Nonlinear transmission line model for the inner ear

Psychoacoustical and physiological measurements of the inner ear in humans and animals indicate that the cochlea operates as a nonlinear and active system. The nature of the cochlear nonlinearity was mainly concluded from our ability to hear distortion products such as 2f₁-f₂ when tones at frequencies f₁ and f₂ are introduced to the ear simultaneously¹. The presence of active properties in the cochlea were strongly supported by the possibility to measure acoustic emissions in humans and animals ear canal2. There are several kinds of measurable cochlear emissions3: (1) Spontaneous otoacoustic emission (SOAE) which is a very narrowband energy emitted from the inner ear to the ear canal without stimulating the ear. It is found in about 30 % of the normal hearing human subjects, but it is rarely found in animals. (2) Tone and click evoked emission (CE), which is a delay response measured in the ear canal following a click or a short tone burst. The spectrum of the delay response is specific to every ear. It can be found in most normal hearing human subjects, but it is rarely found in animals. (3) Acoustic distortion products (ADPs), which are nonlinear products emitted from the inner ear to the ear canal when a two-tone stimulus is introduced to the ear. ADPs are found in most healthy ears (animals and humans), however the amplitude of ADP relative to the amplitude of the primary tones is much higher than found in humans. In the present study we introduce a model that suggests an explanation for the great variability in cochlear emission properties among different species and within individuals in the same species, and predict many measured properties of cochlear emissions.