There was a recent presentation given at the National Hearing Conservation Association (NHCA) conference last month questioning the integrity of the ER-15 uniform attenuator for musicians. The ER-15 custom made earplug has been available since 1988 and provides a much more uniform attenuation (lessening of sound energy) across the frequency region than other types of hearing protectors that have been designed with industry in mind.
My short response to this presentation is that the data was either measured incorrectly or just outright wrong. I have been fitting the ER-15 earplug (and its cousins, the ER-25 and ER-9) for almost 25 years and the actual musician data and subjective responses are amazing. If a musician loses their pair of ER-15 earplugs, this is an “emergency situation” and I even get calls at home to have them replaced as soon as possible. Subjectively, placing one’s finger over the small diaphragm opening will demonstrate the benefit of the ER-15 earplug. This clinical trick temporarily removes the benefit of the ER-15 and simulates a conventional foam earplug that may be used in industry. The audiologist just needs to say the sound /sssss/ which has high frequency energy starting at 4000 Hz + while the audiologist removes their finger from the diaphragm; immediately the musician smiles and says the right side of the piano keyboard just came back.
But let me now give a longer answer with some data. And I must apologize to my secretary who pulled 50 random musician files (and who now needs to refile them). Fifty files were examined and the real ear measurement data of the actual attenuation of the ER-15 were recorded.
Like hearing aid fittings, verification is the cornerstone of a good fitting. Hearing aid attenuation values with the ER-15 (or any earplug) is just as important, and it needs to be done correctly.
So, let’s start with the correct way to do this. Calibrate the real ear measurement system in the normal fashion and then set the stimulus level to 70 dB SPL. Perform a real ear measurement of the unoccluded ear canal (known as the Real Ear Unaided Response or REUR) and then without removing the probe tube microphone, perform a second real ear measurement of the “aided” response with the ER-15 in place. The difference is the real ear attenuation of the ER-15 for that individual.
Even though modern real ear measurement systems call this an “aided” response, when we are dealing with hearing protectors, we are referring to “negative gain” or attenuation. It is as if the hearing protector was a hearing aid working in reverse- to attenuate sound.
The stimulus level needs to be at least 70 dB SPL because unlike hearing aids the real ear “aided” response with the ER-15 will be a lower number- that is, the ER-15 lessens or attenuates the sound. If I recall my grade 3 math, 70 dB – 15 dB = 55 dB. Values around 45 dB and higher are easily measured by real ear measurement systems, whereas quieter sounds are not. This is because all real ear measurement systems have internal noise and this noise floor can result in a testing artifact. If a level of 50 or 55 dB SPL, which is normal for hearing aid work, was used as the stimulus, minimal attenuation would actually be measured in the higher frequency region, since erroneously we would really only be measuring the internal noise of the real ear measurement machine and not the hearing protector.
In order to improve reliability, all stimuli should be presented from 45 degrees off to the side, and 45 degree above the musician’s head. There is nothing magic about this loudspeaker location- it simply has been studied over the last 30 year and found to be the best location, for a number of reasons.
Like any assessment tool, its strengths and weaknesses have to be known.
Average attenuations for 50 pairs of ER-15 hearing protectors showing the maximum and minimum values
I have included a graph of the 50 ER-15 earplugs- red data is for the right ear and blue, for the left. Standard deviations are all less than 2 dB but in this graph the maximum and minimum data points are shown for all of the test frequencies. Indeed, the ER-15 is a flat or uniform hearing protector. Some musicians have a slight roll-off in the higher frequency regions and others do not. But like any hearing protector, these data are well within the published specifications for the ER-15.
One note is worthy of attention. Whenever there is a probe tube inserted in an ear canal with a hearing protector (or a hearing aid) in place, there may be a small “slit leak” formed between the ear canal wall and the device being assessed. As can be seen in the figure, for some musicians, this has reduced the measured attenuation in the lower frequency region (250 Hz and 500 Hz). This is an artifact of my testing and is a well-known artifact of real ear measurement with any device. In many cases (but not all) I had used a bit of Vasoline around the edges of the probe tube to minimize this. Clearly I wasn’t always successful, but for the most part, this worked well.
In short…. or is it too late,… ER-15 musician hearing protectors are as advertised and they do indeed provide a uniform attenuation for the musician. These data are not routinely published in the literature for the same reason as real ear measurement data is not routinely published. This is part and parcel of a routine clinical audiology assessment and is part of the verification process which any hearing health care professional uses regardless of whether it is for positive gain (hearing aids) or negative gain (hearing protectors).
Marshall,
Loved the last para beginning with,…In short…or is it too late…!!
Very funny. Liked your clinical assessemnt with the probe tube. Good objective measures are a key and must when truly gathering data. Nice work.
Let me a subjective overview, I had a pair of the custom molds made about 9 months ago and “love them”. Mine, and several of our staff had them made by Westone. They had an exceptional fit and to listen to a live concert, the sound was aweesome. The comfort level mad a huge difference. I highly recommend them to all those that want to not only protect their hearing but to enjoy the concert to a whole new (lower) level .
Nice post Marshall! Like your humor. 🙂
Indeed the ER-9/15/25 works as advertised; and also the $11.95 ER-20 “Ety•Plug” does a stellar job for the money, especially for high school marching bands, and for concert-goers.
However, there’s a serious issue not often discussed when discussing hearing protection: Below about 150 Hz any earplug doesn’t work, due to the acoustic path being short-circuited by bone conduction.
The way to test for it is to use behavioral thresholds, not probe mics: You’ll need a big speaker, perhaps a subwoofer, for testing below 80 Hz. Speaker placement is not critical at those long wavelengths; however you’ll need a good sound meter to actually measure the SPL, as room resonant modes come into play; and typical sound booths are only good down to maybe 200 Hz.
In situ low frequency attenuation is a problem with *all* hearing “protection;” and perversely, the higher the NRR figure, the more damage can occur: The wearer (and the industrial hygienist or audiologist) is lulled into complacency with a false sense of security. An analogous situation is putting on sunblock with an SPF of 20, 30 or even 50; but instead of actual sunblock all that’s in the bottle is plain hand lotion: The higher the SPF on the label, the more sunburn will occur.
Dan Schwartz
Editor, The Hearing Blog
PS: I learned about this earplug issue the hard way in 1980, when I was 19, while designing, lab testing & on-site fine tuning a massive subwoofer system upgrade at the sister club to Studio 54 in NJ: At full power the system could hit 146 dB SPL (re 20 µBar) with a -3dB point of 21.5 Hz; and in fact was later throttled back because it was knocking drinks off the bar 80 feet off the dance floor. And Yes, despite my wearing NRR 31 hearing protection, my own hearing was stolen from me. #FML
But, I was fortunate: The original -3dB point I selected was 18 Hz, not 21.5 Hz: I opted for a 6th order Chebyshev instead of 8th order Butterworth system response. I’ll leave it as a reader exercise as to why I was actually lucky.
PPS: Now, before you musicians start yelling that the lowest note on a kick drum or bass guitar is 40 Hz, in fact I used a dbx 120 Subharmonic Synthesizer (still made to this day! https://www.dbxpro.com/en-US/product_families/subharmonic-synthesis ), which filtered sounds below 85 Hz, divided them in half, and mixed them back in; so those ~40 Hz kick drum thumps turned into bone-rattling pulses.
PPPS: And, if the dbx 120 subharmonic synthesizer from 1980 looks strangely familiar, you’re right: Go to SpectralIQ dot com and see for yourself! ~DLS