tell the pitch
of a sound by its frequency
. How do they tell the difference between frequencies though? How are they 'coded
'? This is where Place theory
The origin of Place Theory is generally credited to Georg von Bekesy, a researcher who performed experiments in the '30s to '40s to answer exactly this question. He made holes in the ears of cadavers in order to be able to observe the basilar membranes. He then played sounds of different frequencies to observe the ripples of waves along the basilar membrane. He noticed that the envelope of waves, 'peaked', and then subsided, like an ocean wave. The location of this peak depended on the frequency of the sound played.
It's easy going from here. If the location depends on the frequency, then the location at which the membrane gets the most stimulation (the place where the most hairs are disturbed) can be used to determine the frequency of sounds. However, this was not the only way the ear codes frequencies. Another theory, Frequency-Matching Theory was required to explain how the ear codes very low sounds which have no characteristic locations.