The level of bone-conducted sound in the auditory meatus is increased at low frequencies by occlusion of the meatus, for example, by the earmold of a hearing aid (HA). Physical measures of this “occlusion effect” (OE) require vibration of the skull. In previous research, either self-voicing or audiometric bone conduction vibrators were used to produce this vibration, with the result that the OE could not be measured for frequencies below 125 Hz. However, frequencies below this can be important for music perception by HA users. The objective was to develop and evaluate a method that gives a lower-bound estimate of the OE for frequencies below 125 Hz.
A low-noise amplifier with extended low-frequency response was used to record the output of a miniature microphone inserted into the meatus of participants. The signal came from sounds of the heartbeat and blood flow of the participant, transmitted via bone conduction through the walls of the meatus. A simultaneous recording was made of the carotid pulse to permit time-locked averaging (and hence noise reduction) of the microphone signal. Recordings were made from 7 otologically and audiometrically normal participants, using clinical probe tips to produce the occlusion. Recordings were also made from an overlapping group of 9 participants, using fast-setting impression material to provide a more consistent degree of occlusion. The difference in level of the recorded signal for unoccluded and occluded conditions provided a lower bound for the magnitude of the OE.
The mean OE increased with decreasing frequency, reaching a plateau of about 40 dB for frequencies below 40 Hz. For some individual recordings, the OE reached 50 dB for frequencies below 20 Hz. With occlusion, the heartbeat became audible for most participants.
The OE can be very large at low frequencies. The use of HAs with closed fittings, which may be used either to prevent acoustic feedback or to allow amplification of low frequencies, may lead to an unacceptable OE. The authors suggest reducing the OE with the use of a seal deep in the meatus, where the wall of the meatus is more rigid.