy.” intermittency over time has been studied of the research revolved around whether

ent and audiology news | JULY/AUGUST 2018 | VOL 27 NO 3 | www.entandaudiologynews.com AUDIOLOGY FEATURE

References “It is frequently difficult to differentiate a noise induced 1. Feuerstein J, Chasin M. Noise exposure and issues in hearing conservation. In: Katz J, Medwetsky L, Burkard, hearing loss from a music induced hearing loss purely on Hood L. (Eds.). Handbook of Clinical Audiology, 6th edition. Philadelphia, PA, USA; Wolters Kluwer Health/ audiometric data.” Lippincott Williams & Wilkins; 2009: 678-98. 2. Chasin M. Changes in Thinking about TTS: Pre- and Post-2000. Hearing Review 2018;25(3):10. 3. Kujawa SG, Liberman MC. Adding insult to injury: cochlear nerve degeneration after “temporary” noise-induced hearing loss. Journal of Neuroscience 2009;29:14077-85. 4. Kujawa SG, Liberman MC. Synaptopathy in the noise-exposed and aging cochlea: Primary neural a measure of TTS could be used to predict hearing loss” and while that can grab degeneration in acquired sensorineural hearing loss. future permanent threshold shift (PTS), headlines, at this point in time, little is Hearing Research 2015;330:191-9. however no research has shown that TTS known about how this can manifest itself in 5. Liberman MC, Kujawa SG. Cochlear synaptopathy in (or the pattern of recovery from TTS) can be , how this can be reliably measured, acquired sensorineural hearing loss:Manifestations and mechanisms. Hearing Research 2017;349:138-47. used as a predictor of future PTS [2]. This is and what the prevalence actually is. 6. Viana LM, O’Malley JT, Burgess BT, et al. Cochlear one of the main arguments against the 5 dB neuropathy in presbycusis:Confocal analysis exchange rate. Hearing protection of hidden hearing loss in post-mortem tissue. Hearing After the year 2000, a number of studies Because of the laws of where high Research 2015;327:78-88. have demonstrated that despite hearing frequency sounds acoustically ‘see’ any 7. Makary CA, Shin J, Kujawa SG, Liberman MC, Merchant SN. Age-related primary cochlear neuronal thresholds returning to the pre-exposure obstruction better than lower , degeneration in human temporal bones. Journal of the level (i.e. no measureable TTS), there can conventional industrial hearing protection Academy of Research in Otolaryngology 2011;12:711-7. be some permanent neural deficits that tends to provide only about 20-25 dB of 8. Berger EH. Methods of measuring the attenuation of may not be immediately detectable. That protection for the lower frequency sounds hearing protection devices. Journal of the Acoustical is, despite a return to normal cochlear but up to 35-40 dB for the higher frequency Society of America 1986;79:1655-87. function (with a normal audiogram), sounds. Because sound energy can enter there can be neural deficits that remain. the skull directly to the cochlea (in the Specifically, the synapse from the cochlear 2000 Hz region), the maximum limit of inner hair cells to the VIII auditory nerve attenuation on any hearing protector is can be permanently altered with a reduced 40 dB. [8]. amplitude on the Wave I on a traditional The musician’s earplug utilises an ABR evoked audiometry paradigm. This has (either Helmholtz or been referred to as “cochlear synaptopathy” wavelength based) to re-establish much of [3-5]. There are currently no accurate the mid and high frequency sound energy; measures of cochlear synaptopathy and its the result being a flat or uniform hearing prevalence is not well defined. protector. Having a uniform attenuation While prevalence estimates of cochlear hearing protector allows the musician to synaptopathy in models are found hear the proper balance between the lower AUTHOR in the literature, it would be erroneous to frequency fundamental (or tonic) energy relate this to humans. There is very little and the higher frequency harmonic energy. data but there has been some research All acoustic energy in the music is reduced on human temporal bones. Viana and identically from a potentially damaging colleagues [6] counted the number of to a non-damaging sound level. Various synapses in five temporal bones. As a manufacturers of musicians’ earplugs function of age, there were fewer synaptic have different strategies to accomplish connections at the time of death. Another this but most provide approximately 15 dB study by Makary and colleagues [7] showed of sound attenuation. While 15 dB does that again, as a function of age in 100 not sound like a lot, every 3 dB reduction Marshall Chasin, AuD, human temporal bones, there was a marked effectively cuts the dose of exposure in Musicians’ Clinics of Canada, decrease in cochlear spiral ganglion cells, half. A musician wearing a 15 dB uniform Toronto, ON, Canada. despite having intact cochlear sensory cell hearing protector can then be in a musical E: [email protected] populations at the time of death. environment for 32 times as long as without www.MusiciansClinics.com Cochlear synaptopathy has colloquially hearing protection – more is not necessarily been referred to in the media as “hidden better.

Table 1. Predicted PTS for a range of exposure levels (in dBA) for a number of studies and models. Adapted from [1]. Sound Level dBA Passchier-Vermeer Robinson Baughn NIOSH ISO R-1999 (1968, 1971) (1968, 1971) (1973) (1973) (1990) 85 dBA 8 6 9 5 6 90 dBA 15 12 14 11 11 95 dBA 23 18 17 20 21

ent and audiology news | JULY/AUGUST 2018 | VOL 27 NO 3 | www.entandaudiologynews.com