for a complete beginner's intro to the fascinating world of neuroscience.
acoustics & signal processing
Here another animation showing the mechanical response of the basilar membrane, but this time the incoming sound is not the sum of two sine waves, but a single "ideal impulse", or click.
The top trace shows the click stimulus. Think of the click as travelling through air, but also impinging on the cochlea at time zero. The bottom trace shows the basilar membrane, with distance from the basal end on the x-axis.
This figure shows acoustic cues to sound source direction. It is a color version of Fig. 5-2 of "Auditory Neuroscience", and is based on acoustic recordings from my own ears carried out by Prof Doris Kistler at the University of Wisconsin at Madison.
This animation shows a simulation of "travelling wave motion" in the basilar membrane in response to a sound composed of two frequencies (1000 and 2500 Hz). The sound waveform is shown in the top panel, the basilar membrane response is shown below. Since the frequency components of the input are separated by more than an octave, they are well separated by the mechanical filtering of the cochlea, producing clearly separated "travelling waves" for each frequency component.
A guitar string, plucked at the centre, will be stretched into a triangular shape before you let it go. Once you let it go it will vibrate in a "triangular sort of way". The thing about its motion is that it can be thought of as a superposition of simple harmonic motions, with harmonically related frequencies, as shown here. The "more or less triangular" vibration in the lowest panel arises as a weighted sum of the three modes of vibration shown above it.
Early hearing aids were often simple "ear trumpets" designed to funnel sound to the ear over a larger area to amplify the sound. But the users of hearing aids have always been concerned about the cosmetic side effects of their devices.