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.     

Effects of low-frequency biasing on spontaneous otoacoustic emissions: amplitude modulation

The dynamic effects of low-frequency biasing on spontaneous otoacoustic emissions (SOAEs) were studied in human subjects under various signal conditions. Results showed a combined suppression and modulation of the SOAE amplitudes at high bias tone levels. Ear-canal acoustic spectra demonstrated a reduction in SOAE amplitude and growths of sidebands while increasing the bias tone level. These effects varied depending on the relative strength of the bias tone to a particular SOAE. The SOAE magnitudes were suppressed when the cochlear partition was biased in both directions. This quasi-static modulation pattern showed a shape consistent with the first derivative of a sigmoid-shaped nonlinear function. In the time domain, the SOAE amplitudes were modulated with the instantaneous phase of the bias tone. For each biasing cycle, the SOAE envelope showed two peaks each corresponded to a zero crossing of the bias tone. The temporal modulation patterns varied systematically with the level and frequency of the bias tone. These dynamic behaviors of the SOAEs are consistent with the shifting of the operating point along the nonlinear transducer function of the cochlea. The results suggest that the nonlinearity in cochlear hair cell transduction may be involved in the generation of SOAEs.     

Behaviors of cubic distortion product otoacoustic emissions evoked by amplitude modulated tones

Distortion product otoacoustic emissions (DPOAEs) were measured using sinusoidal amplitude modulation (AM) tones. When one of the primary stimuli (f(1) or f(2), f(1)?< f(2)) was amplitude modulated, a series of changes in the cubic difference tone (CDT) were observed. In the frequency domain, multiple sidebands were present around the CDT and their sizes grew with the modulation depth of the AM stimulus. In the time domain, the CDT showed different modulation patterns between two major signal conditions: the AM tone was used as the f(1) or the f(2). The CDT amplitude followed the AM tone when the f(1) was amplitude modulated. However, when the AM tone acted as the f(2), the CDT showed a more complex modulation pattern with a notch present at the AM tone peak. The relatively linear dependence of CDT on f(1) and the nonlinear relation with f(2) can be explained with a variable gain-control model representing hair cell functions at the DPOAE generation site. It is likely that processing of AM signals at a particular cochlear location depends on whether the hair cells are tuned to the frequency of the carrier. Nonlinear modulation is related to on-frequency carriers and off-frequency carriers are processed relatively linearly.     

Cochlear compression: effects of low-frequency biasing on quadratic distortion product otoacoustic emission

Distortion product otoacoustic emissions (DPOAEs) are generated from the nonlinear transduction n cochlear outer hair cells. The transducer function demonstrating a compressive nonlinearity can be estimated from low-frequency modulation of DPOAEs. Experimental results from the gerbils showed that the magnitude of quadratic difference tone (QDT, f2-f1) was either enhanced or suppressed depending on the phase of the low-frequency bias tone. Within one period of the bias tone, QDT magnitudes exhibited two similar modulation patterns, each resembling the absolute value of the second derivative of the transducer function. In the time domain, the center notches of the modulation patterns occurred around the zero crossings of the bias pressure, whereas peaks corresponded to the increase or decrease in bias pressure. Evaluated with respect to the bias pressure, modulated QDT magnitude displayed a double-modulation pattern marked by a separation of the center notches. Loading/unloading of the cochlear transducer or rise/fall in bias pressure shifted the center notch to positive or negative sound pressures, indicating a mechanical hysteresis. These results suggest that QDT arises from the compression that coexists with the active hysteresis in cochlear transduction. Modulation of QDT magnitude reflects the dynamic regulation of cochlear transducer gain and compression.     

Constructing a cochlear transducer function from the summating potential using a low-frequency bias tone

A new method is developed to construct a cochlear transducer function using modulation of the summating potential (SP), a dc component of the electrical response of the cochlea to a sinusoid. It is mathematically shown that the magnitude of the SP is determined by the even-order terms of the power series representing a nonlinear function. The relationship between the SP magnitudes and the second derivative of the transducer function was determined by using a low-frequency bias tone to position a high-frequency probe tone at different places along the cochlear transducer function. Two probe tones (6 kHz and 12 kHz) ranging from 70 to 90 dB SPL and a 25-Hz bias tone at 130 dB SPL were simultaneously presented. Electric responses from the cochlea were recorded by an electrode placed at the round window to obtain the SP magnitudes. The experimental results from eight animals demonstrated that the SP magnitudes as a function of bias levels are essentially proportional to the second derivative of a sigmoidal Boltzmann function. This suggests that the low-frequency modulated SP amplitude can be used to construct a cochlear transducer function.     

Influence of primary frequencies ratio on distortion product otoacoustic emissions amplitude. II. Interrelations between multicomponent DPOAEs, tone-burst-evoked OAEs, and spontaneous OAEs

Distortion product otoacoustic emissions (DPOAEs) are used widely in humans to assess cochlear function. It is well known that 2f1-f2 DPOAE amplitude increases as the f2/f1 ratio increases from 1.0 to about 1.20, and then decreases as the f2/f1 ratio increases above 1.20, showing an amplitude ratio function, which is thought to be related to cochlear filtering properties. Different lower sideband DPOAEs are believed to show the same amplitude ratio functions as the 2f1-f2 DPOAE, with a magnitude peak situated at a constant DPOAE frequency relative to f2. More recently, several studies have suggested the involvement of a DPOAE component coming from its own distortion product place as well as the DPOAE component coming from the f2 place. To investigate DPOAE generation sites and the importance of the DPOAE frequency place, amplitude ratio functions of 2f1-f2, 3f1-2f2, 4f1-3f2 and 2f2-f1, 3f2-2f1, 4f2-3f1 DPOAE components have been systematically studied in 18 normally hearing subjects, using an f2 fixed, f1 sweep method, and an f1 fixed, f2 sweep method, at ten different f2 frequencies. Results show a dependency of the distortion magnitude peak on f2 frequency for each lower sideband DPOAE, and a small frequency shift of the distortion peak for the high order lower sideband DPOAE components. Strong correlation between the different lower sideband DPOAE amplitude were obtained, whether they were recorded with the same f1 (and a different f2) or with the same f2 (and a different f1), suggesting that lower side-band DPOAE amplitude does not depend on small variations in the f2 frequency. Moreover, correlations between DPOAE amplitude and tone-burst evoked otoacoustic emissions (TBOAEs) are highly significant for TBOAEs centered at the f2 frequency and at 1/2 octave below the f2 frequency, suggesting some degree of importance of the cochlear status at frequencies below f2 in DPOAE amplitude. Subjects presenting spontaneous otoacoustic emissions showed a greater lower sideband DPOAE amplitude recorded for low f2/f1 ratios, and a distortion magnitude peak shifted towards higher frequencies. The best correlation between upper sideband DPOAE amplitude and lower sideband DPOAE amplitude occurred for lower sideband DPOAEs generated by an f2 frequency 1/2 octave to 1 octave below the primaries used to generate upper sideband DPOAEs, suggesting a site of generation basal to f2 for the upper sideband DPOAEs. Correlations between TBOAE amplitude and upper sideband DPOAE amplitude agreed with a site of upper sideband DPOAE generation basal to f2, and which would move with the DPOAE frequency itself.     

Deriving a cochlear transducer function from low-frequency modulation of distortion product otoacoustic emissions

In this paper, a new method is introduced to derive a cochlear transducer function from measuring distortion product otoacoustic emissions (DPOAEs). It is shown that the cubic difference tone (CDT, 2f1-f2) is produced from the odd-order terms of a power series that approximates a nonlinear function characterizing cochlear transduction. Exploring the underlying mathematical formulation, it is found that the CDT is proportional to the third derivative of the transduction function when the primary levels are sufficiently small. DPOAEs were measured from nine gerbils in response to two-tone signals biased by a low-frequency tone with different amplitudes. The CDT magnitude was obtained at the peak regions of the bias tone. The results of the experiment demonstrated that the shape of the CDT magnitudes as a function of bias levels was similar to the absolute value of the third derivative of a sigmoidal function. A second-order Boltzmann function was derived from curve fitting the CDT data with an equation that represents the third derivative of the Boltzmann function. Both the CDT-bias function and the derived nonlinear transducer function showed effects of primary levels. The results of the study indicate that the low-frequency modulated DPOAEs can be used to estimate the cochlear transducer function.     

Maturation of medial efferent system function in humans

Otoacoustic emissions are typically reduced in amplitude when broadband noise is presented to the contralateral ear. This contralateral suppression is attributed to activation of the medial olivocochlear system, which has an inhibitory effect on outer hair-cell activity. By studying the effects of contralateral noise on cochlear output at different stages of auditory maturation in human neonates, it is possible to describe the timecourse for development of medial efferent system function in humans. The present study recorded 2 f1-f2 distortion product otoacoustic emissions (DPOAE) in human adults, term and premature neonates at three f2 frequencies: 1500, 3000, and 6000 Hz, using fixed primary tone frequency ratio (f2/f1 = 1.2) and level separation (10 dB, L1 > L2). Average DPOAE growth functions were recorded with and without contralateral broadband noise. Results indicate that contralateral suppression of DPOAEs is absent at 6000 Hz, but present at 1500 and 3000 Hz for all ages. However, DPOAE amplitude from premature neonates was not altered by noise in an adult-like manner; in this age group, DPOAE amplitude was equally likely to by suppressed or enhanced by noise presented contralaterally. Contralateral enhancement may reflect a temporary stage of immaturity in outer hair cell-medial efferent fiber synapses just prior to term birth.     

Evidence for basal distortion-product otoacoustic emission components

Distortion-product otoacoustic emissions (DPOAEs) were measured with traditional DP-grams and level/phase (L/P) maps in rabbits with either normal cochlear function or unique sound-induced cochlear losses that were characterized as either low-frequency or notched configurations. To demonstrate that emission generators distributed basal to the f(2) primary-tone contribute, in general, to DPOAE levels and phases, a high-frequency interference tone (IT) was presented at 1/3 of an octave (oct) above the f(2) primary-tone, and DPOAEs were re-measured as "augmented" DP-grams (ADP-grams) and L/P maps. The vector difference between the control and augmented functions was then computed to derive residual DP-grams (RDP-grams) and L/P maps. The resulting RDP-grams and L/P maps, which described the DPOAEs removed by the IT, supported the notion that basal DPOAE components routinely contribute to the generation of standard measures of DPOAEs. Separate experiments demonstrated that these components could not be attributed to the effects of the 1/3-oct IT on f(2), or DPOAEs generated by the addition of a third interfering tone. These basal components can "fill in" the lesion estimated by the commonly employed DP-gram. Thus, ADP-grams more accurately reveal the pattern of cochlear damage and may eventually lead to an improved DP-gram procedure.     

Modeling the growth rate of distortion product otoacoustic emissions by active nonlinear oscillators

In this work, growth-rate curves of the 2 f1-f2 distortion product otoacoustic emission (DPOAE) are analyzed in a population of 30 noise exposed subjects, including both normal-hearing and hearing impaired subjects. A particular embedded limit-cycle oscillator equation is used to model the cochlear resonant response at the cochlear places of the primary and secondary tone frequencies (f2 and 2 f1-f2). The parameters of the oscillator equation can be directly interpreted in terms of effectiveness of the cochlear feedback mechanisms associated with the active filter amplification. A two-sources paradigm is included in the model, in agreement with experimental evidence and with the assumptions of more detailed full cochlear models based on the transmission line formalism. According to this paradigm, DPOAEs are nonlinearly generated at the cochlear place that is resonant at frequency f2, and coherently reflected at the 2 f1-f2 place. The analysis shows that the model, which had been previously used to describe the relaxation dynamics of transient evoked otoacoustic emissions (TEOAEs), also correctly predicts the observed growth rate of the DPOAE response as a function of the primary tones amplitude. A significant difference is observed between normal and impaired ears. The comparison between the growth rate curves at different frequencies provides information about the dependence of cochlear tuning on frequency.     

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 source unmixing: a test of the two-mechanism model for DPOAE generation

This paper tests key predictions of the "two-mechanism model" for the generation of distortion-product otoacoustic emissions (DPOAEs). The two-mechanism model asserts that lower-sideband DPOAEs constitute a mixture of emissions arising not simply from two distinct cochlear locations (as is now well established) but, more importantly, by two fundamentally different mechanisms: nonlinear distortion induced by the traveling wave and linear coherent reflection off pre-existing micromechanical impedance perturbations. The model predicts that (1) DPOAEs evoked by frequency-scaled stimuli (e.g., at fixed f2/f1) can be unmixed into putative distortion- and reflection-source components with the frequency dependence of their phases consistent with the presumed mechanisms of generation; (2) The putative reflection-source component of the total DPOAE closely matches the reflection-source emission (e.g., low level stimulus-frequency emission) measured at the same frequency under similar conditions. These predictions were tested by unmixing DPOAEs into components using two completely different methods: (a) selective suppression of the putative reflection source using a third tone near the distortion-product frequency and (b) spectral smoothing (or, equivalently, time-domain windowing). Although the two methods unmix in very different ways, they yield similar DPOAE components. The properties of the two DPOAE components are consistent with the predictions of the two-mechanism model.     

Hybrid measurement of auditory steady-state responses and distortion product otoacoustic emissions using an amplitude-modulated primary tone

A maximum auditory steady-state response (ASSR) amplitude is yielded when the ASSR is elicited by an amplitude-modulated tone (f(c)) with a fixed modulation frequency (f(m) = 40 Hz), whereas the maximum distortion product otoacoustic emission (DPOAE) level is yielded when the DPOAE is elicited using a fixed frequency ratio of the primary tones (f2/f1 = 1.2). When eliciting the DPOAE and ASSR by the same tone pair, optimal stimulation is present for either DPOAE or ASSR and thus adequate simultaneous DPOAE/ASSR measurement is not possible across test frequency f2 or f(c), respectively. The purpose of the present study was to determine whether the ASSR and DPOAE can be measured simultaneously without notable restrictions using a DPOAE stimulus setting in which one primary tone is amplitude modulated. A DPOAE of frequency 2f1-f2 and ASSR of modulation frequency 41 Hz were measured in ten normal hearing subjects at a test frequency between 0.5 and 8 kHz (f2 = f(c)). The decrease in the DPOAE level and the loss in ASSR amplitude during hybrid mode stimulation amounted, on average, to only 2.60 dB [standard deviation (SD) = 1.38 dB] and 1.83 dB (SD = 2.38 dB), respectively. These findings suggest simultaneous DPOAE and ASSR measurements to be feasible across all test frequencies when using a DPOAE stimulus setting where the primary tone f2 is amplitude modulated.     

Suppression of distortion product otoacoustic emissions in the anuran ear

When a two-tone stimulus is presented to the ear, so-called distortion product otoacoustic emissions (DPOAEs) are evoked. Adding an interference tone (IT) to these two DPOAE-evoking primaries affects normal DPOAE generation. The "effectiveness" of interference depends on the frequency of the IT in relation to the primary frequencies and this provides clues about the locus of emission generation within the inner ear. Here results are presented on the effects of ITs on DPOAEs thought to originate from the basilar papilla (BP) of a frog species. It is found that the IT always resulted in a reduction of the recorded DPOAE amplitude: DPOAE enhancement was not observed. Furthermore, iso-suppression curves (ISCs) exhibited two relative minima suggesting that the DPOAEs arise at different loci in the inner ear. These minima occurred at fixed frequencies, which coincided with those primary frequencies that resulted in maxima in DPOAE audiograms. The occurrence of two minima suggests that DPOAEs, which are presumed to originate exclusively from the BP, partially arise from the amphibian papilla as well. Finally, the finding that the minima in the ISCs are independent of the primary or DPOAE frequencies provides support for the notion that the BP functions as a single auditory filter.     

Multiple internal reflections in the cochlea and their effect on DPOAE fine structure

In recent years, evidence has accumulated in support of a two-source model of distortion product otoacoustic emissions (DPOAEs). According to such models DPOAEs recorded in the ear canal are associated with two separate sources of cochlear origin. It is the interference between the contributions from the two sources that gives rise to the DPOAE fine structure (a pseudoperiodic change in DPOAE level or group delay with frequency). Multiple internal reflections between the base of the cochlea (oval window) and the DP tonotopic place can add additional significant components for certain stimulus conditions and thus modify the DPOAE fine structure. DPOAEs, at frequency increments between 4 and 8 Hz, were recorded at fixed f2/f1 ratios of 1.053, 1.065, 1.08, 1.11, 1.14, 1.18, 1.22, 1.26, 1.30, 1.32, 1.34, and 1.36 from four subjects. The resulting patterns of DPOAE amplitude and group delay (the negative of the slope of phase) revealed several previously unreported patterns in addition to the commonly reported log sine variation with frequency. These observed "exotic" patterns are predicted in computational simulations when multiple internal reflections are included. An inverse FFT algorithm was used to convert DPOAE data from the frequency to the "time" domain. Comparison of data in the time and frequency domains confirmed the occurrence of these "exotic" patterns in conjunction with the presence of multiple internal reflections. Multiple internal reflections were observed more commonly for high primary ratios (f2/f1 > or = 1.3). These results indicate that a full interpretation of the DPOAE level and phase (group delay) must include not only the two generation sources, but also multiple internal reflections.     

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.     

Interaction between adenosine triphosphate and mechanically induced modulation of electrically evoked otoacoustic emissions

It was shown previously that electrically evoked otoacoustic emissions (EEOAEs) can be amplitude modulated by low-frequency bias tones and enhanced by application of adenosine triphosphate (ATP) to scala media. These effects were attributed, respectively, to the mechano-electrical transduction (MET) channels and ATP-gated ion channels on outer hair cell (OHC) stereocilia, two conductance pathways that appear to be functionally independent and additive in their effects on ionic current through the OHC. In the experiments described here, the separate influences of ATP and MET channel bias on EEOAEs did not combine linearly. Modulated EEOAEs increased in amplitude, but lost modulation at the phase and frequency of the bias tone (except at very high sound levels) after application of ATP to scala media, even though spectral components at the modulation sideband frequencies were still present. Some sidebands underwent phase shifts after ATP. In EEOAEs modulated by tones at lower sound levels, substitution of the original phase values restored modulation to the waveform, which then resembled a linear summation of the separate effects of ATP and low-frequency bias. While the physiological meaning of this procedure is not clear, the result raises the possibility that a secondary effect of ATP on one or more nonlinear stages in the transduction process, which may have caused the phase shifts, obscured linear summation at lower sound levels. In addition, "acoustic enhancement" of the EEOAE may have introduced nonlinear interaction at higher levels of the bias tones.     

Pure-tone threshold estimation from extrapolated distortion product otoacoustic emission I/O-functions in normal and cochlear hearing loss ears

A new method for direct pure-tone threshold estimation from input/output functions of distortion product otoacoustic emissions (DPOAEs) in humans is presented. Previous methods use statistical models relating DPOAE level to hearing threshold including additional parameters e.g., age or slope of DPOAE I/O-function. Here we derive a DPOAE threshold from extrapolated DPOAE I/O-functions directly. Cubic 2 f1-f2 distortion products and pure-tone threshold at f2 were measured at 51 frequencies between f2=500 Hz and 8 kHz at up to ten primary tone levels between L2=65 and 20 dB SPL in 30 normally hearing and 119 sensorineural hearing loss ears. Using an optimized primary tone level setting (L1 = 0.4L2 + 39 dB) that accounts for the nonlinear interaction of the two primaries at the DPOAE generation site at f2, the pressure of the 2 f1-f2 distortion product pDP is a linear function of the primary tone level L2. Linear regression yields correlation coefficients higher than 0.8 in the majority of the DPOAE I/O-functions. The linear behavior is sufficiently fulfilled for all frequencies in normal and impaired hearing. This suggests that the observed linear functional dependency is quite general. Extrapolating towards pDP=0 yields the DPOAE threshold for L2. There is a significant correlation between DPOAE threshold and pure-tone threshold (r=0.65, p<0.001). Thus, the DPOAEs that reflect the functioning of an essential element of peripheral sound processing enable a reliable estimation of cochlear hearing threshold up to hearing losses of 50 dBHL without any statistical data.     

Recovery of distortion-product otoacoustic emissions after a 2-kHz monaural sound-exposure in humans: effects on fine structures

A better understanding of the vulnerability of the fine structures of distortion-product otoacoustic emissions (DPOAEs) after acoustic overexposure may improve the knowledge about DPOAE generation, cochlear damage, and lead to more efficient diagnostic tools. It is studied whether the DPOAE fine structures of 16 normal-hearing human subjects are systematically affected after a moderate monaural sound-exposure of 10 min to a 2-kHz tone normalized to an exposure level L(EX,8h) of 80 dBA. DPOAEs were measured before and in the following 70 min after the exposure. The experimental protocol allowed measurements with high time and frequency resolution in a 1/3-octave band centered at 3 kHz. On average, DPOAE levels were reduced approximately 5 dB in the entire measured frequency-range. Statistically significant differences in pre- and post-exposure DPOAE levels were observed up to 70 min after the end of the sound exposure. The results show that the effects on fine structures are highly individual and no systematic change was observed.     

Effects of middle-ear immaturity on distortion product otoacoustic emission suppression tuning in infant ears

Distortion product otoacoustic emission (DPOAE) measures of cochlear function, including DPOAE suppression tuning curves and input/output (I/O) functions, are not adultlike in human infants. These findings suggest the cochlear amplifier might be functionally immature in newborns. However, many noncochlear factors influence DPOAEs and must be considered. This study examines whether age differences in DPOAE I/O functions recorded from infant and adult ears reflect maturation of ear-canal/middle-ear function or cochlear mechanics. A model based on linear middle-ear transmission and nonlinear cochlear generation was developed to fit the adult DPOAE I/O data. By varying only those model parameters related to middle-ear transmission (and holding cochlear parameters at adult values), the model successfully fitted I/O data from infants at birth through age 6 months. This suggests that cochlear mechanics are mature at birth. The model predicted an attenuation of stimulus energy through the immature ear canal and middle ear, and evaluated whether immaturities in forward transmission could explain the differences consistently observed between infant and adult DPOAE suppression. Results show that once the immaturity was compensated for by providing infants with a relative increase in primary tone level, DPOAE suppression tuning at f2= 6000 Hz was similar in adults and infants.