There are more sounds in the world than the human ear can ever hear

Our ears are impressively sensitive pieces of apparatus. To make itself heard, a sound need only be strong enough to deflect our eardrum by 0.00000001 mm (40 thousand millionths in). Our ears can pick up a rich variety of sounds from the breathing of a baby to the deafening boom of a supersonic jet. But compared with some of our fellow animals, even those of us with perfect hearing are effectively half deaf.

Sound is a vibration of the air and travels as a series of waves. The 'frequency' of the waves - the number of waves per second - determines whether the sound is high-pitched like a scream or low as a bass drum.

The human ear by no means picks up all the sound vibrations in the outside world. It can register waves of between 20 and 20 000 vibrations a second (v/s) - the highest C on a piano is 4096 v/s, so 20 000 is just a shrill hiss. But dogs can hear ultrasonic whistles of 35 000 v/s, which are totally inaudible to human beings. Bats are even sharper of hearing: they respond to frequencies of up to 75 000 v/s.

Yet no land animal can match the remarkable bottle-nosed dolphin. As dolphins communicate in their complex language of clicks and whistles, they can discriminate sounds from 20 up to a startling 150 000 v/s - nearly eight times the human limit. It is no use, however, wishing that dolphins could 'lend us their ears'. Extraordinary, they hear through their jaws and throat, which pick up the high-frequency vibrations. Their ears are virtually atrophied.