Gray Scale Propagation


This animation shows a gray-scale representation of the pressure in the cochlea. The previous two animations showed the pressure along the basilar membrane as a sound propagated down its length. This animations now looks at the pressure in the fluid. As the pressure wave travels down the Basilar Membrane it exerts pressure on the fluid. The pressure field is shown in this animation for a single audio input tone. The basilar membrane is shown along the center line.

The change in propagation speed means that the long-wave, or one-dimensional, approximation is only valid in the early part of the wave's travel. Near the response peak the wavelength is short enough that energy can flow both down the cochlea and perpendicular to the membrane, so a two-dimensional model is needed. This Mathematica animation shows the pressure wave due to a pure tone as it travels down the cochlea. Darker and lighter regions correspond to pressures above and below the average.

On the left, where the propagation speed is high, the wave in each chamber is essentially one dimensional and the wave travels entirely along the direction of the basilar membrane. As the wave slows down the energy starts to bunch up and there is now a variation in pressure in the direction perpendicular to the length of the cochlea.

After a short distance the energy in the wave is completely dissipated. On the right, very faint waves are actually traveling at an angle, carrying their energy into the basilar membrane where the energy is dissipated in membrane losses.

Time Delay