Otoacoustic Emissions In Humans Research Articles

Examining the Factors that Contribute to Non-Monotonic Growth of the [Formula: see text]Otoacoustic Emission in Humans.

Cubic distortion product otoacoustic emission input-output functions in humans show a complex pattern of growth. To further investigate the growth of the [Formula: see text] otoacoustic emission, magnitude and phase input-output functions were obtained from human subjects using a range of stimulus levels, frequencies, and frequency ratios. Three factors related to cochlear nonlinearity may produce non-monotonic input-output functions: a two-component interaction, an operating point shift, and two-tone suppression. To complement data interpretation, a local model of distortion product otoacoustic emission generation was fit to the magnitude spectrum of the averaged ear canal sound pressure recording to quantify operating point shift. Results obtained are consistent with non-monotonic growth occurring primarily as a result of two-tone suppression and/or a two-component interaction. These two mechanisms are expected to operate at different stimulus levels, with different signature magnitude and phase patterns, and are unlikely to overlap in producing non-monotonic growth. An operating point shift was suggested in three cases. These results support multiple factors contributing to the complexity of growth of the [Formula: see text] otoacoustic emission in humans and highlight the importance of looking at phase in addition to magnitude when interpreting distortion product otoacoustic emission growth.

Investigation of distortion product otoacoustic emission in humans using a nonlinear mechano-electro-acoustic model of the cochlea

Nonlinear response component (2f1-f2) measured in the ear canal when two primary tones at f1 and f2 are presented to the mammalian cochlea, is termed as distortion product otoacoustic emission (DPOAE). Whether the mechanism of propagation of distortion product (DP) from the cochlea toward the middle ear, via fast compression waves in fluid or via slow traveling waves on the basilar membrane, is still being debated. In the current study, the computational mechano-electro-acoustic nonlinear cochlea model is used to predict the location and propagation of DP. The three-dimensional model has two fluid-filled ducts coupled with BM and OHC somatic feedback motility coupled to nonlinear HB mechanotransduction, which is the source of nonlinear distortion in the model. The Puria-Allen transmission line model is used for the middle ear. For solving the nonlinear cochlea model dynamics, the alternating time-frequency method is used. The model parameters are adapted to the human cochlea to help understand DPOAE in humans. Preliminary results based on the model will be discussed.

Medial olivocochlear reflex effects on amplitude growth functions of long- and short-latency components of click-evoked otoacoustic emissions in humans.

Functional outcomes of medial olivocochlear reflex (MOCR) activation, such as improved hearing in background noise and protection from noise damage, involve moderate to high sound levels. Previous noninvasive measurements of MOCR in humans focused primarily on otoacoustic emissions (OAEs) evoked at low sound levels. Interpreting MOCR effects on OAEs at higher levels is complicated by the possibility of the middle-ear muscle reflex and by components of OAEs arising from different locations along the length of the cochlear spiral. We overcame these issues by presenting click stimuli at a very slow rate and by time-frequency windowing the resulting click-evoked (CE)OAEs into short-latency (SL) and long-latency (LL) components. We characterized the effects of MOCR on CEOAE components using multiple measures to more comprehensively assess these effects throughout much of the dynamic range of hearing. These measures included CEOAE amplitude attenuation, equivalent input attenuation, phase, and slope of growth functions. Results show that MOCR effects are smaller on SL components than LL components, consistent with SL components being generated slightly basal of the characteristic frequency region. Amplitude attenuation measures showed the largest effects at the lowest stimulus levels, but slope change and equivalent input attenuation measures did not decrease at higher stimulus levels. These latter measures are less commonly reported and may provide insight into the variability in listening performance and noise susceptibility seen across individuals.NEW & NOTEWORTHY The auditory efferent system, operating at moderate to high sound levels, may improve hearing in background noise and provide protection from noise damage. We used otoacoustic emissions to measure these efferent effects across a wide range of sound levels and identified level-dependent and independent effects. Previous reports have focused on level-dependent measures. The level-independent effects identified here may provide new insights into the functional relevance of auditory efferent activity in humans.

Sweep-tone evoked stimulus frequency otoacoustic emissions in humans: Development of a noise-rejection algorithm and normative features

In recent years, there has been a growing interest to measure stimulus frequency otoacoustic emissions (SFOAEs) using sweep tones. While there are several advantages of the sweep-tone technique, one of the major problems with sweep-tone methodologies is the lack of an objective analysis procedure that considers and rejects individual noisy recordings or noisy segments. A new efficient data-driven method for rejecting noisy segments in SFOAE analysis is proposed and the normative features of SFOAEs are characterized in fifty normal-hearing young adults. The automated procedure involved phase detrending with a low-order polynomial and application of median and interquartile ranges for data outlier rejection from individual recordings. The SFOAE level and phase were analyzed using the least-squared fit method, and the noise floor was estimated using the error of the mean of the sweep level. Overall, the results of this study demonstrated the effectiveness of the automated noise rejection procedure and described the normative features of sweep-tone evoked SFOAEs in human adults.

Profiles of Stimulus-Frequency Otoacoustic Emissions from 0.5 to 20kHz in Humans.

The characteristics of human otoacoustic emissions (OAEs) have not been thoroughly examined above the standard audiometric frequency range (>8kHz). This is despite the fact that deterioration of cochlear function often starts at the basal, high-frequency end of the cochlea before progressing apically. Here, stimulus-frequency OAEs (SFOAEs) were obtained from 0.5 to 20kHz in 23 young, audiometrically normal female adults and three individuals with abnormal audiograms, using a low-to-moderate probe level of 36dB forward pressure level (FPL). In audiometrically normal ears, SFOAEs were measurable at frequencies approaching the start of the steeply sloping high-frequency portion of the audiogram (∼12-15kHz), though their amplitudes often declined substantially above ∼7kHz, rarely exceeding 0dB SPL above 8kHz. This amplitude decline was typically abrupt and occurred at a frequency that was variable across subjects and not strongly related to the audiogram. In contrast, certain ears with elevated mid-frequency thresholds but regions of normal high-frequency sensitivity could possess surprisingly large SFOAEs (>10dB SPL) above 7kHz. When also measured, distortion-product OAEs (DPOAEs) usually remained stronger at higher stimulus frequencies and mirrored the audiogram more closely than SFOAEs. However, the high-frequency extent of SFOAE and DPOAE responses was similar when compared as a function of the response frequency, suggesting that middle ear transmission may be a common limiting factor at high frequencies. Nevertheless, cochlear factors are more likely responsible for complexities observed in high-frequency SFOAE spectra, such as abrupt amplitude changes and narrowly defined response peaks above 10kHz, as well as the large responses in abnormal ears. These factors may include altered cochlear reflectivity due to subtle damage or the reduced spatial extent of the SFOAE generation region at the cochlear base. The use of higher probe levels is necessary to further evaluate the characteristics and potential utility of high-frequency SFOAE measurements.

Oxidative stress biomarkers and otoacoustic emissions in humans exposed to styrene and noise

Objective: Evaluating the correlation between otoacoustic emission levels, styrene exposure, and oxidative stress biomarkers concentration in styrene-exposed subjects, to investigate the role of oxidative stress in outer hair cell damage. Design: Distortion product otoacoustic emissions were measured in the exposed workers and in a control group. Separation between the distortion and reflection otoacoustic components was performed by time-frequency-domain filtering. The urinary concentration of the DNA and RNA oxidation products, namely 8-oxo-7,8-dihydroguanine (oxoGua), 8-oxo-7,8-dihydro-2′-deoxyguanosine (oxodGuo), and 8-oxo-7,8-dihydroguanosine (oxoGuo), were evaluated. Study sample: Nine subjects exposed to styrene in a fiberglass factory, eight control subjects. The two groups were statistically equivalent in mean age. Results: Statistically significant differences were found in the distortion component levels between the exposed and the control group. High levels of the oxidative damage biomarkers were found in the workers exposed to high levels of styrene. Significant negative correlation was found between the otoacoustic emission distortion component levels and the concentration of the oxoGuo biomarker. Conclusions: Exposure-induced damage of the cochlear amplifier is shown in the mid-frequency range, confirming animal experiments, in which hair cells in the cochlear middle turn were damaged. Hearing damage is consistent with the outer hair cell apoptosis pathway associated with oxidative stress.

Multi-tone suppression of distortion-product otoacoustic emissions in humans.

The purpose of this study was to investigate the combined effect of multiple suppressors. Distortion-product otoacoustic emission (DPOAE) measurements were made in normal-hearing participants. Primary tones had fixed frequencies (f2 = 4000 Hz; f1 / f2 = 1.22) and a range of levels. Suppressor tones were at three frequencies (fs = 2828, 4100, 4300 Hz) and range of levels. Decrement was defined as the attenuation in DPOAE level due to the presence of a suppressor. A measure of suppression called suppressive intensity was calculated by an equation previously shown to fit DPOAE suppression data. Suppressor pairs, which were the combination of two different frequencies, were presented at levels selected to have equal single-suppressor decrements. A hybrid model that represents a continuum between additive intensity and additive attenuation best described the results. The suppressor pair with the smallest frequency ratio produced decrements that were more consistent with additive intensity. The suppressor pair with the largest frequency ratio produced decrements at the highest level that were consistent with additive attenuation. Other suppressor-pair conditions produced decrements that were intermediate between these two alternative models. The hybrid model provides a useful framework for representing the observed range of interaction when two suppressors are combined.

Level dependence of the nonlinear-distortion component of distortion-product otoacoustic emissions in humans.

Distortion-product otoacoustic emissions (DPOAEs) emerge when presenting two primary tones with different frequencies f1 and f2 to the cochlea and are commonly used in diagnosis and research to evaluate the functional state of the cochlea. Optimal primary-tone stimulus levels accounting for the different level dependencies of the traveling-wave amplitudes of the two primary tones near the f2-tonotopic place on the basilar membrane are often used to maximize DPOAE amplitudes. However, parameters defining the optimal levels can be affected by wave interference between the nonlinear-distortion and coherent-reflection components of the DPOAE. Here, the components were separated in the time domain using a pulsed stimulus paradigm and optimal levels determined. Based on the amplitude dependence of the nonlinear-distortion components on primary-tone stimulus levels, level parameters yielding maximum DPOAE amplitudes were derived for six normal-hearing adults and compared to data recorded with continuous two-tone stimulation. The level parameters resulting from analysis of the nonlinear-distortion components show dependence on stimulus frequency and small standard deviations. DPOAE input/output functions derived for optimal levels exhibit larger slopes, wider dynamic range and less variability across subjects than those derived for conventional stimulus and analysis conditions, potentially increasing their reliability and sensitivity for assessing cochlea function.

Effects of sevoflurane and desflurane on otoacoustic emissions in humans.

Otoacoustic emissions (OAEs) are non-invasive, easy to apply and objective test methods which are widely used to determine the presence of hearing in audiology clinics. Under certain circumstances, the study should be applied under general anesthesia. The aim of this study was to determine the influence of new short-acting inhalation agents, desflurane and sevoflurane, on OAE in humans. These short-acting agents are widely used in general anesthesia. Thirty-one healthy patients who underwent septoplasty and turbinoplasty surgery were included in this study. Unpremedicated patients were anesthetized and monitored by a standard protocol except the inhalation agents. Desflurane and sevoflurane were added to the inhaled gas mixture at ~1MAC, 5-6 % and 1.5-2 %, respectively. Transient evoked otoacoustic emissions and distortion product otoacoustic emissions measured in both ears of each patient preoperatively in the operating room before induction, 5 min after induction, after the completion of surgical procedure while the anesthetic agents are still given and 1 h after surgery in the ward. Between-group and within-group comparisons and correlations with hemodynamic parameters were performed for statistical analysis. The measurements of 26 ears in desflurane group and 28 ears in sevoflurane group were evaluated. There were no differences in initial measurements between groups (p > 0.05). Both groups presented significant decrease in intraoperative measurements and changes in time were statistically significant (p < 0.05). The changes in OAEs were similar to changes in systemic blood pressures. Correlation between OAEs and systemic blood pressures were significant (p < 0.05). In conclusion, sevoflurane and desflurane decreased OAEs around 2-3 dB; OAEs are still measurable under inhalation agents. This provides some findings about the OAE status of patient, but the evaluations should be done with the impact of anesthetic agents in mind.

Human otoacoustic emissions generated by active outer hair cells

In the present study, human otoacoustic emissions (OAEs) documented in the literature are shown to agree with predictions based on the hypothesis that the main OAE sources are active cochlear outer hair cells (OHCs) of three different functional categories, namely (1) OHCs enabled by oscillating internal organ-of-Corti resonators (IOCRs) to feed mechanical energy into forward-traveling cochlear waves generated by the acoustic stimuli used, (2) OHCs driving spontaneous localized feedback-generated cochlear-partition vibrations involving standing evanescent sound-pressure waves in the liquids above and below the partition, and (3) OHCs causing nonlinear restoring forces enabling pairs of stationary tones to generate distortion products (DPs). The corresponding predictions of OAE properties are based on cochlear maps, i.e., on certain functions x(f), where f is the frequency of a tone and x is a related distance from the cochlear base, measured along the cochlear channel.

Suppression tuning of distortion product otoacoustic emissions in humans: results from cochlear mechanics simulation

Is human hearing more sharply tuned than other mammals? This has been a heavily debated subject in the field of cochlear mechanics. The debate continues partially due to lack of non-invasive methods to estimate human cochlear tuning accurately. Recently, Gorga et al. (2011, JASA) derived tuning curves from suppression of distortion product (DP) otoacoustic emissions (OAEs) in normal-hearing human ears. Frequencies of the primary tones were varied from 0.5 to 8 kHz. Sharpness of tuning was analyzed in terms of the Q-value of equivalent rectangular bandwith, which ranged from 4 to 10 at the lowest stimulus level tested. The Q-values were similar to that of psychoacoustic tuning but lower than inferred from latencies of stimulus-frequency OAEs (Shera et al. 2002, PNAS). In the present work, we simulate DPOAE suppression based on a computer model of cochlear mechanics (Liu and Neely, 2010, JASA). The simulated DPOAE suppression tuning curves (STCs) resemble those obtained in experiments but discrepancies remain. At high frequencies, the simulated DPOAE STCs are not as sharply tuned as the magnitude response of traveling waves in the model. Confounding factors and interpretation of results will be discussed. (Supported by Taiwan's NSC and NTHU)

Low-frequency modulated quadratic and cubic distortion product otoacoustic emissions in humans

Previous studies have used low-frequency tones to modulate distortion product otoacoustic emissions (DPOAEs). The cubic DPOAE (CDPOAE) is mostly chosen because amplitudes sufficient for modulation can be evoked with moderate sound pressure levels. Quadratic DPOAEs (QDPOAEs) however, are more sensitive to minute changes of the cochlear operating point (OP) and are better suited to assess changes of the cochlear OP.Here, we compare the properties of low-frequency (30 Hz, 80–120 dB SPL) modulated CDPOAE and QDPOAEs evoked with f2 = 2 and 5 kHz in human subjects with normal hearing. The modulation depth was quantified with the modulation index (MI), a measure which considers both amplitude and phase.Modulated CDPOAEs evoked with f2 = 2 kHz have amplitude maxima at the zero crossings and amplitude minima at the extremes of the biasing tone (BT) which correlate positively with the BT level. CDPOAEs evoked with f2 = 5 kHz were recorded during biasing in exactly the same way as described before. At the highest BT levels used (120 dB SPL), very little modulation could be detected. Not only the depth, but also the shape of the QDPOAE modulation pattern is correlated with the BT level. At moderate BT levels (about 90–100 dB SPL) QDPOAEs evoked with f2 = 5 kHz show one amplitude notch around the zero crossing of the positive going flank of the BT (a single modulation pattern). At and above a BT level of about 105 dB SPL, the pattern reverses and shows a double modulation pattern. At the highest BT level used (120 dB SPL), quadratic MIs exceed cubic MIs (2.0 ± 0.5 and 0.97 ± 0.06, respectively).Patterns of low-frequency modulated QDPOAEs in humans are similar to the modulation seen in animal studies and as predicted by mathematical models. Human low-frequency modulated QDPOAEs are ideally suited to estimate cochlear OP shifts because of their high sensitivity to the OP shift.

Interrelationships between spontaneous and low-level stimulus-frequency otoacoustic emissions in humans

It has been proposed that OAEs be classified not on the basis of the stimuli used to evoke them, but on the mechanisms that produce them (Shera and Guinan, 1999). One branch of this taxonomy focuses on a coherent reflection model and explicitly describes interrelationships between spontaneous emissions (SOAEs) and stimulus-frequency emissions (SFOAEs). The present study empirically examines SOAEs and SFOAEs from individual ears within the context of model predictions, using a low stimulus level (20 dB SPL) to evoke SFOAEs. Emissions were recorded from ears of normal-hearing young adults, both with and without prominent SOAE activity. When spontaneous activity was observed, SFOAEs demonstrated a localized increase about the SOAE peaks. The converse was not necessarily true though, i.e., robust SFOAEs could be measured where no SOAE peaks were observed. There was no significant difference in SFOAE phase-gradient delays between those with and without observable SOAE activity. However, delays were larger for a 20 dB SPL stimulus level than those previously reported for 40 dB SPL. The total amount of SFOAE phase accumulation occurring between adjacent SOAE peaks tended to cluster about an integral number of cycles. Overall, the present data appear congruous with predictions stemming from the coherent reflection model and support the notion that such comparisons ideally are made with emissions evoked using relatively lower stimulus levels.

On- and Off-Frequency Forward Masking by Schroeder-Phase Complexes

Forward masking by harmonic tone complexes was measured for on- and off-frequency maskers as a function of masker phase curvature for two masker durations (30 and 200 ms). For the lowest signal frequency (1 kHz), the results matched predictions based on the expected interactions between the phase curvature and amplitude compression of peripheral auditory filtering. For the higher signal frequencies (2 and 6 kHz), the data increasingly departed from predictions in two respects. First, the effects of the masker phase curvature became stronger with increasing masker duration, inconsistent with the expected effects of the fast-acting compression and time-invariant phase response of basilar membrane filtering. Second, significant effects of masker phase curvature were observed for the off-frequency masker using a 6-kHz signal, inconsistent with predictions based on linear processing of stimuli well below the signal frequency. New predictions were generated assuming an additional effect with a longer time constant, consistent with the influence of medial olivocochlear efferent activation on otoacoustic emissions in humans. Reasonable agreement between the predicted and the measured effects suggests that efferent activation is a potential candidate mechanism to explain certain spectro-temporal masking effects in human hearing.

Influence of contralateral acoustic stimulation on the quadratic distortion product f2– f1 in humans

Contralateral acoustic stimulation is known to activate the medial olivocochlear system which is capable of modulating the amplification process in the outer hair cells of the inner ear. We investigated the influence of different levels of contralateral broadband noise on distortion product otoacoustic emissions in humans, with a particular focus on the quadratic distortion product at f2– f1. The primary stimulus frequency ratio was optimized to yield maximum f2– f1 level. While the cubic distortion product at 2 f1 –f2 was not significantly affected during contralateral noise stimulation, the level of f2– f1 was reduced by up to 4.8 dB on average (maximum: 10.1 dB), with significant suppression occurring for noise levels as low as 40 dB SPL. In addition, a significant phase lead was observed. Quadratic distortions are minimal at a symmetrical position of the transfer function of the cochlear amplifier. The observed sensitivity of f2– f1 to contralateral noise stimulation could hence be resulting from a shift of the operating state and/or a change in the gain of the cochlear amplification due to contralateral induced efferent modulation of the outer hair cell properties.

Contralateral noise has possible asymmetric frequency-sensitive effect on the 2F1-F2 otoacoustic emission in humans

Contralateral acoustic stimulation has been shown to produce a suppressive effect on the 2F1-F2 distortion-product otoacoustic emission (DPOAE). According to reports by other studies, there is a frequency-specific suppressive region between 1 and 2kHz when the contralateral broadband or narrowband noise stimulation contains energy close to or near the F2 frequencies in that region. In general the broader the bandwidth of the noise stimulation, the greater is the suppressive effect. A frequency-sensitive region between 1 and 2kHz has also been suggested as narrowband noises with center frequencies remote from the F2 frequency seems to produce suppression in the same defined region. In order to test this hypothesis, narrowband noises wide enough to elicit suppression with center frequencies unmatched to the F2 frequencies were employed in the present study. For the right ear, a strong frequency-sensitive suppressive region was noted for the F2 frequency at 2kHz, but not at 1 and 4kHz. The left ear had some suppression at 1kHz, but this effect was not as robust as seen in the right ear. The apparent suppressive differences between ears suggest a possible MOC reflex asymmetry that has not previously been described for DPOAEs. These results suggest that physiological differences between ears and possibly handedness must be taken into account when examining contralateral suppression to narrowband noises. Furthermore, the frequency contributions of the contralateral stimulus may have a greater complex interaction with the cochlear physiology than previously considered.

Low-frequency modulation of distortion product otoacoustic emissions in humans

Low-frequency modulation of distortion product otoacoustic emissions (DPOAEs) was measured from the human ears. In the frequency domain, increasing the bias tone level resulted in a suppression of the cubic difference tone (CDT) and an increase in the magnitudes of the modulation sidebands. Higher-frequency bias tones were more efficient in producing the suppression and modulation. Quasi-static modulation patterns were derived from measuring the CDT amplitude at the peaks and troughs of bias tones with various amplitudes. The asymmetric bell-shaped pattern resembled the absolute value of the third derivative of a nonlinear cochlear transducer function. Temporal modulation patterns were obtained from inverse FFT of the spectral contents around the DPOAE. The period modulation pattern, averaged over multiple bias tone cycles, showed two CDT peaks each correlated with the zero-crossings of the bias tone. The typical period modulation pattern varied and the two CDT peaks emerged with the reduction in bias tone level. The present study replicated the previous experimental results in gerbils. This noninvasive technique is capable of revealing the static position and dynamic motion of the cochlear partition. Moreover, the results of the present study suggest that this technique could potentially be applied in the differential diagnosis of cochlear pathologies.

Overexposure effects of a 1-kHz tone on the distortion product otoacoustic emission in humans

The effects of overexposure on the properties of distortion product otoacoustic emissions (DPOAEs) are investigated. In total, 39 normal-hearing humans were monaurally exposed to a 1-kHz tone lasting for 3 min at an equivalent threshold sound-pressure level of 105.5 dB. The effects of overexposure were studied in two experiments (1) on the broadband DPOAE and (2) on the DPOAE fine structure, measured using a higher frequency resolution in a narrower frequency range. The obtained DPOAE shifts were compared to temporary threshold shift (TTS) obtained after a similar exposure. Similarities between DPOAE shifts and TTS were found in the affected frequency range and the time course of recovery. The amount of TTS was higher in the early recovery time (1-4-min postexposure), but similar to the DPOAE shift (even in absolute terms) at later recovery times (5-20-min postexposure). The DPOAE fine structure was not systematically changed after the exposure.

Mechanisms of generation of the 2f2–f1 distortion product otoacoustic emission in humans

The 2f1-f2 distortion product otoacoustic emission (DPOAE) is considered to consist of two components in normally hearing ears, one having constant phase with changing DP frequency (wave fixed) and one having an increasing phase lag with increasing frequency (place fixed). The aim was to identify the wave-fixed and place-fixed components of both 2f1-f2 and 2f2-f1 DPs, and, in particular, to show whether a wave-fixed 2f2-f1 DP exists in normally hearing adults. DPOAE recordings were made in 20 ears of normally hearing young adults. Four frequency ratios were used and recording entailed fixed frequency-ratio sweeps. A separation into wave-fixed and place-fixed components was carried out using a time-window separation method. A method for estimating the noise floor after data processing was developed. Results confirmed the presence of wave-fixed and place-fixed components for 2f1-f2, consistent with previous studies. Both components were also present for 2f2-f1 in virtually all subjects. This latter finding conflicts with current models of DPOAE generation, and so a modified model is proposed. Unlike the 2f1-f2 emission, which has a wave-fixed component that is strongly dependent on the frequency ratio, neither component of the 2f2-f1 emission showed such a dependence. The proposed model explains these findings in terms of the overlap of the primary frequency traveling waves.

Reducing reflected contributions to ear-canal distortion product otoacoustic emissions in humans

Distortion product otoacoustic emission (DPOAE) fine structure has been attributed to the interaction of two cochlear-source mechanisms (distortion and reflection sources). A suppressor presented near the 2f1-f2 frequency reduces the reflection-source contribution and, therefore, DPOAE fine structure. Optimal relationships between stimulus and suppressor conditions, however, have not been described. In this study, the relationship between suppressor level (L3) and stimulus level (L2) was evaluated to determine the L3 that was most effective at reducing fine structure. Subjects were initially screened to find individuals who produced DPOAE fine structure. A difference in the prevalence of fine structure in two frequency intervals was observed. At 2 kHz, 11 of 12 subjects exhibited fine structure, as compared to 5 of 22 subjects at 4 kHz. Only subjects demonstrating fine structure participated in subsequent measurements. DPOAE responses were evaluated in 1/3-octave intervals centered at 2 or 4 kHz, with 4 subjects contributing data at each interval. Multiple L3's were evaluated for each L2, which ranged from 20 to 80 dB SPL. The results indicated that one or more L3's at each L2 were roughly equally effective at reducing DPOAE fine structure. However, no single L3 was effective at all L2's in every subject.