When fitting musician earplugs, the clinical rule of thumb is the longer the better. This has to do with the issue of stability as well as minimizing the occlusion effect.
Simply stated, the occlusion effect is caused by the wearer’s own voice (or vibration energy from their instrument that is transduced through the mandible and condyle to the boney portion of the outer ear canal. The medial part of the ear canal, being bone, tends to be a great loudspeaker but only for the lower frequencies. With an unoccluded ear canal, this low frequency energy simply is lost to the environment and we are not aware of it. Once the outer ear is occluded, this low frequency energy is trapped and heard as an echoey sound. That is, the boney portion of the ear canal functions as a loudspeaker cone and generates this low frequency sound energy. Having a long obstructing ear mold (either for a hearing protector or a hearing aid) essentially sits on this “loudspeaker cone” preventing it from building up a sound vibration in the first place. This functions in the same way as what would happen if you pressed your hand into a normal loudspeaker cone- sound would stop, or at least be significantly lessened.
The use of a long bore into the ear canal works in the same way- low frequency energy just cannot be generated in the occluded ear canal. This is certainly nothing new and there have been clinical tests for this phenomenon for years. Personally I used the [a]/[i] test. These are the vowels in ‘father’ (a] and ‘seat’ [i]. For those that like linguistics (and even for those who do not like linguistics), the vowel [a] has no significant energy below 500 Hz (its first formant). In contrast, all high vowels such as [i] have low frequency first formants which has significant energy at 125 Hz. Normally, when we articulate the two vowels [a] and [i] they are roughly equally loud. However when one occludes their ears, the loudness of the [i] skyrockets in comparison. That is, the low frequency energy of the vowel [i] is enhanced by the bone conducted transmission of vibration to the occluded ear canal. The vowel [a] which has no significant energy below 500 Hz is only slightly enhanced but its occluded loudness pales in comparison to the vowel [i].
You can also verify the occlusion effect using a probe microphone device- simply have the client say the vowel [i] and measure their spectrum and do the same for [a]. Then plug up the ear and do the same and you will be able to see the low frequency enhancement due to the occlusion effect if there is low frequency to be enhanced- that’s why you will see the effect more with [i] than with [a], but we are getting away from the message of this blog, so ignore the above!
Clinically if a hearing aid client, when asked which is louder [a] or [i] says [i], then our job is not done. The hearing aids are occluding this person’s ears and changes need to be made. The [a]/[i] trick is also useful for hearing protectors such as musician earplugs. If the musician says that the [i] is louder then expect to see them again after they play their instrument.
The modifications of the musicians’ earplugs would be the same for a hearing aid- either make the canal bore longer or reduce the occlusion. In many cases the length of the canal bore cannot be made longer for logistical reasons or simply because of comfort reasons, and that’s where a vent comes in.
For short bored ear canal devices one may require a 3 or 4 mm vent to completely minimize the occlusion effect, however, for long bored devices (that still occlude) only a 1.4 mm vent is necessary.
Musician earplugs should be made with as long of a bore as is possible but sometimes even this is not perfect and some venting is required. The last thing that anyone wants to do with hearing protector is to have a large vent- this will obviate the entire reason for the hearing protector in the first place, but a 1.4 mm vent seems to be a great compromise between too much occlusion effect and too much loss of low frequency attenuation.
I would rather have a client or a musician wearing something that isn’t 100% efficient acoustically but is wearable than the converse.
It turns out when it comes to music that the occlusion effect is not a large issue with string players or percussionists. And this is quite reasonable. For violins, violas, cellos, guitars and bass the sound is not transmitted to the ear by way of bone conduction- merely air conduction from the outside.
However when it comes to woodwind instruments with mouthpieces being in contact with the teeth (e.g. clarinet or saxophone), significant low frequency vibrational energy can be transduced to the boney portion of the outer ear. This is also true of the generated vibration from the lips for trumpet and other brass players. It is the brass and mouthpiece bearing woodwinds that require either a very long sound bore into the ear canal or a 1.4 mm vent.
My 1.4 mm drill bit is every bit as valuable as my probe tube microphone system.