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.
Highlights
The 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 human ear canal is a tube with one end open to the air and the other end terminated by the tympanic membrane (TM) that separates the outer ear from the middle ear
It was demonstrated that the standing wave in the ear canal has large impacts on the quantification of sound energy entering the middle ear
Summary
The 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 enhancements or cancellations between the forward and backward waveforms can form a standing wave (or stationary wave) in the ear canal, characterized by positions where the sound pressure level (SPL) appears to be standing still. Without taking into account the effects of standing waves, the hearing thresholds of standard audiogram tests may be questionable [5,6], the acoustic measurements of hearing aid fittings could cause over amplification and discomfort of the patients [7,8], and the results of otoacoustic emissions could be unreliable [9,10,11,12].the actual sound energy delivered to the middle ear should be measured to quantify the stimulus level presented to the ear in clinical applications. A commonly accepted reference of the delivered sound energy is the TM pressure measured within a few millimeters from the TM [1,11]
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