The occlusion effect occurs when an object fills the outer portion of a person's

ear canal The ear canal (external acoustic meatus, external auditory meatus, EAM) is a pathway running from the outer ear to the middle ear. The adult human ear canal extends from the pinna to the eardrum and is about in length and in diameter. Struc ...

, causing that person to perceive echo-like "hollow" or "booming" sounds generated from their own voice. The bone-conducted sound travels to the cochlea through different pathways. The outer ear pathway corresponds to the sound pressure generated in the ear canal cavity due to the vibration of the ear canal wall, which constitutes the source of the occlusion effect. At low frequencies, the outer ear pathway is negligible when the ear canal is open but dominates when it is occluded. The occlusion effect is thus objectively characterized by an acoustic pressure increase in the occluded ear canal at low frequencies and which can be measured with a probe-tube microphone. Considering that the vibrating ear canal wall acts as an ideal source of volume velocity (also known as

volumetric flow rate In physics and engineering, in particular fluid dynamics, the volumetric flow rate (also known as volume flow rate, or volume velocity) is the volume of fluid which passes per unit time; usually it is represented by the symbol (sometimes ). I ...

), the occlusion device increases the “opposition” of the ear canal cavity to the volume velocity imposed by its wall and thus increases the amplitude of the acoustic pressure that is generated in reaction, leading to the occlusion effect. The

acoustic impedance Acoustic impedance and specific acoustic impedance are measures of the opposition that a system presents to the acoustic flow resulting from an acoustic pressure applied to the system. The SI unit of acoustic impedance is the pascal-second per cub ...

of the ear canal cavity represents its “opposition” to the volume velocity transfer and governs its reaction in terms of acoustic pressure. In other words, the occlusion effect is mainly due to the increase of the acoustic impedance of the ear canal cavity when it is occluded. 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 A hearing aid is a device designed to improve hearing by making sound audible to a person with hearing loss. Hearing aids are classified as medical devices in most countries, and regulated by the respective regulations. Small audio amplifiers su ...

users who only have a mild to moderate high-frequency

hearing loss Hearing loss is a partial or total inability to Hearing, hear. Hearing loss may be present at birth or acquired at any time afterwards. Hearing loss may occur in one or both ears. In children, hearing problems can affect the ability to Language ...

, but use hearing aids which block the entire

. The occlusion effect is also deemed to be a notable source of discomfort to workers wearing shallowly inserted passive occlusion devices such as

earplug An earplug is a device that is inserted in the ear canal to protect the user's ears from loud noises, intrusion of water, foreign bodies, dust or excessive wind. Since they reduce the sound volume, earplugs are often used to help prevent hear ...

s. Active occlusion algorithms are needed to help people with severe hearing loss adequately. If a person suffers from "near-normal low-frequency hearing and mild to moderate hearing loss of up to 70 dB at mid and high frequencies," hearing aids with increased vent size or hollow ear-molds/domes are more suitable for them in lessening the extent of the occlusion effect. In the latter case, the open-fitting decreases the ear canal acoustic impedance and thus the occlusion effect. For earplug users, an incomplete seal has a similar effect at frequencies lower than the Helmholtz resonance formed by the system (the neck of the resonator corresponding to the incomplete seal at the earplug/ear canal wall interface and the resonator cavity being the partially occluded ear canal). In the general case, the deep-fitting reduces the occlusion effect because the volume velocity imposed by the ear canal wall to the occluded ear canal cavity decreases since the surface as well as the vibration amplitude of the remaining ear canal wall diminish with the insertion depth.

Notes and references

Otology {{biology-stub

See also

Notes and references