The clinical utility of expressing hearing thresholds in terms of the forward-going sound pressure wave.

OBJECTIVE: To assess the clinical utility of quantifying pure-tone hearing thresholds in terms of the forward-going sound pressure wave. DESIGN: Sound pressure measurements in the ear canal were used to derive, with hearing threshold measurements, hearing thresholds expressed in terms of the forward-going sound pressure wave, hearing thresholds based on coupler-based calibration, and hearing thresholds expressed in terms of the sound pressure measured at the microphone. STUDY SAMPLE: Fifty-two adults, 18 to 34 years of age, served as the study group. RESULTS: Audiogram configurations were similar up to 2000 Hz for the three expressions of hearing threshold, consistent with the ear canal acting as a simple volume up to this frequency. Above 2000 Hz, notable differences in hearing threshold were found, consistent with the acoustic input impedance of the ear differing from a rigid, hard-walled cavity. Repeat testing showed all three expressions of hearing threshold to be repeatable. High density measurements of hearing threshold from 3000 to 6000 Hz provided qualified support for the derivation of the forward-going sound pressure wave. CONCLUSIONS: Hearing thresholds expressed in terms of the forward-going sound pressure wave are repeatable, and with in-situ calibration, may be superior to the current coupler-based method.

Wideband reflectance in newborns: normative regions and relationship to hearing-screening results.

OBJECTIVES: To develop normative data for wideband middle-ear reflectance in a newborn hearing-screening population and to compare test performance with 1-kHz tympanometry for prediction of otoacoustic emission (OAE) screening outcome. DESIGN: Wideband middle-ear reflectance (using both tone and chirp stimuli from 0.2 to 6 kHz), 1-kHz tympanometry, and distortion-product (DP) OAEs were measured in 324 infants at two test sites. Ears were categorized into DP pass and DP refer groups. RESULTS: Normative reflectance values were defined over various frequency regions for both tone and chirp stimuli in ambient pressure conditions, and for reflectance area indices integrated over various frequency ranges. Receiver operating characteristic analyses showed that reflectance provides the best discriminability of DP status in frequency ranges involving 2 kHz and greater discriminability of DP status than 1-kHz tympanometry. Repeated-measures analyses of variance established that (a) there were significant differences in reflectance as a function of DP status and frequency but not sex or ear; (b) tone and chirp stimulus reflectance values are essentially indistinguishable; and (c) newborns from two geographic sites had similar reflectance patterns above 1 kHz. Birth type and weight did not contribute to differences in reflectance. CONCLUSIONS: Referrals in OAE-based infant hearing screening were strongly associated with increased wideband reflectance, suggesting middle-ear dysfunction at birth. Reflectance improved significantly during the first 4 days after birth with normalization of middle-ear function. Reflectance scores can be achieved within seconds using the same equipment used for OAE screening. Newborns with high reflectance scores at stage I screening should be rescreened within a few hours to a few days, because most middle-ear problems are transient and resolve spontaneously. If reflectance and OAE are not passed upon stage II screening, referral to an otologist for ear examination is suggested along with diagnostic testing. Newborns with normal reflectance and a refer result for the OAE screen should be referred immediately to an audiologist for diagnostic testing with threshold auditory brainstem response because of higher risk for permanent hearing loss.

An in situ calibration for hearing thresholds.

Quantifying how the sound delivered to the ear canal relates to hearing threshold has historically relied on acoustic calibration in physical assemblies with an input impedance intended to match the human ear (e.g., a Zwislocki coupler). The variation in the input impedance of the human ear makes such a method of calibration questionable. It is preferable to calibrate the acoustic signal in each ear individually. By using a calibrated sound source and microphone, the acoustic input impedance of the ear can be determined, and the sound delivered to the ear calibrated in terms of either (i) the incident sound pressure wave or (ii) that portion of the incident sound pressure wave transmitted to the middle ear and cochlea. Hearing thresholds expressed in terms of these quantities are reported, these in situ calibrations not being confounded by ear canal standing waves. Either would serve as a suitable replacement for the current practice of hearing thresholds expressed in terms of sound pressure level calibrated in a 6cc or 2cc coupler.

Evaluation of human middle ear function via an acoustic power assessment.

Measurements of middle ear (ME) acoustic power flow (power reflectance, power absorption, and transmittance) and normalized impedance (acoustic resistance, acoustic reactance, and impedance magnitude) were compared for their utility in clinical applications. Transmittance, a measure of the acoustic power absorbed by the ME, was found to have several important advantages over other measures of acoustic power flow. In addition to its simple and audiologically relevant physical interpretation (absorbed power), the normal transmittance curve has a simple shape that is visually similar to the ME transfer function. The acoustic impedance measures (resistance and reactance) provided important additional information about ME status and supplemented transmittance measurements. Together these measurements can help identify unusual conditions such as eardrum perforations. While this article is largely a review of the development of a commercial power reflectance measurement system, previously unpublished experimental results are presented.