Abstract
Because cartilage conduction—the transmission of sound via the aural cartilage—has different auditory pathways from well-known air and bone conduction, how the output volume in the external auditory canal is stimulated remains unknown. To develop a simulator approximating the conduction of sound in ear cartilage, the vibrations of the pinna and sound in the external auditory canal were measured using pinna simulators made of silicon rubbers of different hardness (A40, A20, A10, A5, A0) as measured by a durometer. The same procedure, as well as a current calibration method for air conduction devices, was applied to an existing pinna simulator, the Head and Torso Simulator (hardness A5). The levels for vibration acceleration and sound pressure from these pinna simulators show spectral peaks at dominant frequencies (below 1.5 kHz) for the conduction of sound in cartilage. These peaks were likely to move to lower frequencies as hardness decreases. On approaching the hardness of actual aural cartilage (A10 to A20), the simulated levels for vibration acceleration and sound pressure approximated the measurements of human ears. The adjustment of the hardness used in pinna simulators is an important factor in simulating accurately the conduction of sound in cartilage.
Highlights
The Vibration acceleration levels (VALs) obtained from the different pinna simulators were compared with those of a human ear (Figure 4) over the same range of frequencies (
From the hardness measurements obtained using the same durometer, the hardness of the human ear at the tragus corresponded to A20 or A10. These results suggest that vibration simulations of the aural cartilage are important when adjusting the hardness of human aural cartilage
Using five pinna simulators differing in hardness, vibrations at the pinna and sound in the external auditory canal were measured and compared with those of a human ear for the purpose of calibrating cartilage conduction of sound
Summary
If the aural cartilage vibrates, sound can be clearly heard [1,2] This phenomenon is termed cartilage conduction, and hearing aids based on cartilage conduction [2,3,4,5] have been marketed in Japan since November 2017. When a small transducer is fixed at the entrance of the ear canal, it can generate sound via the aural cartilage into the external auditory canal (Figure 1) [6,7,8,9]. That is, this cartilage acts as a diaphragm and the transducer functions as a voice coil of a loudspeaker. The vibrations propagating through skull bone are small enough that their contribution to hearing can be ignored when the cartilage is stimulated [5]
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