Influence of in situ, sound-level calibration on distortion-product otoacoustic emission variability

Standing waves can cause errors during in-the-ear calibration of sound pressure level (SPL), affecting both stimulus magnitude and distortion-product otoacoustic emission (DPOAE) level. Sound intensity level (SIL) and forward pressure level (FPL) are two measurements theoretically unaffected by standing waves. SPL, SIL, and FPL in situ calibrations were compared by determining sensitivity of DPOAE level to probe-insertion depth (deep and "shallow") for a range of stimulus frequencies (1-8 kHz) and levels (20-60 dB). Probe-insertion depth was manipulated with the intent to shift the frequencies with standing-wave minima at the emission probe, introducing variability during SPL calibration. The absolute difference in DPOAE level between insertions was evaluated after correcting for an incidental change caused by the effect of ear-canal impedance on the emission traveling from the cochlea. A three-way analysis of variance found significant main effects for stimulus level, stimulus frequency, and calibration method, as well as significant interactions involving calibration method. All calibration methods exhibited changes in DPOAE level due to the insertion depth, especially above 4 kHz. However, SPL demonstrated the greatest changes across all stimulus levels for frequencies above 2 kHz, suggesting that SIL and FPL provide more consistent measurements of DPOAEs for frequencies susceptible to standing-wave calibration errors.     

Further assessment of forward pressure level for in situ calibration

Quantifying ear-canal sound level in forward pressure has been suggested as a more accurate and practical alternative to sound pressure level (SPL) calibrations used in clinical settings. The mathematical isolation of forward (and reverse) pressure requires defining the The?venin-equivalent impedance and pressure of the sound source and characteristic impedance of the load; however, the extent to which inaccuracies in characterizing the source and/or load impact forward pressure level (FPL) calibrations has not been specifically evaluated. This study examined how commercially available probe tips and estimates of characteristic impedance impact the calculation of forward and reverse pressure in a number of test cavities with dimensions chosen to reflect human ear-canal dimensions. Results demonstrate that FPL calibration, which has already been shown to be more accurate than in situ SPL calibration, can be improved particularly around standing-wave null frequencies by refining estimates of characteristic impedance. Better estimates allow FPL to be accurately calculated at least through 10 kHz using a variety of probe tips in test cavities of different sizes, suggesting that FPL calibration can be performed in ear canals of all sizes. Additionally, FPL calibration appears a reasonable option when quantifying the levels of extended high-frequency (10-18 kHz) stimuli.     

Acoustics of ear canal measurement of eardrum SPL in simulators

The effect of standing waves on the ear canal measurement of eardrum sound pressure level (SPL) was determined by both calculation and measurement. Transmission line calculations of the standing wave were made using the dimensions of the ANSI S3.25-1979 ear simulator and three different eardrum impedances. Standing wave curves have been obtained for the standard eardrum impedance at 1-kHz intervals in the range of 1-8 kHz. The changes in standing wave position due to each of the three eardrum impedances and their effects on ear canal measurements of SPL were computed for each of the eardrum impedances. Ear canal SPL measurements conducted on simulators modified to correspond to the eardrum impedances used in the calculations were compared to the computed values. Differences between eardrum SPLs and those measured at different locations in the ear canal approached a standing wave ratio (SWR) of 10-12 dB as the position of the measuring probe approached the standing wave minimum at each frequency. These maximum differences compared favorably with data developed by other investigators from real ears. Differences due to the eardrum impedance were found to be significant only in the frequency region of 2-5 kHz. Calibration of probes in a standard or modified ANSI simulator at the same distance from the eardrum as in the real ear reduces the eardrum SPL measurement errors to those resulting from differences in eardrum impedance.     

The reliability of auditory thresholds in the 8- to 20-kHz range using a prototype audiometer

This study was designed to evaluate both intra- and intertester reliability of auditory thresholds in the 8- to 20-kHz range using a recently developed high-frequency audiometer [Stevens et al., J. Acoust. Soc. Am. 81, 470-484 (1987)]. With this device, signals from a high-frequency transducer are introduced into the ear canal via a plastic tube. A calibration function is calculated for each ear and used to estimate the sound-pressure level (SPL) at the tympanic membrane. Twenty normal-hearing listeners were tested four times, twice by each of two examiners. In the higher frequencies, accurate calibration functions could not be obtained for many subjects; in these cases, values extrapolated from lower frequencies were used to estimate SPL. Findings reveal that the standard error of measurement for both intra- and intertester measures increases as a function of frequency. Intertester variability was only slightly higher than intratester variability. In most cases, variability of threshold estimates in dB SPL was higher than that observed for the uncorrected attenuator settings. Exclusion of extrapolated values improved reliability substantially.     

High-frequency audiometric assessment of a young adult population

The hearing thresholds of 37 young adults (18-26 years) were measured at 13 frequencies (8, 9,10,...,20 kHz) using a newly developed high-frequency audiometer. All subjects were screened at 15 dB HL at the low audiometric frequencies, had tympanometry within normal limits, and had no history of significant hearing problems. The audiometer delivers sound from a driver unit to the ear canal through a lossy tube and earpiece providing a source impedance essentially equal to the characteristic impedance of the tube. A small microphone located within the earpiece is used to measure the response of the ear canal when an impulse is applied at the driver unit. From this response, a gain function is calculated relating the equivalent sound-pressure level of the source to the SPL at the medial end of the ear canal. For the subjects tested, this gain function showed a gradual increase from 2 to 12 dB over the frequency range. The standard deviation of the gain function was about 2.5 dB across subjects in the lower frequency region (8-14 kHz) and about 4 dB at the higher frequencies. Cross modes and poor fit of the earpiece to the ear canal prevented accurate calibration for some subjects at the highest frequencies. The average SPL at threshold was 23 dB at 8 kHz, 30 dB at 12 kHz, and 87 dB at 18 kHz. Despite the homogeneous nature of the sample, the younger subjects in the sample had reliably better thresholds than the older subjects. Repeated measurements of threshold over an interval as long as 1 month showed a standard deviation of 2.5 dB at the lower frequencies (8-14 kHz) and 4.5 dB at the higher frequencies.     

Comparing behavioral and physiological measures of combination tones: Sex and race differences

Both distortion-product otoacoustic emissions (DPOAEs) and performance in an auditory-masking task involving combination tones were measured in the same frequency region in the same ears. In the behavioral task, a signal of 3.6?kHz (duration 300?ms, rise/fall time 20?ms) was masked by a 3.0-kHz tone (62?dB SPL, continuously presented). These two frequencies can produce a combination tone at 2.4?kHz. When a narrowband noise (2.0-2.8?kHz, 17?dB spectrum level) was added as a second masker, detection of the 3.6-kHz signal worsened by 6-9?dB (the Greenwood effect), revealing that listeners had been using the combination tone at 2.4?kHz as a cue for detection at 3.6?kHz. Several outcomes differed markedly by sex and racial background. The Greenwood effect was substantially larger in females than in males, but only for the White group. When the magnitude of the Greenwood effect was compared with the magnitude of the DPOAE measured in the 2.4?kHz region, the correlations typically were modest, but were high for Non-White males. For many subjects, then, most of the DPOAE measured in the ear canal apparently is not related to the combination-tone cue that is masked by the narrowband noise.     

Quantitative estimation of minor conductive hearing loss with distortion product otoacoustic emissions in the guinea pig

Subclinical conductive hearing losses (CHLs) can affect otoacoustic emissions and therefore limit their potential in the assessment of the cochlear function. Theoretical considerations to estimate a minor CHL from DPOAE measurements [Kummer et al. (2006). HNO 54, 457-467] are evaluated experimentally. They are based on the fact, that the level difference of the stimulus tones L(1) and L(2) for optimal excitation of the inner ear is given by L(1)=aL(2)+b. A CHL is presumed to attenuate both L(1) and L(2) to the same extent such that excitation of the inner ear is no longer optimal. From the change of L(1) that is necessary to restore optimal excitation of the inner ear and thus to produce maximal DPOAE levels, the CHL can be estimated. In 10 guinea pig ears an experimental CHL was produced, quantified by determination of compound action potential (CAP) thresholds at 8 kHz (CHL(CAP)) and estimated from DPOAE measurements at 8 kHz (CHL(DPOAE)). CHLs up to 12 dB could be assessed. CHL(DPOAE) correlated well with CHL(CAP) (R=0.741, p=0.0142). Mean difference between CHL(DPOAE) and CHL(CAP) was 4.2±2.6 dB. Estimation of minor CHL from DPOAE measurements might help to increase the diagnostic value of DPOAEs.     

Middle ear cavity and ear canal pressure-driven stapes velocity responses in human cadaveric temporal bones

Drive pressure to stapes velocity (V(st)) transfer function measurements are collected and compared for human cadaveric temporal bones with the drive pressure alternately on the ear canal (EC) and middle ear cavity (MEC) sides of the tympanic membrane (TM), in order to predict the performance of proposed middle-ear implantable acoustic hearing aids, as well as provide additional data for examining human middle ear mechanics. The chief finding is that, in terms of the V(st) response, MEC stimulation performs at least as well as EC stimulation below 8 kHz, provided that the EC is unplugged. Plugging the EC causes a reduced response for MEC drive below 2 kHz, due to a corresponding reduction of the pressure difference between the two sides of the TM. Between 8 and 11 kHz, the MEC drive transfer functions feature an approximately 17 dB drop in magnitude below the EC drive case, the cause of which remains unknown. The EC drive transfer functions reported here feature significantly less magnitude roll-off above 1 kHz than previous studies [with a slope of -2.3 vs -6.7 dB/octave for Aibara et al., Hear. Res. 152, 100-109 (2001)], and significantly more phase group delay (134 vs 62 micros for Aibara et al.).     

Cochlear generation of intermodulation distortion revealed by DPOAE frequency functions in normal and impaired ears

Distortion product otoacoustic emission (DPOAE) frequency functions were measured in normal-hearing and hearing-impaired ears. A fixed-f2/swept-f1 paradigm was used with f2 fixed at half-octave intervals from 1 to 8 kHz. L1 was always 10 dB greater than L2, and L2 was varied from 65 to 10 dB SPL in 5-dB steps. The responses were quantified by the frequency and amplitude of the peak response. Peak responses were closer to f2 in higher frequency regions and for lower intensity stimulation. Results from hearing-impaired subjects suggest that audiometric thresholds at the distortion product frequency, fdp, in addition to hearing status at f2, can affect DPOAE results. Results are discussed in terms of several manifestations of a second resonance model, as well as a dual source model for the generation of DPOAEs as measured in the ear canal of humans. It appears that a dual source model accounts for the data better than second filter models.     

High-frequency audiometry: test reliability and procedural considerations

This study compared the reliability of a recently developed high-frequency audiometer (HFA) [Stevens et al., J. Acoust. Soc. Am. 81, 470-484 (1987)] with a less complicated system that uses supraaural earphones (Koss system). The new approach permits calibration on an individual basis, making it possible to express thresholds at high frequencies in dB SPL. Data obtained from 50 normal-hearing subjects, ranging in age from 10-60 years, were used to evaluate the effects on reliability of threshold variance, earpiece/earphone fitting variance, and the variance associated with the HFA calibration process. Without earpiece/earphone replacement, the reliability of thresholds for the two systems is similar. With replacement, the HFA showed poorer reliability than the Koss system above 11 kHz, largely due to errors in estimating the calibration function. HFA reliability is greater for subjects with valid calibration functions over the entire frequency range. When average correction factors are applied to the Koss data in an effort to convert threshold estimates to dB SPL, individual transfer functions are not represented accurately. Thus the benefit of being able to express thresholds at high frequencies in dB SPL must be weighed against the additional source of variability introduced by the HFA calibration process.     

Sources of DPOAEs revealed by suppression experiments,inverse fast Fourier transforms,and SFOAEs in impaired ears

DPOAE sources are modeled by intermodulation distortion generated near the f2 place and a reflection of this distortion near the DP place. In a previous paper, inverse fast Fourier transforms (IFFTs) of DPOAE filter functions in normal ears were consistent with this model [Konrad-Martin et al., J. Acoust. Soc. Am. 109, 2862-2879 (2001)]. In the present article, similar measurements were made in ears with specific hearing-loss configurations. It was hypothesized that hearing loss at f2 or DP frequencies would influence the relative contributions to the DPOAE from the corresponding basilar membrane places, and would affect the relative magnitudes of SFOAEs at frequencies equal to f2 and fDP. DPOAEs were measured with f2 = 4 kHz, f1 varied, and a suppressor near fDP. L2 was 25-55 dB SPL (L1 = L2 + 10 dB). SFOAEs were measured at f2 and at 2.7 kHz (the average fDP produced by the f1 sweep) for stimulus levels of 20-60 dB SPL. SFOAE results supported predictions of the pattern of amplitude differences between SFOAEs at 4 and 2.7 kHz for sloping losses, but did not support predictions for the rising- and flat-loss categories. Unsuppressed IFFTs for rising losses typically had one peak. IFFTs for flat or sloping losses typically have two or more peaks; later peaks were more prominent in ears with sloping losses compared to normal ears. Specific predictions were unambiguously supported by the results for only four of ten cases, and were generally supported in two additional cases. Therefore, the relative contributions of the two DPOAE sources often were abnormal in impaired ears, but not always in the predicted manner.     

The influence of systematic primary-tone level variation L2-L1 on the acoustic distortion product emission 2f1-f2 in normal human ears

The purpose of the present study was to determine the effect of primary-tone level variation, L2--L1, on the amplitude of distortion-product otoacoustic emissions (DPOAEs). The DPOAE at the frequency 2f1--f2 (f2 greater than f1) was measured in 20 ears of ten normally hearing subjects. Acoustic distortion products were generated by primaries f1 and f2 with geometric mean frequencies of 1, 2, and 4 kHz. The f2/f1 ratios were 1.25 (1 kHz), 1.23 (2 kHz), and 1.21 (4 kHz). The primary-tone level L1 was kept constant at either 65 or 75 dB SPL while the second primary-tone level L2 was varied between 20 and 90 dB SPL in 5-dB steps. The level differences L2--L1 generating maximal DPOAE amplitudes depended on L1 and on the geometric mean frequency of f1 and f2. There were large interindividual differences. Overall, the L2--L1 evoking maximal mean DPOAE amplitudes was --10 dB for geometric mean frequencies of 1 and 2 kHz with both L1 = 65 dB SPL and L1 = 75 dB SPL. For 4 kHz, L2-L1 was --5 dB with L1 = 65 dB SPL and 0 dB with L1 = 75 dB SPL. The mean slopes of the DPOAE growth functions in the initial linearly increasing portions were steeper at higher stimulus frequencies, increasing from 0.52 at 1 kHz to 0.72 at 4 kHz for L1 = 65 dB SPL and from 0.48 at 1 kHz to 0.72 at 4 kHz for L1 = 75 dB SPL.     

Breaking away: violation of distortion emission phase-frequency invariance at low frequencies

The phase versus frequency function of the distortion product otoacoustic emission (DPOAE) at 2f(1) - f(2) is approximately invariant at frequencies above 1.5 kHz in human subjects when recorded with a constant f(2)/f(1). However, a secular break from this invariance has been observed at lower frequencies where the phase-gradient becomes markedly steeper. Apical DPOAEs, such as 2f(1)?- f(2), are known to contain contributions from multiple sources. This experiment asked whether the phase behavior of the ear canal DPOAE at low frequencies is driven by the phase of the component from the distortion product (DP) region at 2f(1)?- f(2), which exhibits rapid phase accumulation. Placing a suppressor tone close in the frequency to 2f(1)?- f(2) reduced the contribution of this component to the ear canal DPOAE in normal-hearing adult human ears. When the contribution of this component was reduced, the phase behavior of the ear canal DPOAE was not altered, suggesting that the breaking from DPOAE phase invariance at low frequencies is an outcome of apical-basal differences in cochlear mechanics. The deviation from DPOAE phase invariance appears to be a manifestation of the breaking from approximate scaling symmetry in the human cochlear apex.     

Critical modulation frequency based on detection of AM versus FM tones

The ratios between the modulation index (eta) for just noticeable FM of a sinusoidally modulated pure tone and the degree of modulation (m) for just noticeable AM at the same carrier and the same modulation frequency were measured at carrier frequencies of 0.125, 0.25, 0.5, 1, 2, 4, and 8 kHz. Signal levels were 20 dB SL and 50 dB SPL or 80 dB SPL. At low modulation frequencies, for example, 8 Hz, AM and FM elicit very different auditory sensations (i.e., a fluctuation in loudness or pitch, respectively). In this case, eta and m show different values for just noticeable modulation. Since both stimuli have almost equal amplitude spectra if eta equals m (m less than 0.3), the difference in detection thresholds reflects differences in the phase relation between carrier and sidebands in AM and FM. With increasing modulation frequency, the eta-m ratio decreases and reaches unity at a modulation frequency called the "critical modulation frequency" (CMF). At modulation frequencies above the CMF, the same modulation thresholds are obtained for AM and FM. Therefore, it can be concluded that the difference in phase between the two types of stimuli is not perceived in this range. At center frequencies below 1 kHz, where phase errors caused by headphones and ear canal presumably are small, the CMF is useful in estimating critical bandwidth.     

Theory of forward and reverse middle-ear transmission applied to otoacoustic emissions in infant and adult ears

The purpose of this study is to understand why otoacoustic emission (OAE) levels are higher in normal-hearing human infants relative to adults. In a previous study, distortion product (DP) OAE input/output (I/O) functions were shown to differ at f2 = 6 kHz in adults compared to infants through 6 months of age. These DPOAE I/0 functions were used to noninvasively assess immaturities in forward/reverse transmission through the ear canal and middle ear [Abdala, C., and Keefe, D. H., (2006). J. Acoust Soc. Am. 120, 3832-3842]. In the present study, ear-canal reflectance and DPOAEs measured in the same ears were analyzed using a scattering-matrix model of forward and reverse transmission in the ear canal, middle ear, and cochlea. Reflectance measurements were sensitive to frequency-dependent effects of ear-canal and middle-ear transmission that differed across OAE type and subject age. Results indicated that DPOAE levels were larger in infants mainly because the reverse middle-ear transmittance level varied with ear-canal area, which differed by more than a factor of 7 between term infants and adults. The forward middle-ear transmittance level was -16 dB less in infants, so that the conductive efficiency was poorer in infants than adults.     

Distortion-product otoacoustic-emission suppression tuning in human infants and adults using absorbed sound power

The greatest difference in distortion product otoacoustic emission (DPOAE) suppression tuning curves (STCs) in infant and adult ears occurs at a stimulus frequency of 6 kHz. These infant and adult STCs are much more similar when constructed using the absorbed power level of the stimulus and suppressor tones rather than using sound pressure level. This procedure incorporates age-related differences in forward and reverse transmission of sound power through the ear canal and middle ear. These results support the theory that the cochlear mechanics underlying DPOAE suppression are substantially mature in full-term infants.     

Further efforts to predict pure-tone thresholds from distortion product otoacoustic emission input/output functions

Recently, Boege and Janssen [J. Acoust. Soc. Am. 111, 1810-1818 (2002)] fit linear equations to distortion product otoacoustic emission (DPOAE) input/output (UO) functions after the DPOAE level (in dB SPL) was converted into pressure (in microPa). Significant correlations were observed between these DPOAE thresholds and audiometric thresholds. The present study extends their work by (1) evaluating the effect of frequency, (2) determining the behavioral thresholds in those conditions that did not meet inclusion criteria, and (3) including a wider range of stimulus levels. DPOAE I/O functions were measured in as many as 278 ears of subjects with normal and impaired hearing. Nine f2 frequencies (500 to 8000 Hz in 1/2-octave steps) were used, L2 ranged from 10 to 85 dB SPL (5-dB steps), and L1 was set according to the equation L1 = 0.4L2 + 39 dB [Kummer et al., J. Acoust. Soc. Am. 103, 3431-3444 (1998)] for L2 levels up to 65 dB SPL, beyond which L1 = L2. For the same conditions as those used by Boege and Janssen, we observed a frequency effect such that correlations were higher for mid-frequency threshold comparisons. In addition, a larger proportion of conditions not meeting inclusion criteria at mid and high frequencies had hearing losses exceeding 30 dB HL, compared to lower frequencies. These results suggest that DPOAE I/O functions can be used to predict audiometric thresholds with greater accuracy at mid and high frequencies, but only when certain inclusion criteria are met. When the SNR inclusion criterion is not met, the expected amount of hearing loss increases. Increasing the range of input levels from 20-65 dB SPL to 10-85 dB SPL increased the number of functions meeting inclusion criteria and increased the overall correlation between DPOAE and behavioral thresholds.     

Level dependence of distortion-product otoacoustic emissions measured at high frequencies in humans

Given that high-frequency hearing is most vulnerable to cochlear pathology, it is important to characterize distortion-product otoacoustic emissions (DPOAEs) measured with higher-frequency stimuli in order to utilize these measures in clinical applications. The purpose of this study was to explore the dependence of DPOAE amplitude on the levels of the evoking stimuli at frequencies greater than 8 kHz, and make comparisons with those data that have been extensively measured with lower-frequency stimuli. To accomplish this, DPOAE amplitudes were measured at six different f2 frequencies (2, 5, 10, 12, 14, and 16 kHz), with a frequency ratio (f2/f1) of 1.2, at five fixed levels (30 to 70 dB SPL) of one primary (either f1 or f2), while the other primary was varied in level (30 to 70 dB SPL). Generally, the level separation between the two primary tones (L1 > L2) generating the largest DPOAE amplitude (referred to as the "optimal level separation") decreased as the level of the fixed primary increased. Additionally, the optimal level separation was frequency dependent, especially at the lower fixed primary tone levels ( < or = 50 dB SPL). In agreement with previous studies, the DPOAE level exhibited greater dependence on L1 than on L2.     

Distortion product otoacoustic emissions for hearing threshold estimation and differentiation between middle-ear and cochlear disorders in neonates

Our aim in the present study was to apply extrapolated DPOAE I/O-functions [J. Acoust. Soc. Am. 111, 1810-1818 (2002); 113, 3275-3284 (2003)] in neonates in order to investigate their ability to estimate hearing thresholds and to differentiate between middle-ear and cochlear disorders. DPOAEs were measured in neonates after birth (mean age = 3.2 days) and 4 weeks later (follow-up) at 11 test frequencies between f2 = 1.5 and 8 kHz and compared to that found in normal hearing subjects and cochlear hearing loss patients. On average, in a single ear hearing threshold estimation was possible at about 2/3 of the test frequencies. A sufficient test performance of the approach is therefore suggested. Thresholds were higher at the first measurement compared to that found at the follow-up measurement. Since thresholds varied with frequency, transitory middle ear dysfunction due to amniotic fluid instead of cochlear immaturity is suggested to be the cause for the change in thresholds. DPOAE behavior in the neonate ears differed from that found in the cochlear hearing loss ears. From a simple model it was concluded that the difference between the estimated DPOAE threshold and the DPOAE detection threshold is able to differentiate between sound conductive and cochlear hearing loss.     

Level dependence of distortion product otoacoustic emission phase is attributed to component mixing

Distortion product otoacoustic emissions (DPOAEs) measured in the ear canal represent the vector sum of components produced at two regions of the basilar membrane by distinct cochlear mechanisms. In this study, the effect of stimulus level on the 2f(1)?- f(2) DPOAE phase was evaluated in 22 adult subjects across a three-octave range. Level effects were examined for the mixed DPOAE signal measured in the ear canal and after unmixing components to assess level effects individually on the distortion (generated at the f(1), f(2) overlap) and reflection (at f(dp)) sources. Results show that ear canal DPOAE phase slope becomes steeper with decreasing level; however, component analysis further explicates this result, indicating that interference between DPOAE components (rather than a shift in mechanics related to distortion generation) drives the level dependence of DPOAE phase measured in the ear canal. The relative contribution from the reflection source increased with decreasing level, producing more component interference and, at times, a reflection-dominated response at the lowest stimulus levels. These results have implications for the use of DPOAE phase to study cochlear mechanics and for the potential application of DPOAE phase for clinical purposes.