Douglas H. Keefe

Ear‐canal impedance and reflection coefficient in human infants and adults

The ear-canal impedance and reflection coefficient were measured in an adult group and in groups of infants of age 1, 3, 6, 12, and 24 months over frequency range 125-10,700 Hz. The development of the external ear canal and middle ear strongly affect input impedance and reflection coefficient responses, and this development is not yet complete at age 24 months. Contributing factors include growth of the area and length of the ear canal, a resonance in the ear-canal walls of younger infants, and a probable influence of growth of the middle-ear cavities. The middle-ear compliance is lower in infants than adults, and the middle-ear resistance is higher. The power transfer into the middle ear of the infant is much less than into that of the adult. Such differences in power transfer directly influence both behavioral and physiological measurements of hearing. The difficulties of interpretation of neonatal tympanograms are shown to be a consequence of ear-canal wall vibration. Impedance and reflectance measurements in the 2-4-kHz range are recommended as a potentially useful clinical tool for circumventing these difficulties.

Acoustical wave propagation in cylindrical ducts: Transmission line parameter approximations for isothermal and nonisothermal boundary conditions

Approximate expressions are given for the characteristic impedance and propagation wavenumber for linear acoustic transmission through a gas enclosed in a rigid cylindrical duct. These expressions are most complicated in the transition zone where the thermoviscous boundary layers are on the order of the tube radius. The approximations are accurate to within 1% for all frequencies and tube diameters except within the transition zone where the approximations are accurate to within 10%. A simple modification of the transmission line parameters is presented for the case where the tube walls are nonisothermal.

Method to measure acoustic impedance and reflection coefficient

A frequency-domain based system for measuring acoustic impedance and reflection coefficient is described. The calibration procedure uses a least-mean-squares approximation to the Thevenin parameters describing the source and receiver characteristics in which the data measured on closed, cylindrical tubes are matched to a viscothermal tube model. The system is intended for use in acoustical measurement in human ear canals, in which the cross-sectional area of the ear canal at the point of insertion is imprecisely known. This area is acoustically estimated from the impedance data, and the reflection coefficient is calculated in terms of this area and the impedance data. Measurements on a variety of closed tubes show the method is accurate over the frequency range investigated (less than 10.7 kHz). The time-domain reflection function is evaluated by transforming the reflection coefficient from the frequency domain, but the finite bandwidth of the measured data limits the accuracy of time-domain response measurements. The method is well suited for frequency-domain measurements in human ear canals.

Maturation of the middle and external ears: acoustic power-based responses and reflectance tympanometry.

Objective: The maturation of the external and middle ear in the human infant has significant effects on the interpretation of measured ear‐canal responses to acoustic stimuli. A tutorial section is presented of power‐based response functions, accompanied by a hierarchy of stimulus specifications contrasting pressure‐based and power‐based responses. An experimental section follows on reflectance tympanometry, the aims of which are to introduce and assess the feasibility of the technique and to discuss implications for tests of hearing development. Design: A tympanometric measurement of admittance is used with an estimate of ear‐canal area to calculate a so‐called reflectance tympanogram as a function of frequency and static pressure in the ear canal. Selected results on 226 Hz reflectance tympanograms are reported for normal‐hearing adults and for infants of age 3 to 6 mo with both normal and flat 226 Hz admittance tympanograms. A multifrequency reflectance tympanogram is reported for an adult. Results: Measured at ambient ear‐canal pressure, the acoustic external‐ and middle‐ear responses of infants of age 1 to 6 mo are compared with those of adults. The admittance level is influenced by the ear‐canal area, the interplay of compliant‐ and inertance‐controlled effects in the middle ear, and the presence of losses. Ear‐canal area is a major factor in distinguishing infant from adult responses. Energy reflectance provides a measure of middle‐ear power transmission that is approximately independent of probe placement in the ear canal and that varies with maturation. These power‐based responses, measured at ambient pressure, are contrasted with tympanometric measurements. Reflectance tympanometry is defined and easily measured in infants and adults. Some infants with flat 226 Hz tympanograms have energy reflectance in the normal range at higher frequencies (2 to 4 kHz). Conclusion: Acoustic measurements of power‐based responses in the ear canal‐reflectance, admittance, and impedance‐provide insight into the maturation of the external and middle ear. Reflectance tympanometry tests the relative accuracy underlying the tympanometric measurement of compensated eardrum admittance and may have clinical utility.

Identification of Neonatal Hearing Impairment: Ear-canal Measurements of Acoustic Admittance and Reflectance in Neonates

Objectives 1) To describe broad bandwidth measurements of acoustic admittance (Y) and energy reflectance (R) in the ear canals of neonates. 2) To describe a means for evaluating when a YR response is valid. 3) To describe the relations between these YR measurements and age, gender, left/right ear, and selected risk factors. Design YR responses were obtained at four test sites in well babies without risk indicators, well babies with at least one risk indicator, and graduates of neonatal intensive care units. YR responses were measured using a chirp stimulus at moderate levels over a frequency range from 250 to 8000 Hz. The system was calibrated based on measurements in a set of cylindrical tubes. The probe assembly was inserted in the ear canal of the neonate, and customized software was used for data acquisition. Results YR responses were measured in over 4000 ears, and half of the responses were used in e-ploratory data analyses. The particular YR variables chosen for analysis were energy reflectance, equivalent volume and acoustic conductance. Based on the view that unduly large negative equivalent volumes at low frequencies were physically impossible, it was concluded that appro-imately 13% of the YR responses showed evidence of improper probe seal in the ear canal. To test how these outliers influenced the overall pattern of YR responses, analyses were conducted both on the full data set (N = 2081) and the data set excluding outliers (N = 1825). The YR responses averaged over frequency varied with conceptional age (conception to date of test), gender, left/right ear, and selected risk factors; in all cases, significant effects were observed more frequently in the data set excluding outliers. After excluding outliers and controlling for conceptional age effects, the dichotomous risk factors accounting for the greatest variance in the YR responses were, in rank order, cleft lip and palate, aminoglycoside therapy, low birth weight, history of ventilation, and low APGAR scores. In separate analyses, YR responses varied in the first few days after birth. An analysis showed that the use of a YR test criterion to assess the quality of probe seal may help control the false-positive rate in evoked otoacoustic emission testing. Conclusions This is the first report of wideband YR responses in neonates. Data were acquired in a few seconds, but the responses are highly sensitive to whether the probe is fully sealed in the ear canal. A real-time acoustic test of probe fit is proposed to better address the probe seal problem. The YR responses provide information on middle-ear status that varies over the neonatal age range and that is sensitive to the presence or absence of risk factors, ear, and gender differences. Thus, a YR test may have potential for use in neonatal screening tests for hearing loss.

On the existence of an age/threshold/frequency interaction in distortion product otoacoustic emissions

Interactions among age, threshold, and frequency in relation to distortion product otoacoustic emissions (DPOAE) have yet to be resolved. The effects of these variables were explored by analyzing DPOAEs in ears with thresholds not exceeding 20 dB HL. Multivariate regression analyses were performed in two different ways. For data to be included in the first analysis, audiometric threshold had to be 20 dB HL or better only at the particular frequency under study, but might exceed 20 dB HL at other half-octave frequencies. Significant main effects were found for age, threshold, and frequency. There was also an age-by-frequency interaction, but a significant age-by-threshold interaction was not observed. DPOAE amplitudes decreased as either age, frequency, or threshold increased. In the second analysis, when a more stringent inclusion criterion was applied (normal thresholds at all frequencies), the main effects for age, threshold, and frequency were not significant. The significant age-by-frequency interaction remained, whereby DPOAE amplitudes decreased as age and frequency increased, but the age-by-threshold interaction again was not significant. The magnitude of DPOAE amplitude change across age, threshold, and frequency and for the age-by-frequency interaction was small but similar for both groups of subjects. Age in association with threshold did not account for observed changes in DPOAE amplitudes for either group. Importantly, the lack of a significant age-by-threshold interaction indicates that there may be processes intrinsic to aging alone that act on DPOAE generation.

Energy transmittance predicts conductive hearing loss in older children and adults

The test performance of a wideband acoustic transfer function (ATF) test and 226-Hz tympanometry was assessed in predicting the presence of conductive hearing loss, based on an air-bone gap of 20 dB or more. Two ATF tests were designed using an improved calibration method over a frequency range (0.25-8 kHz): an ambient-pressure test and a tympanometric test using an excess static pressure in the ear canal. Wideband responses were objectively classified using moment analyses of energy transmittance, which was a more appropriate test variable than energy reflectance. Subjects included adults and children of age 10 years and up, with 42 normal-functioning ears and 18 ears with a conductive hearing loss. Predictors were based on the magnitudes of the moment deviations from the 10th to 90th percentiles of the normal group. Comparing tests at a fixed specificity of 0.90, the sensitivities were 0.28 for peak-compensated static acoustic admittance at 226 Hz, 0.72 for ambient-pressure ATF, and 0.94 for pressurized ATF. Pressurized ATF was accurate at predicting conductive hearing loss with an area under the receiver operating characteristic curve of 0.95. Ambient-pressure ATF may have sufficient accuracy to use in some hearing-screening applications, whereas pressurized ATF has additional accuracy that may be appropriate for hearing-diagnostic applications.

High-frequency click-evoked otoacoustic emissions and behavioral thresholds in humans

Relationships between click-evoked otoacoustic emissions (CEOAEs) and behavioral thresholds have not been explored above 5 kHz due to limitations in CEOAE measurement procedures. New techniques were used to measure behavioral thresholds and CEOAEs up to 16 kHz. A long cylindrical tube of 8 mm diameter, serving as a reflectionless termination, was used to calibrate audiometric stimuli and design a wideband CEOAE stimulus. A second click was presented 15 dB above a probe click level that varied over a 44 dB range, and a nonlinear residual procedure extracted a CEOAE from these click responses. In some subjects (age 14-29 years) with normal hearing up to 8 kHz, CEOAE spectral energy and latency were measured up to 16 kHz. Audiometric thresholds were measured using an adaptive yes-no procedure. Comparison of CEOAE and behavioral thresholds suggested a clinical potential of using CEOAEs to screen for high-frequency hearing loss. CEOAE latencies determined from the peak of averaged, filtered temporal envelopes decreased to 1 ms with increasing frequency up to 16 kHz. Individual CEOAE envelopes included both compressively growing longer-delay components consistent with a coherent-reflection source and linearly or expansively growing shorter-delay components consistent with a distortion source. Envelope delays of both components were approximately invariant with level.

Double-evoked otoacoustic emissions. I. Measurement theory and nonlinear coherence

An evoked otoacoustic emission (OAE) measurement technique is proposed to better control for probe distortion. Each double-evoking (2E) stimulus sequence includes three, equal-duration subsequences defined as follows: s1(t) is a single chirp or click, s2(t)=es1(t−τ) is a copy of s1 with relative amplitude e and delay τ, and s12(t)=s1(t)+s2(t) is the superposition of the first two stimuli. The pressure response to each subsequence is p1, p2, and p12, respectively. The double chirp-evoked distortion product (2ChDP) and double click-evoked otoacoustic emission (2CEOAE) are defined by pD=p12−(p1+p2). The 2ChDP response may be time compressed to analyze as an equivalent 2CEOAE response. The 2E response family provides a complementary representation between DP measurements and double click-evoked OAE measurements. A technique based on nonlinear coherence quantifies random noise in terms of a nonlinear signal-to-noise ratio.

Sound-conduction effects on distortion-product otoacoustic emission screening outcomes in newborn infants: test performance of wideband acoustic transfer functions and 1-kHz tympanometry.

Objective: Universal newborn hearing screening (UNHS) test outcomes can be influenced by conditions affecting the sound conduction pathway, including ear canal and/or middle ear function. The purpose of this study was to evaluate the test performance of wideband (WB) acoustic transfer functions and 1-kHz tympanometry in terms of their ability to predict the status of the sound conduction pathway for ears that passed or were referred in a UNHS program. Design: A distortion-product otoacoustic emission (DPOAE) test was used to determine the UNHS status of 455 infant ears (375 passed and 80 referred). WB and 1-kHz tests were performed immediately after the infant’s first DPOAE test (day 1). Of the 80 infants referred on day 1, 67 infants were evaluated again after a second UNHS DPOAE test the next day (day 2). WB data were acquired under ambient and tympanometric (pressurized) ear canal conditions. Clinical decision theory analysis was used to assess the test performance of WB and 1-kHz tests in terms of their ability to classify ears that passed or were referred, using DPOAE UNHS test outcomes as the “gold standard.” Specifically, performance was assessed using previously published measurement criteria and a maximum-likelihood procedure for 1-kHz tympanometry and WB measurements, respectively. Results: For measurements from day 1, the highest area under the receiver operating characteristic curve was 0.87 for an ambient WB test predictor. The highest area under the receiver operating characteristic curve among several variables derived from 1-kHz tympanometry was 0.75. In general, ears that passed the DPOAE UNHS test had higher energy absorbance compared with those that were referred, indicating that infants who passed the DPOAE UNHS had a more acoustically efficient conductive pathway. Conclusions: Results showed that (1) WB tests had better performance in classifying UNHS DPOAE outcomes than 1-kHz tympanometry; (2) WB tests provide data to suggest that many UNHS referrals are a consequence of transient conditions affecting the sound conduction pathway; (3) WB data reveal changes in sound conduction during the first 2 days of life; and (4) because WB measurements used in the present study are objective and quick it may be feasible to consider implementing such measurements in conjunction with UNHS programs.