The ER-15 earplug was the first to have uniform attenuation and it has been commercially available since 1988. Its sister earplug, the ER-25 which is more appropriate for drummers and other percussionists reached the marketplace in 1992. Since that time, the ER-15 earplugs have become the mainstay of hearing loss prevention for those in the performing arts. More recently other earplugs have come on to the market with similar attenuation characteristics such as the Dynamic Ear (DM white filter).
In both cases there is an interplay between the attenuator button and the properties of the earmold that is used. Specifically, a broadly tuned resonance is created between the compliance inherent in the attenuator button and the mass of air in the sound bore of the custom earmold. This is typically around 2700 Hz in order to offset the natural insertion loss (attenuation) when an occluding earplug is placed in the ear. The net effect is a flat or uniform earplug that treats bass, mid-range, and treble notes identically.
From time to time, modifications need to be made to these earplugs. What if the individual’s ear canal resonance (REUR) is not at 2700 Hz. What types of modifications can be done to bring the resonance in line with your client’s outer ear requirements? We can run to the audiologist’s favourite formula:
Fres α (Length/cross sectional area)
This means that the resonant frequency of a resonant system is proportional to the length of the bore divided by its cross sectional area. A long, narrow bore will have a higher resonant frequency and a short, wide bore will have a lower one. The way I remember it is to remember the old 1930s comedians Laurel and Hardy. The tall thin one is higher pitched than the short fat one. If your client is a large truck driver with a 32 mm long ear canal (vs. 28 mm), the odds are that his REUR will be around 2500 Hz (rather than 2700 Hz). The ER-15 needs to be modified by having a shorter and wider bore than the earmold laboratory would normally use for a “typical” ER-15 earplug. The reverse would be true for a “small ear-ed” person. This of course can, and should be verified using real ear measurement. It should be pointed out that this modification may ensure that the REUR is offset appropriately, but there will be a high frequency alteration in the attenuation that occurs above the resonant frequency.
Before going onto the second most common modification of the ER-15 it may be worthwhile reviewing how one does this with your real ear measurement system. After calibrating in the normal fashion, set the stimulus level to 70-75 dB SPL. It needs to be this high to ensure that the stimulus level – attenuation is above the noise floor of the real ear measurement system. Once done, disable the reference microphone and the loudspeaker. With the Audioscan set the stimulus level to 0 dB and with the Frye system, set the stimulus level to “off”. You now have an in situ sound level meter that can be used to measure attenuation. There is an application note on this very issue that I wrote in 1998. It can be found at http://www.audioscan.com/resources/appnotes/AppNote_98-05.pdf.
Another modification of the ER-15 earplug is the placement of a 1.4 mm vent parallel to the main sound bore. A vent of this size has two effects. The first is a complete resolution of the occlusion effect (as long as the vent bore is sufficiently long- at, or past the second bend in the earcanal) and the second is to minimize the sound level of instrument-related sounds such as lip vibration with brass instruments. And indeed this modification is routinely used with brass players (and some vocalists).
Well, that is all that I know about this. If you have anything more to add, please feel free to comment.