Hearing with your chest

Evelyn Glennie, from her website.As well as explaining evolution (see my last post), analog feedback may explain the ability of deaf people to hear through their bodies as the percussionist Evelyn Glennie learned to do. According to her biography,

Evelyn spent a lot of time when she was young (with the help of Ron Forbes her percussion teacher at school) refining her ability to detect vibrations. She would stand with her hands against the classroom wall while Ron played notes on the timpani (timpani produce a lot of vibrations). Eventually Evelyn managed to distinguish the rough pitch of notes by associating where on her body she felt the sound with the sense of perfect pitch she had before losing her hearing. The low sounds she feels mainly in her legs and feet and high sounds might be particular places on her face, neck and chest.

This is pretty amazing. What her brain managed to do was to take physical structures in her body that were not ‘designed’ or evolved to be sensors, but use them that way anyway.

This is only possible because we are analog systems and physical objects. We cannot help but feel vibrations: our bones and chest cavity have natural frequencies at which they will vibrate. These vibrations cannot help but be passed on by our nervous system. Of course, we normally ignore these sensations. But our brain can, if necessary, focus in on these ‘signals’ and use them to make sense of the world.

In a digital system there would almost certainly be no such path. No engineer would design a leg based on the idea that it might one day have to be used for something bizarre like hearing. And even if one did put some vibration sensors on the leg, for some other purpose, the digital threshold might well be too high for the relevant information to get through.

One of the disadvantages of analog systems, its sensitivity to noise, is also an advantage. Noise is mostly just stuff you didn’t think you wanted to measure, but got stuck with anyway. Usually it’s a pain. But when you suddenly discover that there is good information hidden in the noise then you still have access to it: unlike in digital systems where you’ve already thrown it away by setting a digital threshold that turns a 0.78 (and a 0.563, and a 0.6724) into a uniform 1.