3.5 What arrives at the oval window

The same signal that entered as a pressure wave at the eardrum now arrives at the oval window as a velocity of the stapes footplate. “Hey Dr. Miles!”, which entered the auditory canal as a fluctuating pressure of perhaps a few hundred microPascals, has been ferried across the impedance mismatch and is now pushing fluid into the scala vestibuli of the cochlea. The middle ear has done its job. The cochlear traveling wave we built in movement 5 starts here.

One last note before we move on. The middle-ear cavity is filled with air at approximately atmospheric pressure, and that pressure has to be maintained. The Eustachian tube, which connects the middle-ear cavity to the back of the throat, equalizes pressure when you swallow or yawn. This is why your ears pop on airplanes. It is also why a blocked Eustachian tube (during a cold) causes hearing loss: the eardrum cannot move freely if the pressure on its medial side does not match the pressure on its lateral side. Conductive hearing loss from middle-ear effusion can easily reach 30 dB — the cost of an unmitigated impedance mismatch.

We now have a velocity at the oval window. The stapes is driving the cochlear fluid. Movement 5 explains, at length, what happens next.