REFLECTIONS FROM WAVELENGTH INDUCED IMPEDANCE DISCONTINUITIES IN A PHYSICAL MODEL - POSSIBLE RELEVANCE TO COCHLEAR MECHANICS

A rigid tube incorporating a longitudinal slot covered by a membrane w ith an increasing compliance per unit length from its source to its te rmination has been investigated. Continuous sinusoidal pressure waves at a number of discrete frequencies were input to the source and the R MS pressure along the length of the tube measured. At each frequency, nodes and antinodes were found between the source and the unique point corresponding to the position where transverse resonance occurs. Tran sverse resonance occurs at a position where the input frequency matche s the natural resonant frequency of the transverse fluid inertance and membrane compliance at that position. Downstream of this position, no pressure waves could be detected. This very simple model, reproduces certain fluid dynamic features of the cochlea since, in so much as whe n the input frequency is increased, the position of transverse resonan ce moves upstream towards the source with no detectable signal downstr eam from the site of resonance. A theoretical model incorporating the wavelength dependent nature of both transverse and axial fluid inertan ces is used to describe the physical model. This theoretical model sho wed the same pattern of nodes and antinodes as the pressure measuremen ts in the tube, and indicated reflections occurring from the position of resonance. These reflections are caused by an impedance jump at the resonant position which occurs when the wavelength becomes short comp ared to the transverse dimensions of the tube, and this, in turn, chan ges the transverse and axial fluid inertances. This phenomenon may pla y a role in the cochlea where wavelengths are considered to become sho rt as the wave approaches the position of resonance and where emission s have been detected from this position.