hearing
In human beings, hearing is performed by the ears, which also perform the function of balance, a sense in itself but not one of the traditional list (due to Aristotle). This is in common with most mammals. Many other organisms also have some form of hearing, either by some sort of ear, or by other structures, or by a combination.
A common rule of thumb used to describe human hearing is that human hearing is sensitive in the range of frequency of 20 Hz to 20 kHz, though this varies significantly with age, occupational hearing damage, and gender; some individuals are able to hear up to 22 kHz and perhaps beyond, while others are limited to about 16 kHz. Frequencies capable of being heard by humans are called audio or referred to as sonic. Frequencies higher than audio are referred to as ultrasonic, while frequencies below audio are referred to as infrasonic.
Some organisms are able to hear ultrasound and/or infrasound. Some bats use ultrasound for echo location while in flight. Dogs are able to hear ultrasound, which is the principle of ’silent’ dog whistles. Snakes sense infrasound through their bellies, and there is evidence that whales and elephants may use it for communication. See sound for hearing ranges of various organisms.
The hearing can be tested using a device or computer program called audiometer.
Localization of sound
Humans can hear the direction of the source of a sound, sometimes with surprising accuracy. Two mechanisms are known to be used.
* The nervous system can resolve time differences as small as the time it takes sound to pass one ear and reach the other.
* For high frequencies, frequencies with a wavelength shorter than the listener’s head, more sound reaches the nearer ear.
Neither of these mechanisms work as well in water, in which the speed of sound is faster than in air.
The arrival time of a sound to a particular ear is given greater weight when localizing than relative intensity, according to an observation known as the Law of the First Wavefront.
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