An in-situ calibration method and the effects on stimulus frequency otoacoustic emissions

Abstract

BackgroundThe interference between the incoming sound wave and the acoustic energy reflected by the tympanic membrane (TM) forms a standing wave in human ear canals. The existence of standing waves causes various problems when measuring otoacoustic emissions (OAEs) that are soft sounds closely related with the functional status of the inner ear. The purpose of this study was to propose an in-situ calibration method to overcome the standing-wave problem and to improve the accuracy of OAE measurements.MethodsIn this study, the sound pressure level (SPL) at the TM was indirectly estimated by measuring the SPL at the entrance of the ear canal and the acoustic characteristics of the earphone system, so that sound energy entering the middle ear could be controlled more precisely. Then an in-situ calibration method based on the estimated TM SPL was proposed to control the stimulus level when measuring the stimulus frequency otoacoustic emissions (SFOAEs) evoked by swept tones. The results of swept-tone SFOAEs with the in-situ calibration were compared with two other calibration methods currently used in the clinic.ResultsOur results showed that the estimate of the SPL at the TM was rather successful with the maximal error less than 3.2 dB across all the six subjects. With the high definition OAE spectra achieved by using swept tones, it was found that the calibration methods currently used in the clinic might over-compensate the sound energy delivered to the middle ear around standing-wave frequencies and the SFOAE amplitude could be elevated by more than 7 dB as a consequence. In contrast, the in-situ calibration did not suffer from the standing-wave problem and the results could reflect the functional status of the inner ear more truthfully.ConclusionsThis study suggests that calibration methods currently used in the clinic may produce unreliable results. The in-situ calibration based on the estimated TM SPL could avoid the standing-wave problem and might be incorporated into clinical OAE measurements for more accurate hearing loss screenings.

High frequency transient-evoked otoacoustic emission measurements using chirp and click stimuli

“…Experiment 2 mainly focused on the use of chirp stimuli that could be presented without system distortion at higher relative levels than was possible with click stimuli. Including chirps with slower sweep rates traversed the qualitative divide between sweep rates used in swept-tone studies of SFOAEs (Chen et al, 2014; Kalluri & Shera, 2013) and the relatively faster sweep rates typically used with chirp OAE measurements. Experiment 2 used a smaller set of test ears than Exp.…”

Compensating for ear-canal acoustics when measuring otoacoustic emissions

“…seen here reflects the combined effects of ear-canal acoustics on both the evoking stimuli and on the emission itself. For instance, the SPL-calibration overcompensates the stimuli level near the quarter-wave null, resulting in stronger stimulation of the cochlea (e.g., Siegel and Hirohata, 1994;Scheperle et al, 2008;Chen et al, 2014). This may explain the increase in DPOAE levels near 2-4 kHz at the deep probe placement, where the f 1 frequency (2.6-5.2 kHz) was near f k/4 (shifted from 2.6 to 3.4 kHz; see Sec.…”

“…1(B) ;Siegel, 1994;Siegel and Hirohata, 1994]. These effects manifest themselves most simply when stimulus and response are at the same frequency; for example, as reduced behavioral thresholds or increased SFOAE amplitudes near f k/4 (Lewis et al, 2009;Chen et al, 2014;Souza et al, 2014). Interpretation of the effects of quarter-wave nulls on DPOAE measurements is more complicated, since DPOAE levels depend on both the absolute and the relative levels of the two evoking tones (L 1 and L 2 ), which are altered differently when either f 1 and/or f 2 approach f k/4 (Whitehead et al, 1995).…”

Comparing otoacoustic emissions evoked by chirp transients with constant absorbed sound power and constant incident pressure magnitude

“…Estimating an equivalent pressure at the TM Swept-tone stimulus-frequency (SF) OAEs have been measured using a procedure with constant equivalent pressure level at the TM under the assumptions that the ear canal has a cylindrical geometry and an immobile TM (Chen et al, 2014). The latter assumption allows estimation of the ear-canal length from a "quarter-length frequency" in the measured sound pressure level.…”

“…In contrast to Chen et al (2014), this technique allows for the presence of TM mobility. A cylindrical model of ear-canal acoustics is used for a lossfree acoustic wave number k 0 ¼ 2pf =k for sound of frequency f and wavelength k traveling in a canal of length x, in which a time dependence e j2pft is assumed.…”