Otoacoustic emission retrograde standing waves caused by eartip foams

Travelling the retrograde path from the inner ear towards the entrance of the ear canal, otoacoustic emissions (OAE) are apt to be affected by the impedance of the eartip foam. Therefore, the purpose of this study was to determine the complex reflection coefficient at the surface of eartip foam specimens. This was carried out by means of a conventional impedance tube. Measurements were performed with a closed end as well as with an open end behind the eartip foam, the latter configuration being considered as a more realistic analogy to the in-situ performance of eartips. For both configurations of the impedance tube, a high reflection coefficient of the foam was measured, representing an acoustically rigid boundary. Retrograde OAE standing waves are, hence, anticipated when modelling the path taken by the OAE signals. These retrograde standing waves would result in an OAE node near the tympanic membrane at the quarter-wavelength frequencies of the ear canal, rendering the measurements inaccurate at that location. Recording OAEs at the entrance of the ear canal is thus preferable. Moreover, the compression of the foam eartips, evaluated using a specially designed foam specimen, proved to have no significant effect on the reflection coefficient.