Animated drawing showing how the sound waves in the external meatus are transferred to the inner ear fluids and to the basilar membrane (organ of Corti).

The sound wave moves the eardrum and attached ossicular chain. The stapes footplate, in the oval window, transfers the vibrations to the perilymphatic compartment (scala vestibuli) and to the inner ear structures. Depending on the frequency, the vibration has a maximum effect (resonance) at a different point along the basilar membrane, accounting for passive tonotopy.
Here, on the left, a high frequency sound affects a basal portion of the cochlea

Here, on the right, a low frequency sound affects a more apical part of the cochlea