The range of hearing
Being able to hear is unremarkable: powerful sounds shake the body and can be detected even by single-celled organisms. But being able to hear as well as we do is little short of miraculous: we can quite easily detect a sound which delivers a power of 10fi15 watts to the eardrums, despite the fact that it moves them only a fraction of the width of a hydrogen atom.
Almost as impressive is the range of sound powers we can hear. The gap between the quietest audible sound level (the threshold of hearing, 0 dB) to the threshold of pain (around 130 dB) is huge: 130 dB is 1013, which is the number of pence in a hundred billion pounds.
We can also hear a fairly wide range of frequencies; about ten octaves, a couple more than a piano keyboard. We can detect, though not truly hear, frequencies well below and above this too, as will be explained in Chapter 6. And our frequency discrimination is excellent: most of us can detect differences of about a quarter of a semitone; with practice and in ideal conditions, a difference of about one-twentieth of a semitone is just distinguishable. Our judgement of directionality, by contrast, is mediocre; even in favourable conditions we can only determine the direction of a sound’s source within about 10° horizontally or 20° vertically; many other animals can do very much better.
14. The ear (the middle and inner ears are greatly enlarged).
Perhaps the most impressive of all our hearing abilities is that we can understand words whose levels are less than 10 per cent of that of background noise level (if that background is a broad spread of frequencies): this far surpasses any machine.
Our ears have two functions, hearing and balance, and balance is taken care of entirely by the semicircular canals (see Figure 14). The rest of the ear has thus been free to evolve the best possible system for our needs. Not so our vocal apparatus; as a relative latecomer in our evolution it had to flt itself in cheek by jowl, sharing structures previously earmarked for breathing and eating, licking and sucking, kissing and flghting. Yet, in a trained actor or accomplished singer, the speech system functions just as perfectly as a Stradivarius violin.