Parts 1 and 2 of this blog series discussed some research about the negative emotional effects of music (or noise versus music). The studies touched on two real-life scenarios: 1. the effects of music as opposed to noise; and 2. the effects of liking one type of music and hating another type. This part of the blog series will look at some “procedural” issues and point out some possibly incorrect “assumptions.”
In both situations, measures of temporary hearing loss (also known as temporary threshold shift or its acronym, TTS) were discussed. In TTS studies, one measurement is taken before exposure to noise or music, and then another is taken after the exposure. The difference in decibels is the degree of temporary hearing loss. For obvious humanitarian and ethical reasons, there are very few studies of permanent hearing loss (or PTS) after exposure in well-controlled laboratory settings. The literature does contain many case reports following human tragedies such as motor vehicle accidents and bombings of buildings. However, understandably the results are more difficult to quantify and only very rough estimates of the noise exposures are obtained.
This brings up the first issue… how temporary is temporary threshold shift? For these types of studies, ideally one would assess some aspect of a subject’s hearing just before and immediately after the exposure. But in reality, even with well-controlled studies, hearing may not be re-assessed until minutes, or even an hour, after the exposure.
In these noise studies, it is well known that sometimes there is a rapid resolution of the TTS, and in other cases resolution is more gradual and prolonged. Measureable TTSs can resolve in minutes or hours.
Some researchers have posited that the rate of resolution of the TTS may be a predictor of hearing loss, but to date there is no evidence to support this view. It is quite possible that subject #1 may have post-exposure hearing tested within one to two minutes and subject #10 would have to wait 50-60 minutes before being tested. There may have been a significant amount of spontaneous resolution for subject #10 that might not have been observed for subject #1. Can subject #10’s results be compared directly with those of subject #1? This tends to create a rather larger standard deviation or spread in the measured results that may obscure a phenomenon from being observed, if indeed it does exist.
Is TTS a predictor of anything important? We have all experienced TTS in our everyday lives; after going to a rock concert, we may notice that our ears feel “numb” or dull. There may be reduced clarity or even tinnitus. This typically resolves in 16 to 18 hours, although it may take the tinnitus several days to abate.
Eventually if TTS occurs frequently enough, it may become permanent. This, at least intuitively, suggests a relationship between permanent hearing loss and temporary threshold shift. However, a blunt statement that “prior to having a permanent hearing loss from noise, one must have had some episodes of TTS” is about all we can say.
It turns out that there is very little relationship between TTS and permanent hearing loss. A person who reports a very significant TTS after a rock concert (and perhaps even tinnitus) is no more or no less likely to suffer an eventual permanent hearing loss than a person who suffers no, or very little TTS, after the concert. TTS cannot be used as a predictor of permanent hearing loss.
This has clinical ramifications. If a panic-stricken concert goer calls my office because of tinnitus and a feeling of dullness after last evening’s concert, it is not that important to see them immediately. I may want to do this to allay their concerns, but in reality there is not much that can be done other than hand holding. This patient is no more in need of audiological services than a friend who also attended the concert and experienced no, or little, TTS.
Research that uses TTS as a measure may be expedient, but it should not be extended to predict permanent hearing loss. To date, we have no predictor of permanent hearing loss other than some very general large-scale models such as the ISO1999. Even models like this may be useful for making general comments about large populations, but they should not be used for predicting hearing loss for individuals.
Having said this, measures of TTS can be used for counselling purposes and to probe our auditory systems. TTS can reflect noise or music exposures that are in excess of some exposure fences (such as 85 dBA), but it really should be a measure of the “dose” of exposure that we concern ourselves with.
A level of 85 dBA for a 5-hour gig may not be damaging, but perhaps an exposure of 100 dBA for 2 hours would be. Dose measurement is typically based on the sound level (in dBA) and the duration. Exceeding 100% of the dose may not be as problematic as exceeding 100% of the radiation dose for an X-ray technician, but it can be used as a guide for counselling, hearing protection, noise control at the source, and environmental changes.
Dosimeters do not weight the exposure according to its emotional content. Perhaps multiplying the measured dose by a factor of 1.5 or 2 for those “negative” or stressful exposures would provide a better estimate of potential damage.