A dual number rating system for hearing protection – part 2

Marshall Chasin
May 28, 2013

In Part 1 of this blog we talked about a single number rating scheme for hearing protectors called the Noise Reduction Rating or NRR.  This number is mandated to be written on the packaging of non-custom hearing protectors by the Environmental Protection Agency or EPA.  The problems with single number rating schemes are that they tend to be simplistic, require many correction factors and don’t show the inherent variability between individual users of the hearing protectors.  The individual variability issue is inherent whenever people are involved and it would be a much easier problem to resolve if it weren’t for people using hearing protection!  However, given that real people need to be involved, I suppose we are stuck with this problem.

In some sense, we could have an entirely different set of problems (but possibly better problems?) with labeling hearing protectors if our ancestral researchers would have decided to approach hearing protection in the same way that they approached hearing aids.  The 2 cc coupler has been around since the 1940’s and is useful for testing standards such as ANSI S3.22 which is used for quality control assessment in hearing aids.  Nowhere in ANSI S3.22 does it specify how a hearing aid should perform- just that it meets certain standards according to a rigid test protocol.  It would take either a functional assessment or a real ear measure on an individual to determine how a hearing aid performs.  The difference between the test standard and how a hearing aid actually performed is sometimes called a Real Ear Coupler Difference or RECD.  Well, hearing protection is the same thing, but instead of having positive gain, it has negative gain.

The hearing protector could be labeled with a number, or numbers such as the attenuation at all standard test frequencies in a coupler, and this could be used to determine an individual’s RECD, just like with hearing aids.  This is easy to do clinically, but hearing protectors are not routinely fit clinically- they are just given out on the factory floor.

However, musicians’ earplugs do follow the professional clinical model of dispensing.  The musician is assessed and fit with custom (or non-custom) hearing protectors in the comfort of the hearing health care professional’s office, replete with real ear measurement assessment tools.  Perhaps, the RECD for hearing protection for musicians should be the standard of care just like the RECD is the standard of care for our hard of hearing clientele?

Another alternative to the hearing protection RECD would be a non-single number rating system.  The EPA did play with the idea of a dual number rating system in from 2005-2010 (ANSI S12.68-2007) but they rejected it after an incredible amount of work had been done. Some very dedicated researchers and experts in the noise control field had come up with the Noise Level Reduction Statistics or NRS.  Specifically the NRSA20 was the noise reduction that could be obtained by the top 20 percentile of wearers who were highly motivated and well-trained to use them.  The NRSG80 was a lower attenuation obtained by the top 80 percentile of wearers who were less than motivated.  Both the absolute values of the NRS values and the different values could be useful.

I am not sure of all of the reasons why the EPA ultimately rejected the NRS as a replacement for the NRR.  There were great advantages over the single number NRR value.  These included estimates of the range of attenuation values that could reasonably be expected for different users; it prevented purchasers from erroneously assuming that a higher number is necessarily better; and it informed the potential wearer about the possibility of over protection where speech communication could be severely restricted.

I don’t want to cry over spilled milk, but there may be other “dual” rating systems out there to be discovered that may be useful for musicians, and ourselves as clinicians to better inform the musicians.  One may be a two number system as follows:

The first number is the maximum attenuation at any frequency.  The second number is the difference between the smallest attenuation and the greatest attenuation at any frequency of the hearing protector.

For example, the ER-15 musicians’ earplug, being a uniform hearing protector, would have the first number of 15 dB (its maximum attenuation), and its second number being 0 dB (maximum difference between the attenuations at different frequencies).  The ER-15 would then be “15;0”.  This would immediately inform the clinician and the musicians that a person can be exposed for 32 times as long as without hearing protection  (15 dB = 32 times as long) and that the hearing protector would be identical for all frequencies so that music would still sound like music.

We could get even fancier and do the 15 dB = 32 times as long, calculation for everyone and the ER-15 musicians’ earplug would then be labelled as 32;0.

Of course there would be individual differences and differences in how an individual person may wear a non-custom version but if our goal is to use this proposed labeling to get us into the ball park, in the same way we chose a hearing aid that can provide a certain amount of gain and output, then this two number labeling scheme may be quite adequate.

Hearing protection may not be routinely assessed with real ear measurement equipment in the factory (although there are approaches that do precisely this) but why should musicians be lumped into the same pot as their industrial colleagues?

Realizing that musicians have differing requirements than industrial workers may be the first step to ensuring that musicians can still play 30 years from now with minimal deterioration of their hearing.

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