Musicians’ earplugs, also known as the ER-15, have been available since 1988. These were based on a patent by Elmer Carlson of Knowles Electronics in the 1970s. When Mead Killion left Knowles to start his own company, Etymotic Research , he licensed from Knowles, and later developed this product. To date, over one and a half million pairs have been sold (as well as over 3 million pairs of the non-custom ETY earplugs). Since then, other players have entered the marketplace, such as the technology from Dynamic Hearing (and marketed in the United States by Westone). Regardless of the manufacturer or type of hearing protector and regardless of whether they are made for the performing artist or the industrial worker, with these earplugs, as with hearing aids, the attenuation should be measured to verify that they are working as advertised.
Historically there have been several approaches to attenuation. One is the Real-Ear Attenuation Threshold (or REAT) in which audiometric thresholds are obtained with and without hearing protection in place- the difference being the frequency-specific attenuations. Another approach, which is more typically used in the clinic, is a difference between a real-ear measure using a probe microphone with and without the hearing protection in place. Both are identical and should, within the limitations of the variability of the data, yield the same result.
In the case of passive hearing protectors, it doesn’t matter whether they are assessed with a low sound pressure level (e.g., near threshold) or supra-threshold at a level of 70 or 80 dB SPL. It is the difference that counts, not the absolute levels. That is, the attenuation with input stimulus of 80 dB SPL should yield an identical attenuation with an input stimulus of 60 dB SPL.
There are, however, two limitations, and both are related to variability. One is the technique of the assessment (Real-Ear Attenuation Threshold (REAT) or probe-microphone measurement), and the second is whether the “technique” was performed correctly.
Let’s first examine the variability associated with two threshold measures, such as those used with a REAT measurement. If a 5-dB measurement step is used, such as with conventional audiometry, then the expected variability is the square root of 52– namely 5 dB- for the unprotected threshold measurement and then another second measurement with the hearing protector in place- another 5 dB. Taken together, the total variability is on the order of the square root of 52 + 52, or about 7 dB. We can improve on this by using 2-dB steps instead of 5-dB steps for both portions of the attenuation measurement, and one can show that the variability would drop to the square root of 22 + 22 or 2.8 dB. Threshold measures presuppose that the testing environment is quiet enough to measure these thresholds reliably, and this is probably the case when an audiometric test booth is used.
However, when a pair of probe-tube microphone measurement is used (with and without the hearing protection in place), as long as the measurements are performed correctly, standard deviations on the order of 1 dB or less (at least up to 4000 Hz) are quite feasible. Let’s take a step back to review the proper usage of real-ear probe tube measurement testing.
Typically for the two portions of the measurement – with and without the hearing protection- the presentation level needs to be on the order of 70 dB SPL or greater. This has nothing to do with being “more realistic” because music is at a higher level than speech. This merely has to do with the internal noise floor of the real-ear measurement system being used. One needs to ensure that any measurement that is taken be significantly above the noise floor, or else one doesn’t really know whether the result is a measurement of the noise or the hearing protector and stimulus.
In the case of hearing protection with a significant amount of attenuation (e.g., 35 dB), then these measurements will be made at a level equal to the stimulus level – 35 dB. If the stimulus level of the real-ear measurement system was only 50 dB, then measurements would be taken at 50 dB SPL – 35 dB = 15 dB SPL. It is quite probable that 15 dB SPL is quieter than the noise floor of the real-ear probe tube measurement system. If, however, one uses a constant stimulus level of 70 dB SPL (with the same hearing protector), the measurements would be made at 70 dB SPL – 35 dB = 35 dB SPL. This most certainly would be above the noise floor of any commercially available real-ear measurement system.
To assess the noise floor on your own real-ear probe-microphone system, calibrate in the normal fashion, and then perform a run with the probe tube blocked (either held tightly between your fingers or by using a pair of pliers). The result is a frequency by frequency measure of the noise floor of your own device; as long as the same probe tube length is used, this should never change.
This figure shows the real-ear measurement of 50 pairs of randomly selected ER-15 musicians’ earplugs. I made all these measurements on musicians at the Musicians Clinics of Canada. Not only is the attenuation quite flat (as advertised), but the standard deviation (shown near the top by a series of small x and o markings) is quite low. The standard deviation calculation is based on n = 50. This would be slightly larger if the sample size were significantly smaller.
The attenuation of the hearing protectors should be identical (within the technique’s variance), regardless of whether they were assessed using a probe microphone, a threshold based technique such as the REAT, or even in a hearing aid test box and coupler.
Of importance is that these data (and standard deviations) would be identical regardless of whether the hearing protectors were measured at 70 dB SPL or 100 dB SPL. As long as the hearing protectors are passive (and do not provide different levels of protection depending on input such as electronic compressor type hearing protectors), one should always obtain identical data.
In parts 2 and 3 of this series of posts, we will address internal noise levels of probe-tube microphones, and the angle and azimuth of the loudspeakers that all real-ear probe tube microphones should use. This is not something new and the salient article was published in Ear and Hearing in 1987, more than a quarter of a century ago.