Identification of noise sources that influence distortion product otoacoustic emission measurements in human neonates.

OBJECTIVE: The objective of this study was to identify individual sources of noise and their contribution to the overall noise that influences valid measurement of otoacoustic emissions in neonates. The hypothesis was that careful selection of eliciting signals and signal processing parameters, unique analysis of measured results, and control of certain subject characteristics would allow isolation of these individual noise sources and determine their relative influence. DESIGN: Eliciting signal parameters were optimized and held constant to minimize equipment noise. Analysis of noise floors in relation to signal level was used to identify equipment-related noise associated with changes in signal parameters. Analysis of noise floor distributions was used to determine whether environmental noise entered the measurements via inadequate coupling of the probe to the ear. The acoustic characteristics of the middle ear were varied via subject selection to determine the influence of middle-ear characteristics on noise floor levels. RESULTS: The two sources of noise associated with the measurement equipment need not contribute to the noise floor for biologically relevant otoacoustic emissions measurements (eliciting signal levels between 30 and 75 dB SPL). Of the two pathways identified for environmental noise, the pathway resulting from an inadequate seal between the probe and the ear canal can be eliminated. One of the two sources of noise related to the subject, noise resulting from biologic activity unrelated to the ear can be minimized. However, the remaining factor, the status of the middle ear, has been shown to contribute as much as 6 dB to the overall noise floor. CONCLUSIONS: Careful selection of signal parameters and additional data analyses and procedural variables can isolate or control several sources of noise that influence distortion product otoacoustic emission measurements in neonates. Tight coupling between the probe unit and the external ear canal should be maintained for all measurements. Middle ear abnormalities can increase noise floors up to 6 dB.

Growth of the 2f1-f2 distortion product otoacoustic emission for low-level stimuli in human neonates.

Distortion product otoacoustic emissions (DPOAEs) for low stimulus levels (< 60 dB SPL) have been reported in adult humans under ideal conditions. In neonates, DPOAEs have been reported only for high-level stimuli. The purpose of this paper was to determine characteristics of the 2f1-f2 DPOAE for low-level stimuli in neonates and to assess the feasibility of obtaining such measures in a noisy environment. Subjects were 19 premature neonates presumed to have normal hearing based on systematic pneumatic otoscopy measures and evoked auditory brainstem responses. For stimuli centered at 2000 and 6000 Hz and presented over a range of 30 to 75 dB SPL, DPOAEs were measured employing linear time averaging for up to 128 time frames at each level. In quiescent subjects, the level of the noise floor was as low as that reported in cooperative adults under ideal conditions (approximately -30 dB SPL), and the functions were identical. That is, valid measures were obtained for very low stimulus levels (30 dB SPL), the rate of growth approached 1 dB/dB, and identical nonmonotonicities (saturation, plateaus, and notches) were observed as those reported for adults. When the noise floor was elevated due to subject activity, no valid data could be obtained for low-level stimuli even though the DPOAEs were at expected levels for high-level stimuli. These results have important implications for the use of such measures in this population because the DPOAEs associated with the metabolically active nonlinear cochlear processes at low stimulus levels may be contaminated with DPOAEs associated with other processes at high stimulus levels.

Growth of distortion product otoacoustic emissions with primary-tone level in humans.

This study addressed the hypotheses that the growth of the level of distortion product otoacoustic emissions (LDP) with primary-tone level reflects the behavior of a third-order nonlinear polynomial system, and that two sources exist for these distortion products. The results indicated that the 2f1-f2 otoacoustic emission in humans can be measured over a much larger stimulus range than reported previously, even for stimuli (L1 = L2) as low as 10 dB SPL (re 20 microPa). The input/output functions are best described as a straight line with a rate of growth of about 1 dB/dB of stimulus level. For stimulus levels at which metabolically active, nonlinear cochlear processes are in operation, the system does not behave as a simple third-order nonlinear polynomial. Small plateaus and sharp discontinuities or 'notches' can occur in the functions at stimulus levels of approximately 55 dB SPL. These characteristics are consistent with the notion of two separate sources of the LDP, one at low stimulus levels, and one at high levels. An alternative explanation is that the measured otoacoustic emission does not represent only the activity at a single location along the basilar membrane but includes the effects of interactions among similar signals arising from multiple locations, or from the original source via multiple paths.

Clinical significance of relative probe-tone levels on distortion product otoacoustic emissions.

The effect of systematic variations in the relative levels (L1, L2) of two primary tones (f1, f2) on the amplitude of the distortion product otoacoustic emission (DPOAE) at 2f1-f2 and f1 < f2 was investigated in 14 ears from 7 normally hearing human subjects. The primary tones (f2:f1 = 1.23) were geometrically centred at the standard clinical frequencies of 0.5, 1, 1.5, 2, 3, 4, 6 and 8 kHz. The quantity L1-L2 was varied systematically from -10 dB through + 10 dB with L2 held constant at 75 dB SPL for negative values, L1 held constant at 75 dB SPL for positive values, and L1 = L2 = 75 dB SPL at 0 dB relative difference. The maximum amplitudes of the distortion products were generated when L1 = L2 at all geometric centre frequencies except 8 kHz. The reduction of the DPOAE with reduction of L1 was linear at a rate that gradually increased as a function of geometric mean frequency. To a lesser extent, the reduction of the DPOAE with reduction of L2 also was linear but at a rate that systematically decreased as a function of geometric mean frequency. The results suggest, that to maximize the level of the distortion product for clinical purposes, the relative levels of the primary tones should be equal to each other, at least when overall stimulus levels are around 75 dB SPL and f2:f1 = 1.23.

Clinical significance of probe-tone frequency ratio on distortion product otoacoustic emissions.

Parametric variations in the ratio of primary-tone frequencies were explored to optimize the amplitude of the 2f1-f2 distortion product otoacoustic emission for clinical purposes. Ten ears from 5 normally hearing human subjects were examined with primary tones geometrically centred around the standard audiometric frequencies of 0.5, 1, 1.5, 2, 3, 4, 6 and 8 kHz. The distortion product at the frequency 2f1-f2 (f1 < f2) was measured at six probe tone frequency ratios (f2:f1) varying between 1.15 and 1.40 using equal level primaries of 75 dB SPL. The results showed that a single f2:f1 ratio between 1.20 and 1.25 provides a reasonable value for clinical use in that it optimizes the magnitude of the distortion product at 2f1-f2, provides for sufficient resolution in the test frequency range, and is applicable to the standard clinical test frequencies.

Neural presbycusis: a diagnostic dilemma.

The combined term, sensorineural, is useful because clinical distinction between sensory and neural lesions is often difficult, and because sensory lesions may lead to secondary neural degeneration or, though rarely, a neural lesion may lead to secondary sensory degeneration. The threat of lawsuits for failure to identify treatable neurologic conditions has led to overuse of tests, while fiscal constraints exert pressure to limit expensive diagnostic testing. The purpose of this review of the relation between sensory and neural hearing loss is to provide a practical method to screen for neural lesions using pure-tone thresholds and a single speech discrimination score. The difference between the articulation index and the word recognition score of a patient provides a statistically reliable index of suspicion that may reduce the diagnostic dilemma of neural presbycusis.

Hearing aid evaluation and fitting.

Although many patients with hearing loss benefit from medical or surgical intervention, the vast majority have noncorrectable hearing disorders for which rehabilitation through amplification is indicated. There are three goals for the application of hearing aids: (1) to amplify normal conversational speech to levels that are maximally understandable to the patient; (2) to help the patient hear other environmental sounds; and (3) to assist in the educational or habilitative process for those children who sustain hearing loss prior to language and speech development. In addition, there are certain issues that require medical consideration when a wearable device is placed in the ear. This article describes current hearing aid technology; reviews its benefits, limitations, and application for typical patients; discusses the medical aspects of hearing aid fitting; and describes new hearing aid technology on the horizon.

Hearing levels of railroad trainmen.

The hearing sensitivity of 9427 railroad train crew members, determined during the first year of a company-wide hearing conservation program, was compared with hearing sensitivity in a control population not exposed to occupational noise. The hearing sensitivity of the trainmen did not differ significantly from that of the control population. Multiple regression analysis, which considered separately the effects of age and years of service, showed significant differences in hearing levels due to age, but no differences in hearing levels due to years of service. Evaluation of the data by risk categories developed by the National Institute for Occupational Safety and Health indicate that this group of trainmen had no risk of occupational noise-induced hearing loss. These analyses, combined with studies of locomotive cab noise, show clearly that trainmen are not typically exposed to hazardous occupational noise.

Factors which affect measures of speech audibility with hearing aids.

Speech audibility may be defined as that proportion of a speech spectrum which is above a person's threshold. To optimize speech audibility with a hearing aid, several measures are needed. These include quantification of a speech spectrum, measures of hearing sensitivity, and measures of the "real ear" gain of the hearing aid. Some procedural factors must be considered to obtain adequate measures in a typical clinical setting. Those considered here are: (1) a modified Articulation Index to quantify a speech spectrum, (2) specification of hearing sensitivity with a narrowband sound field reference where the out-of-band rejection rate of the sound field stimulus is greater than twice the slope of the hearing loss, and (3) use of functional gain (measured directly or estimated using earphone and sound field results provided that the sound field stimulus has the required characteristics for measuring hearing sensitivity in the sound field) as a measure of the real ear gain of the hearing aid. Guidelines are given for the practical measurement of speech audibility in a typical clinical setting. The guidelines are appropriate for all measures of real ear gain including those obtained with all probe tube systems.

Development of an ear-level digital hearing aid and computer-assisted fitting procedure: an interim report.

Recent progress on the development of an ear-level digital hearing aid is described. The work includes development of a body-wearable digital hearing aid and a computer-based hearing evaluation system that exploits the flexibility afforded by digital signal processing. The prescriptive criteria and fitting procedure used with the digital hearing aid are described briefly. Design considerations in the development of VLSI chips for an ear-level unit are discussed.

Audiologic findings in young patients with hypophosphatemic bone disease.

Audiometric tests were conducted on 19 subjects with hypophosphatemic bone disease to investigate whether auditory impairment like that reported in affected adults occurs in young patients. No hearing loss or significant auditory findings were noted among the children or young adult patients. However, sensorineural hearing loss of cochlear origin was identified in the three oldest subjects (40 to 58 years), although a history of noise exposure in two of them could explain the observations. The results indicate that if an association exists between hypophosphatemic bone disease and hearing impairment, the auditory signs will not develop until adulthood in treated patients. Additional studies of large populations of affected adults are needed to identify the incidence and mechanism(s) of the auditory system abnormalities and to assess any effect of medical therapy for the metabolic bone disease.

Comparison of hearing-aid gain using functional, coupler, and probe-tube measurements.

Measurements of functional gain were compared first to coupler gain for 57 subjects using one of three hearing aid-earmold combinations and second to probe-tube gain for 12 subjects using in-the-ear hearing aids. The average difference between functional and coupler gain plotted as a function of frequency yielded results that were similar to previous reports, with the greatest effects occurring at 3000 and 4000 Hz. Significant differences were seen among hearing aid-earmold combinations at 3000, 4000, and 6000 Hz. Standard deviations for measurements between 750 and 2000 Hz were less than 5 dB and could be explained by variability of functional gain measures associated with test-retest variability of thresholds measured in a sound field. Below 750 Hz and above 2000 Hz, standard deviations exceeded 5 dB. The greater variability may be explained by differences in earmold venting, acoustic characteristics of the ear canal, and stimuli used to measure functional and coupler gain. Neither room nor hearing-aid noise appeared to affect the results significantly. When functional gain was compared to insertion gain measured with a probe-tube system, the average difference across frequencies was less than 1 dB. The variability of the differences at all frequencies, with the exception of 6000 Hz, was within the range reported for functional gain measurements. It was concluded that functional gain can be accurately estimated using probe-tube measurements.

Acoustic immittance measures: terminology and instrumentation.

After a varied history over the past 15 years, basic acoustic immittance measures now include certain physical measures and certain physiological measures. This article reviews current concepts in the terminology and the instrumentation used for these basic measures. It is designed to provide an understanding of standard acoustic immittance measures and a framework for interpreting the results of special acoustic immittance procedures discussed in this issue.

Audiologic findings in a child with a single-channel cochlear implant.

Audiologic findings in a child who received a single-channel cochlear implant are presented. The measures used were threshold sensitivity to frequency-specific stimuli and results on various subtests from the Test of Auditory Comprehension (Trammel, 1976), the Monosyllable, Trochee, Spondee Test (Erber & Alencewicz, 1976), and the Minimal Auditory Capabilities Battery (Owens, Kessler, Telleen, & Schubert, 1981). The results for the implanted ear indicated uneven change in performance across measures compared to results with conventional amplification prior to receiving the implant. Performance of the implanted ear did not approach performance of the better contralateral ear. Observations by teachers and guardians indicated that there was no apparent change in auditory performance even after the cochlear implant had been worn for 6 months. Overall there was no evidence that the cochlear implant worn for 6 months provided any practical benefit to this child.

Frequency selectivity and thresholds of brief stimuli suitable for electric response audiometry.

Auditory evoked potentials are nearly all on-effects and the 'effective stimuli' for them are necessarily brief. Their frequency specificity is therefore limited, especially for the brainstem responses, because of the well-known trade-off between duration and frequency specificity. Brainstem responses are of special interest because they are unchanged in the sleep-like sedation that is required for difficult-to-test children. The middle-latency responses do not meet this requirement. Two patterns of tone burst that are appropriate and promising for the slow cortical potentials and for brainstem potentials, respectively, have rise and fall times of 2 periods of the modulated tone and plateaus of 10 (or 7) periods and 1 period, respectively. Their behavioral thresholds are nearly insensitive to difference in repetition rate between 4 and 40 stimuli/s. Their peak equivalent SPL threshold values at 500, 1 000, 2 000 and 4 000 Hz have been determined for 16 otologically normal ears. Using these reference levels, audiograms have been obtained for subjects with impaired hearing. The audiograms for 'flat' hearing losses do not differ significantly from the corresponding conventional pure-tone audiograms. The slopes for steep high-frequency hearing losses are underestimated, however, particularly with the brief (2-1-2) pattern. Nevertheless, the 2-1-2 pattern appears to be close to the best possible compromise.

The significance of acoustic admittance procedures in the audiologic evaluation of multiply-handicapped children.

The specific contribution of admittance procedures in the diagnosis of hearing impairment was studied in a group of 53 handicapped children. The value of admittance procedures was assessed in relation to that of otoscopy and pure tone audiometry. Admittance procedures and otoscopy were successfully performed in all but one of the children and indicated conductive pathology in about 40% of the subjects, whereas pure tone audiometry was not feasible or inconclusive in 30.2% of the subjects. In 5.7% of the subjects admittance procedures proved the only diagnostic tool.

Tympanometry in normal neonates.

Acoustic conductance and susceptance tympanograms were obtained at 220 and 660 Hz in 34 neonates. The neonates were categorized into three age groups (8-24 hours, 24-60 hours, and 60-96 hours). Single-peaked, double-peaked, and monotonically increasing tympanograms were found. Static values for conductance, susceptance, admittance, resistance, reactance and impedance at the lateral surface of the tympanic membrane were computed from the tympanograms. There were no significant differences in mean static values among the three groups. At 220 Hz, the individual static reactance values were usually smaller than the static resistance values and often assumed a positive sign. At 660 Hz, the individual static reactance values always assumed a negative sign and were approximately equal to the static resistance values. The single- and double-peaked tympanograms apparently were the result of previously identified interactions between static resistance and reactance values. The data were compared to those of infants and adults. Tympanograms at 220 Hz were obtained for 13 of the original subjects at the age of three to four months. The data collected in this group were consistent with those reported in the literature for the same age group.

Effect of sensorineural hearing loss on acoustic stapedius reflex growth functions.

The growth function of the acoustic stapedius reflex was measured in subjects with normal hearing and sensorineural hearing loss of cochlear origin. The effects of age and magnitude of hearing loss were controlled. Activating stimuli were 500, 1000, and 2000 Hz tones and broadband noise. Stapedius muscle activity was inferred from acoustic impedance measures in the contralateral ear. The mean growth functions for tones were essentially linear in log-log plots with the rate of growth equal for the two groups. The mean growth function for the noise signal was curvilinear for the normal hearing groud had linear for the hearing loss group. Comparison of slope functions derived from the fitted data indicated that the rate of reflex growth for the noise signal, over a limited range above reflex threshold, is greater in ears with cochlear lesions than normal ears. For higher level noise signals, however, the rate of reflex growth is similar for normal and pathological ears. The effect of a cochlear lesion on the input-output function of the cochlea for both tonal and noise stimuli is to maintain the rate of reflex growth but shift the function along the intensity axis of a tonal signal and the response axis for a noise signal.