A model of the occlusion effect with bone-conducted stimulation

Abstract

An acoustical model using simplified ear anatomy was designed to predict the ear-canal sound pressure occlusion effect in humans. These predictions were compared perceptually as well as with ear-canal sound pressure occlusion effect measurements using a foam earplug with shallow insertion, a foam earplug with deep insertion into the bony part of the ear canal, and a circumaural earmuff. There was good resemblance between model predictions and ear-canal sound pressure measurements. It was also found that all occlusion positions, even deep ear-canal occlusion, produced noticeable occlusion effects. With the bone-conduction transducer at the forehead, the perceived occlusion effect was close to that obtained from ear-canal sound pressure data in the 0.3 to 2 kHz frequency range; when the stimulation was at the mastoid the difference between the perceived and measured ear-canal sound pressure occlusion effect was around 10 dB at frequencies below 1 kHz. Further, the occlusion effect was obtained in two clinical settings: with supra-aural earphones (TDH39), and insert earphones (CIR22). Although both transducers produced occlusion effects, insert earphones produced a greater effect than surpa-aural earphones at the low frequencies.

Estimation of bone conduction skull transmission by hearing thresholds and ear-canal sound pressure

“…This difference was believed to originate in the same difference as discussed above where stimulation close to the cochlea produced a cochlear response direction that was more effective than when the stimulation was further away. Trends of less perceived occlusion effect when the stimulation is at the mastoid compared with other positions have been indicated previously (Dean et al, 2000;Klodd et al, 1977;Stenfelt and Reinfeldt, 2007). Stenfelt and Goode (2005a) divided the BC transmission of the head into three frequency ranges: (1) below approximately 0.3 kHz the skull moves as a true rigid body, (2) between 0.3 and 1 kHz the skull motion can be approximated by a mass-spring system, and (3) above 1 kHz the skull moves in all directions determined by the wave transmission in the skull bone.…”

“…This similarity originates in the effect of occlusion. When occluded, the ECSP during BC stimulation was increased at frequencies below 2 kHz and dominated the perceptual BC response (Stenfelt et al, 2003a;Stenfelt and Reinfeldt, 2007). Consequently, the similarity at these frequencies was not a result of the ECSP reflecting the BC perceptual response but the opposite; the BC perceptual response was caused by the ECSP.…”

“…It has often been reported that the perceived occlusion effect was about 10 dB lower than the corresponding change of ECSP (Berger et al, 1983;Huizing, 1960;Stenfelt and Reinfeldt, 2007). These comparisons between perceived occlusion effect and ECSP changes relate to frequencies below 2 kHz where the occlusion effect was present.…”

“…At the higher frequencies, above 2 kHz, there was a general trend of lower ECSP occlusion effect values when compared with the threshold based occlusion effect for all three stimulation positions (Table 3). This was due to the reduction of ECSP caused by the occlusion at higher frequencies, while perception of BC sound was not influenced by occlusion (and therefore insensitive to the decrease in ECSP) due to the contribution of other BC pathways to stimulus perception (Stenfelt and Goode, 2005b;Stenfelt and Reinfeldt, 2007).…”

“…Such contributors can be the sound in the ear-canal during BC stimulation (Bárány, 1938;Stenfelt et al, 2003a) or the inertial effects of the middle ear ossicles (Homma et al, 2009;Huizing, 1960;Stenfelt et al, 2002); although possible, it is not clear that they are related to the cochlear vibration. The middle ear ossicles may contribute to the BC perception at mid-frequencies (1.5-3 kHz) (Stenfelt, 2006), while the outer ear only is believed to be a main contributor at low frequencies (below 2 kHz) when the ear-canal is occluded (Stenfelt et al, 2003a;Stenfelt et al, 2007). Another contributor that seems unrelated to the cochlear vibration is the proposed dynamic pressure transmission from the skull interior (e.g.…”