The vowel which has no significant energy below 500 Hz is only slightly enhanced but its occluded loudness pales in comparison to the vowel. That is, the low frequency energy of the vowel is enhanced by the bone conducted transmission of vibration to the occluded ear canal. However when one occludes their ears, the loudness of the skyrockets in comparison. Normally, when we articulate the two vowels and they are roughly equally loud. In contrast, all high vowels such as have low frequency first formants which has significant energy at 125 Hz. For those that like linguistics (and even for those who do not like linguistics), the vowel has no significant energy below 500 Hz (its first formant). These are the vowels in ‘f ather’ (a] and ‘s eat’. This is certainly nothing new and there have been clinical tests for this phenomenon for years. 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 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. 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. That is, the boney portion of the ear canal functions as a loudspeaker cone and generates this low frequency sound energy. Once the outer ear is occluded, this low frequency energy is trapped and heard as an echoey sound. With an unoccluded ear canal, this low frequency energy simply is lost to the environment and we are not aware of it. The medial part of the ear canal, being bone, tends to be a great loudspeaker but only for the lower frequencies. 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. This has to do with the issue of stability as well as minimizing the occlusion effect. When fitting musician earplugs, the clinical rule of thumb is the longer the better.

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