Effects of the finite speed of sound in—and surgical modification of—the cochlea

Abstract

Most analyses of the mechanics of the cochlear to date have involved the assumption that the cochlea is a rigid chamber filled with incompressible fluids. However, in vivo measurements of the cochlear fluid pressure or basilar membrane motion necessarily must be performed by first surgically modifying the cochlea through removing part of the cochlear bone. This modification violates the assumption of a rigid chamber, and can significantly modify the mechanics of the cochlea. Because of anatomical constraints, many of these measurements are performed near the base of the cochlea. For the species that are used (guinea pig, gerbil, chinchilla, etc.), measurements near the base necessitate the use of stimulus tones with frequencies that are too high for the assumption of an incompressible fluid to be valid. In this paper, we discuss the modifications to the cochlear mechanics necessary to accommodate both the finite speed of sound and the surgical modification of the cochlea. We show that both of these modifications can easily be incorporated into numerical cochlear model codes. Semianalytic solutions based the WKB method are also discussed.