In addition, in studies designed to elucidate the mechanisms of STC hearing
[7][20][21][22], the external auditory canal of the participants was usually occluded with an ear plug in order to exclude the possibility that the participant would respond to the AC sounds accompanying the STC stimulus delivered by a bone vibrator, and in order to reduce external masking sounds. However, in the presence of the occluding ear plugs, the occlusion effect (OE) would likely be elicited. It has been shown that the OE results from vibrations of the walls of the external auditory canal
[23][24] which are induced by the vibrations of the soft tissues initiated by the external bone vibrator, or by the intrinsic body sounds resulting, for example, from the heartbeat. In fact, when the clinician uses a stethoscope to detect these intrinsic body sounds (e.g., heartbeat, pulmonary air flow) in their physical examination of a patient, they are making use of the intrinsic vibrations, and this serves as a clear and obvious confirmation of the existence of soft tissue vibrations and soft tissue conduction
[7]. Since soft tissue (skin) provides the immediate lining of the cavity of the canal including both the cartilaginous and the bony parts of the canal, it is likely that the OE is the result of the vibrations of the more compliant soft tissue-cartilaginous walls of the canal
[7]. These vibrations produce air pressures in the occluded cavity which drive the tympanic membrane and the middle ear ossicles, and excite the inner ear by a mechanism similar to that in response to AC stimulation
[7]. Furthermore, the hearing of self-vocalizations
[5][8] and of one’s own heartbeat and blood flow
[4] when the external canal is occluded is also a result of the OE, in which the vibrations of the vocal cords during vocalization or the vibrations of the heart and blood flow are conducted by the soft tissues to the walls of the external auditory canal, leading to its vibration. It has also been shown that the OE elicited in response to the low frequency vibrations induced by the heartbeat and resulting blood flow reaches a magnitude of 40 to 50 dB
[4], i.e., the sound pressure in the occluded external canal is 100 times greater than that in the open canal; and this would require relatively large excursions of an extensive area of the canal wall, probably the more compliant soft tissue-cartilaginous wall. The air pressures induced in the occluded canal drive the tympanic membrane and the middle ear ossicles, in a pathway similar to that in AC hearing
[7] (see
Figure 1). In addition, in the studies conducted on BAHA participants
[11][25], the external auditory canal in the tested ear was occluded with an ear plug. Therefore, the OE was likely elicited, enhancing the sensitivity (reducing the threshold) of the BAHA participant. Thus, the STC vibrations induced by the bone vibrator at the neck STC site were conducted by means of STC to the external canal walls, causing their vibration. In the presence of the occluding ear plug, the sound pressure in the occluded ear canal would be elevated, driving the tympanic membrane, middle ear ossicles, exciting the inner ear by a mechanism based on a sequence of events similar to an AC pathway, i.e., leading to a traveling wave. Thus, while the OE is considered one of the component mechanisms of BC
[1], the OE is likely the result of the vibrations of the soft tissue-cartilaginous part of the canal wall
[7], and not of the bony part. This may also be the mechanism leading to hearing in response to the delivery of vibratory stimuli to fluid applied to the external canal (which is also a form of STC), which was effective mainly to the lower frequencies
[26].