I guess most of you know how the ear perceives sound, but here's a quick biology lesson anyway. The cochlea of the inner ear is filled with a fluid which is made to vibrate by the eardrum, the same as the sound vibrations falling on it. Sticking up into the fluid are lots (millions? billions?) of little bundles of tiny, tiny fibres, each bundle being a little shorter than the next. Each bundle resonates at a specific frequency, and has a nerve attached: when the fluid vibrates at that frequency, the fibre bundle is 'tweaked', and stimulates the nerve. The total of all these nerve signals is turned by the brain into what we hear.

Still with me? Good! The smallest difference in pitch we can discriminate depends on how accurate and how close to each other (in pitch) these bundles are. As we get older, those at the extremes, especially the high end, stop working so well (or altogether), and this reduces the range of frequencies we can hear. They can be damaged by loud nise - don't stand so close to the p.a., and turn down your headphones!

They are pretty good at discriminating between two pitches, which is why you can hear wow and flutter in your hi-fi, unless it's very hi; and it lets you hear vibrato, where the pitch of a note is 'warbled' up and down ever so slightly by the singer or musician.

It should be possible to train yourself to identify different frequencies with a good degree of accuracy - not just notes on the (any) scale, although it sounds pretty boring to me. Any research physiologists out there care to comment on any of this?

Steve