Ellisoidal reflector for measuring otoacoustic emissions

Otoacoustic emissions (OAEs) are low-intensity sounds present in the ear canal, generated by mechanical processing in the cochlear in the inner ear. OAEs provide a noninvasive technique to sense the mechanical processing of sound in the inner ear. These signals are commonly measured by placing a miniature microphone into the ear canal of the listener.

Such small microphones have however a self-noise at frequencies below 1 kHz that is compareable in intensity with typical intensities of OAEs at frequencies above 1 kHz. Due to this fact, not much is known about the presence or absence of OAEs, and especially SOAE at these low frequencies. In addition, blocking of the ear canal changes the impedance of the middle ear, potentially changing the transmission of acoustical energy from the inner ear to the ear canal, hampering the interpretation of the data in terms of normal listening conditions with open ear canal.

This study presents the design and evaluation of a truncated prolate ellipsoidal reflector in combination with a large-diaphragm low-noise microphone to measure OAEs in the open ear canal of human listeners. The reflector was designed to gain information about BM processing at low frequencies where miniature microphones are not easily applicable.

Acoustical evaluation of the reflector shows a focusing effect of sound from one focal point into the other focal point. Partial removal of elements of the reflector allow to control multiple reflections between the microphone membrane and the ear canal. Spontanoeus-and disortion-product OAEs show similar amplitudes and an improved noise floor at frequencies below 2 kHz compared to a commercially available OAE probe.

The advantages and physical limitations of this system to measure OAEs in listeners with open ear canal and at low frequencies will be discussed.