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.
[1] Considering that the vibrating ear canal wall acts as an ideal source of volume velocity (also known as volumetric flow rate), 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.
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.
[6][7] Active occlusion algorithms are needed to help people with severe hearing loss adequately.
[8] In the latter case, the open-fitting decreases the ear canal acoustic impedance and thus the occlusion effect.