Effects of Intermittent Noise on Real Ear Measurements in Hearing Aid Fitting

When fitting a hearing aid, real ear measurements (REM) can prove a valuable tool for adjusting the gain of the hearing aid. In REM a probe microphone is inserted into the ear canal carefully placing the probe microphone tip a few millimeters from the tympanic membrane. Typically a reference microphone is present outside the ear canal near the pinna. Proper calibration before the REM measurement ensures a flat response of the probe microphone when placing the tip at the same positions as the reference microphone.
The real ear unaided gain (REUG) is measured as the level difference between the reference microphone and the probe microphone placed near the tympanic membrane. Inserting the hearing aid the real ear aided gain (REAG) can be measured similarly and the real ear insertion gain (REIG) is calculated as the difference between the REAG and the REUG. Fitting a hearing aid typically involves matching the hearing aid gain to match a REIG prescription target derived by the patients audiogram and occasionally other diagnostic data. To ensure matching the REIG to the prescription target care must be taken to ensure accuracy in the REM measurements. Inspecting a database of thousands of clinically recorded REM a handful of anomalies were detected and the present study seek to reproduce the observed effects under controlled laboratory conditions.
The study involves measuring REM using manikins as well as human subjects. A semi-automated test setup was developed to synchronise the measurement equipment and the noise disturbance in order to investigate the effects of different types of intermittent noise on REM. Preliminary results indicate that intermittent noise burst even shorter than 1s in duration can cause detrimental effects on the real ear measurement.