The occlusion effect occurs when an object fills the outer portion of a person's ear canal, and that person perceives "hollow" or "booming" echo-like sounds of their own voice. It is caused by bone-conducted sound vibrations reverberating off the object filling the ear canal. When talking or chewing, these vibrations normally escape through an open ear canal; most people are unaware of their existence. When the ear canal is blocked, the vibrations are reflected back toward the eardrum. Compared to a completely open ear canal, the occlusion effect can boost low frequency (usually below 500 Hz) sound pressure in the ear canal by 20 dB or more. This effect can be measured with a probe-tube microphone.
A person with normal hearing can experience this by sticking their fingers into their ears and talking. Otherwise, this effect is often experienced by hearing aid users who only have a mild to moderate high-frequency hearing loss, but use hearing aids which block the entire ear canal.
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