Wave Interference in the Generation of Reflection- and Distortion-source Emissions

Although mammalian otoacoustic emissions (OAEs) have generally been regarded as originating through nonlinear electromechanical distortion, measurements of OAE phase made using frequency-scaled stimuli (e.g., a pure tone or a pair of tones with fixed frequency ratio f2/f1) are now understood as providing clear evidence for two fundamentally different types of emission [11,6]: distortion-source (or “wave-fixed”) emissions, which arise by nonlinear distortion induced by the traveling wave, and reflection-source (or “place-fixed”) emissions, which arise via linear reflection off micromechanical impedance perturbations such as variations in the number, geometry, or mechanical characteristics of outer hair cells [14,20]. Although distortionand reflection-source emissions are often idealized as originating from isolated, point-like sources, both types of OAEs arise from a spatially distributed region. In this paper, we discuss how the distributed nature of the emission sources—specifically, interference among the many wavelets originating from within the source region—contributes essentially to the characteristics of reflectionand distortion-source emissions.