Test-retest reliabiligy of real ear measurements for open canal hearing aid fittings.
University of Southampton, Institute of Sound and Vibration Research,
When fitting hearing aids with conventional earmoulds using real ear measurements (REM) it is recommended to use the modified pressure with concurrent equalization method whereby a reference microphone is used to monitor the REM loudspeaker output level. The review of the literature indicates good test-retest reliability for REM using conventional earmoulds. However, in REM using open-fit, another sound field equalization method (the modified pressure with stored equalization method) should be used to minimize any inaccurate measurements resulting from the amplified sound leakage when the open-fit hearing aid is used. In addition the sound is delivered via a generic ear tip rather than a custom made earmould. Therefore, the test-retest reliability for conventional occluded earmould REM cannot be generalized to the open-fit measures.
Twenty otologically normal participants were tested to investigate short-term test-retest reliability for open-fit REM, and for conventional earmoulds at both 0o and 45o head-to-loudspeaker azimuths by performing repeated measurements at the same participant visit but after removing both the probe tube and the hearing aid and reinserting them. It was found that open-fit REM have good short-term test-retest reliability (mean 1.57 dB, SD 1.10 dB) at both azimuths and are in agreement with measured (mean 2.12 dB, SD 1.45 dB) and reported (SD ranged from 1 to 3.2 dB) conventional earmould REM test-retest variability values in the 0.25 to 4 kHz frequency range (Ringdahl A & Leijon A, 1984, The reliability of insertion gain measurements using probe microphones in the ear canal, Scandinavian Audiology, 13, 173-8). This finding is clinically significant as open fittings are widely fitted using REM to hearing impaired patients.
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