If you have been reading earlier parts of this blog series, I had only promised to write a 2-parter. Well, here is part 3!
The reason for this rogue insertion of yet another installment to this series is the number of emails that I have received about how hearing protection can actually be assessed using real ear measurement – hence, part 3.
I have written about this in earlier blogs but it’ best to have all of this in one place. Using real ear measurement to assess the extent and frequency dependence of hearing protection is quite similar to using it for assessing gain and output with hearing aid amplification.
Courtesy of www.Audioscan.com
There are however two (potential) differences.
One is that most modern hearing aids are “level dependent” where different amounts of gain are generated depending on the level of the input – hearing aids generate more gain for soft sounds and less for louder sounds. There are level dependent hearing protection devices and these should be assessed using the identical protocols for level dependent hearing aids, but these are rather rare in the realm of industrial or music exposure. The vast majority of hearing protection devices are linear and not level dependent.
The other difference is that with hearing amplification, the output is at a relatively high level; certainly higher than the internal noise floor of the real ear measurement device. This is not the case with many forms of hearing protection devices such as Musicians’ Earplugs.
For example, if a 50 dB SPL stimulus level was to be used and the hearing protection device provided 30 dB of attenuation, then the output would be on the order of 20 dB SPL. The vast majority of real ear measurement systems have a noise floor that is greater than 20 dB SPL.
One can easily measure the noise floor (or low sound level limit) of their own real ear measurement system by doing the following: Calibrate the real ear measurement system in the normal fashion and then disable the loudspeaker and reference microphone. Different manufacturers have differing terminologies for this – with Audioscan setting the stimulus level to 0 dB, and Frye setting the stimulus level to “off”. In both cases, the loudspeaker and the reference microphone are both disabled.
Courtesy of www.iso.org
Plug up the end of the probe tube either by pinching it between your fingers or using a pair of plyers. Perform a “run” of the real ear measurement system as you would for a hearing aid. The result is a frequency by frequency measurement of the internal noise floor of your own real ear measurement system. One needs to be at least 10 dB above the noise floor in order to be able to trust your results.
All this means is that instead of using a 50 dB SPL stimulus level, one needs to use a higher level one. I use 70 dB SPL as my stimulus level for assessing the real ear attenuation of hearing protection devices. Of course one needs to be circumspect about the low frequency results due to potential slit leaks between the hearing protection device and the ear canal wall caused by the presence of the probe tube. Using Vaseline or other “gooey” substance around the outer edges of the probe tube will resolve this potential issue.
Using a 70 dB SPL stimulus level will ensure that the output measured by the real ear measurement system will be far above the noise floor.
Up to 80dB SPL can be used for best dynamic range without without harm to the unplugged auditory system.
On the other hand, I maybe wrong. I did not consider the open ear resonance between @000 and 3000 Hz.