Otoacoustic emissions (OAEs) of all types are widely assumed to arise by a common mechanism: nonlinear electromechanical distortion within the cochlea. In this view, both stimulus-frequency (SFOAEs) and distortion-product emissions (DPOAEs) arise because nonlinearities in the mechanics act as "sources" of backward-traveling waves. This unified picture is tested by analyzing measurements of emission phase using a simple phenomenological description of the nonlinear re-emission process. The analysis framework is independent of the detailed form of the emission sources and the nonlinearities that produce them. The analysis demonstrates that the common assumption that SFOAEs originate by nonlinear distortion requires that SFOAE phase be essentially independent of frequency, in striking contradiction with experiment. This contradiction implies that evoked otoacoustic emissions arise by two fundamentally different mechanisms within the cochlea. These two mechanisms (linear reflection versus nonlinear distortion) are described and two broad classes of emissions--reflection-source and distortion-source emissions--are distinguished based on the mechanisms of their generation. The implications of this OAE taxonomy for the measurement, interpretation, and clinical use of otoacoustic emissions as noninvasive probes of cochlear function are discussed.

We develop an objective, noninvasive method for determining the frequency selectivity of cochlear tuning at low and moderate sound levels. Applicable in humans at frequencies of 1 kHz and above, the method is based on the measurement of stimulus-frequency otoacoustic emissions and, unlike previous noninvasive physiological methods, does not depend on the frequency selectivity of masking or suppression. The otoacoustic measurements indicate that at low sound levels human cochlear tuning is more than twice as sharp as implied by standard behavioral studies and has a different dependence on frequency. New behavioral measurements designed to minimize the influence of nonlinear effects such as suppression agree with the emission-based values. A comparison of cochlear tuning in cat, guinea pig, and human indicates that, contrary to common belief, tuning in the human cochlea is considerably sharper than that found in the other mammals. The sharper tuning may facilitate human speech communication.T he mammalian cochlea acts as an acoustic prism, mechanically separating the frequency components of sound so that they stimulate different populations of sensory cells. As a consequence of this frequency separation, or filtering, each sensory cell within the cochlea responds preferentially to sound energy within a limited frequency range. In its role as a frequency analyzer, the cochlea has been likened to a bank of overlapping bandpass filters, often referred to as ''cochlear filters.'' The frequency tuning of these filters plays a critical role in our ability to distinguish and perceptually segregate different sounds. For instance, hearing loss is often accompanied by a degradation in cochlear tuning, or a broadening of the cochlear filters. Although quiet sounds can be restored to audibility with appropriate hearing-aid amplification, the loss of cochlear tuning leads to pronounced, and as yet largely uncorrectable, deficits in the ability of hearing-impaired listeners to extract meaningful sounds from background noise (1).The bandwidths of cochlear filters have been measured directly in anesthetized, non-human mammals by recording from the auditory-nerve fibers that contact the sensory cells (2). Filter bandwidths in humans, however, must be determined indirectly from noninvasive measurements. Traditionally, such studies have relied on psychophysical (i.e., behavioral) measures of filter bandwidth based on the phenomenon of masking; that is, the ability of one sound to interfere with, or ''mask,'' the perception of another. Strong masking is interpreted as indicating that frequency components of the masker fall within the passband of the cochlear filter whose output is used to detect the signal. Interference then occurs because both signal and masker stimulate an overlapping group of sensory cells. Since the pioneering work of Harvey Fletcher (3), filter bandwidths have been obtained by measuring listeners' thresholds for detection of a pure tone in background noises with particular spectral characteristics. These tone th...