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In response to sinusoidal pressure applied to the stapes (tones) the basilar membrane vibrates producing the phenomenon of traveling waves.
 
spiral.gif The basilar membrane is internally formed by thin elastic fibers tensed across the cochlear duct. The fibers are short and closely packed in the basal region, i.e. close to the stapes, and become longer and sparse proceeding towards the apex of the cochlea, where the basilar membrane ends in a foramen that joins the two partions of the spiral canal (see Anatomy). Being under tension, the fibers can vibrate like the strings of a musical instrument.

Traveling waves peak at frequency-dependent locations, higher frequencies peaking closer to more basal location. Peak position is an exponential function
of input frequency because of the exponentially graded stiffness of the basilar membrane. Part of the stiffness change is due to the increasing width of the membrane and part to its decreasing thickness.
 
 
References
  1. Cabezudo LM (1978) The ultrastructure of the basilar membrane in the cat. Acta Otolaryngol. 86: 160-175.
  2. Fernandez C (1952) Dimensions of the cochlea (guinea-pig). J. Acoust. Soc. Am. 24: 519-523.
  3. Greenwood DD (1990) A cochear frequency-postion function for several species - 29 years later. J. Acoust. Soc. Am. 87: 2592-2605.
  4. Gummer AW, Johnstone BM and Armstrong NJ (1981) Direct measurement of basilar membrane stiffness in the guinea pig. J. Acoust. Soc. Am. 70: 1298-1309.
  5. Iurato S (1962) Functional implication of the nature and submicroscopic stucture of the tectorial and basilar membranes. J. Acoust. Soc. Am. 62: 1386-1385.
  6. Olson ES and Mountain DC (1991) In vivo measurement of basilar membrane stiffness. J.Acoust.Soc.Am. 89 (3):1262-1275.