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This phenomena is well-known to audiologists and hearing-air providers.
An occlusion effect occurs when some object (like an unvented earmold) completely fills the outer portion of the ear canal. What this does is trap the bone-conducted sound vibrations of a person's own voice in the space between the tip of the earmold and the eardrum. Ordinarily, when people talk (or chew) these vibrations escape through an open ear canal and the person is unaware of their existence. But when the ear canal is blocked by an earmold, the vibrations are reflected back toward the eardrum and increases the loudness perception of their own voice. Compared to a completely open ear canal, the occlusion effect may boost the low frequency (usually below 500 Hz) sound pressure in the ear canal by 20 dB or more. This is a real and measurable increase in sound. One way that it can be determined is with a probe-tube microphone (a device that should be used, in my judgment, in just about every hearing aid fitting). A probe-tube microphone consists of a very fine, flexible tube that is connected to various types of sound measurement equipment. The tube is inserted in the ear canal and the audiologist measures the sound levels in the ear canal while the hearing aid user utters some standard vowel (like "ee"). Then, with the tube still in place, an earmold (or hearing aid shell) is inserted in the ear canal, making sure that the tube extends a few millimeters past the earmold tip. Then, with the hearing aid turned off, the audiologist again measures the sound levels when the hearing aid user says the same vowel (at the same loudness level). The difference between the sound levels occurring when the ear is open and when it is closed with a hearing aid is a measure of the amount of the occlusion effect. As I noted above, differences of 20 dB or more are common.
. It's also the way "balanced" audio electrical equipment works to miminise noise in lines and mic leads.