Characterization of Cubic Intermodulation Distortion Products in the Cat External Auditory Meatus

We have made extensive measurements of the lower frequency cubic intermodulation distortion product sound pressure in the external auditory meatus of cats. This distortion product has a frequency, f_D = 2f₁-f₂, where f₁ is the lower frequency of two pure tone input sounds. A₁ and A₂ are the amplitudes of the pure tone inputs at frequencies f₁ and f₂ respectively. The evidence is substantial that this distortion product is generated in the cochlea and that it propagates back out into the ear canal (e.g., Kim, 1980). Our data is presented here as Thevenin equivalent source pressures rather than as raw ear canal pressures. The Thevenin equivalent source pressure is a measurement of the properties of the source (assuming that a system is approximately linear in its overall characteristic) while the raw ear canal pressure depends upon not only the source but also the impedances of the sound delivery system and the animal’s input impedance at the place of pressure measurement. (In animal studies, this is frequently the input impedance at the tympanic membrane.)