16.04.2010
An objective hearing test with otoacoustic emissions (OAE) provides information on the inner ear sensory cell performance (cochlear function), even if no cooperation of the subject (e.g. babies) can be expected. OAEs are evoked as a response to an acoustic stimulus and indicate a healthy inner ear. Usually, the stimuli are presented in the ear canal with miniature loudspeakers. An alternative stimulation by means of bone vibrators, however, can provide additional information on the function of the ear.
When two pure tones are presented during an OAE hearing test, the sensory cells in the inner ear generate an additional deeper tone. This tone is known as the distortion-product otoacoustic emission (DPOAE) and can be recorded by means of a miniature microphone placed in the ear canal.
If the stimuli are presented with miniature loudspeakers in the ear canal, an intact middle ear chain is necessary for the detection of DPOAE. Only under this assumption is it possible to assess the function of the sensory cells. For certain types of hearing loss (conductive hearing loss), however, the acoustic stimuli are attenuated before they reach the sensory cells and might fail to evoke a DPOAE in spite of intact hair cells.
An alternative and rather less investigated constellation is the bone-conduction stimulation of DPOAE. For this kind of stimulation, so-called bone vibrators are placed on the cranial bone of the patient. The presented stimuli reach the inner ear directly via the bone-conduction path and the influence of the middle-ear chain on the stimuli is largely avoided.
In a feasibility study, the applicability of the bone-conduction stimulation of DPOAE was assessed using commercially available bone vibrators. DPOAE, stimulated through bone conduction, could be recorded in the ears of all test subjects at least for one frequency. With respect to the calibration of the stimuli, this kind of stimulation provides the advantage that simple microphone probes are sufficient to record the DPOAE. Furthermore, known interference due to evanescent waves of the probe loudspeakers affecting the probe microphone simply does not exist, since the bone vibrators and the probe microphone are spatially separated.
Figure 1: Example of an evoked DPOAE by two pure tones f1 = 1000 Hz and f2 = 1200 Hz. The DPOAE occurs at a frequency fdp of 800 Hz.
Point at the loudspeaker symbols and listen to the stimuli (f1 , f2) and the elicited DPOAE (fdp) (amplified, otherwise not audible!).
As you can hear, the pitch of the ear-generated DPOAE corresponds to the missing fundamental of a major triad. The ratio between the two stimuli (f2 / f1) corresponds to a minor third and (f1 / fdp) to a major third.
Contact person:
Makram A. Zebian, Dept. 1.6, WG 1.61, E-mail: makram.a.zebian@ptb.de