A probe earmold system for measuring eardrum SPL under hearing-aid conditions.

A miniature probe-microphone assembly for use with earmolds was developed to measure the sound pressure level (SPL) of a signal at the eardrum under hearing-aid conditions. The probe-earmold (PE) system was designed with the earmold as an integral part of the acoustic measurement system so that no part of the probe itself projects into the ear canal. Data obtained during a previous investigation with an ear simulator indicated that the PE system could be used to measure eardrum SPL accurately (+/- 3 dB) up to 6.0 kHz. In the current investigation, a behavioral experiment was conducted to validate the accuracy of the PE system on human ears. Comparisons were made between pure-tone thresholds obtained with a conventional earphone and those measured by the PE system using a hearing-aid receiver into an earmold-occluded ear. The basis for the study was the premise that the estimates of the eardrum SPL obtained from pure-tone thresholds are equivalent when the thresholds are determined by sound generated by an earphone into an unobstructed ear or by a hearing aid receiver into an earmold-occluded ear. Results of the study showed no statistically significant differences between the thresholds as determined by the two procedures confirming the simulator data. These results on human ears, together with the physical measurements in a simulator, suggest that the PE system can be used to measure eardrum SPL up to 6.0 kHz in an occluded ear.