Comparing Ear and Mic

The first big difference to notice is that the microphone has a single input and output. It has only one diaphragm, and one mike cable. Right off, by comparison, we have two ears. A big part of what goes on in the brain before the neurological information is presented to our consciousness is the integration of the data from both ears into a single illusion. This is, of course, why stereo recording has worked out so wonderfully! But even two microphones don't do the trick, because the mixing console isn't a brain, in case you hadn't noticed. And there's more: each microphone has a single output - a single electrical waveform traveling down a wire. Each basilar membrane has about 30,000 outputs! Each of those is a nerve that is carrying pulse data (i.e. like digital signals) about a specific audible frequency. What is happening is that the basilar membrane varies in thickness and tension along its length, and as a result different frequencies cause different areas of the basilar membrane to vibrate. The 30,000 or so nerve endings are spread out across the membrane, so that each nerve ending ends up representing a different frequency, sort of. This is (in part) how we can discriminate pitch and harmonies. Visualize the microphone with a filter that divides the incoming signal into 30,000 different sine waves and transmits the loudness (and phase of low frequency signals) of each such sine wave down a separate cable to the console! Some filter. Some cable! Some console!!!

But let's go back through the ear a little. The equivalent part to the microphone diaphragm in the ear is the eardrum, or tympanum. It is at the inner end of a tunnel coming in through the skull from the outside world. The eardrum is a thin membrane stretched across the end of the tunnel. Past it is the middle ear, a hollow cavity. Like the microphone diaphragm, it vibrates in response to sound waves coming into the ear. Unlike the microphone diaphragm, it doesn't just sit there and take it. Instead, it contracts or relaxes (it actually is supported by a muscle) in response to signals from the brain regarding how loud the music is, effectively turning up or down the intensity of sound reaching the basilar membrane. Meanwhile, the brain compensates for this level adjustment so that we don't consciously hear this compensation taking place. (This was another little cutie I noticed on my little drugged-out medical misadventure - sudden loud sounds were a whole lot louder, I mean, like painful!).

And there's more: in the middle ear three interconnected bones (the so-call hammer, anvil and stirrup) mechanically transmit sound from the tympanum to the inner ear (the cochlea). The motion of these bones adds some mechanical advantage to the motion, so that the bone structure can be thought of as a mechanical amplifier pumping about 20 dB of gain into the mechanical signal. But, when the mechanical motion becomes too large, the elastic cartilage holding the bones together begins to stretch, causing the motion to be limited, so this mechanical amplifier also is a peak limiter, with some rather short time constants (rapid attack and release times, to you propeller heads), in comparison to the comparatively slow integration time of the muscle operating the tympanum.

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