Hearing (sense)

Hearing (or audition; adjectival form: "auditory" or "aural") is the ability to perceive sound by detecting vibrations through an organ such as the ear. It is one of the traditional five senses. The inability to hear is called deafness.

In humans and other vertebrates, hearing is performed primarily by the auditory system: vibrations are detected by the ear and transduced into nerve impulses that are perceived by the brain (primarily in the temporal lobe). Like touch, audition requires sensitivity to the movement of molecules in the world outside the organism. Both hearing and touch are types of mechanosensation.

Hearing mechanism
The eardrum of an ear simplifies incoming air pressure waves to a single channel of amplitude. In the inner ear, the distribution of vibrations along the length of the basilar membrane is detected by hair cells. The location and intensity of vibrations in the basilar membrane is transmitted to the brain through the auditory nerve.

Hearing tests
Hearing can be measured by behavioral tests using an audiometer. Electrophysiological tests of hearing can provide accurate measurements of hearing thresholds even in unconscious subjects. Such tests include auditory brainstem evoked potentials (ABR), otoacoustic emissions (OAE) and electrocochleography (EchoG). Technical advances in these tests have allowed hearing screening for infants to become widespread.

Defense mechanism
The hearing structures of many species have defense mechanisms against injury. For example, the muscles of the middle ear (e.g. the tensor tympani muscle) in many mammals contract reflexively in reaction to loud sounds which may injure the hearing ability of the organism.

The idea that the acoustic reflex is a defense mechanism has been a topic for debate in recent years. Commonly cited counterarguments include:
 * The types of sound that produce hearing loss (impact and continuous noise), were not present during the evolutionary history of mammals.
 * The muscles that kink the ossicles are among the smallest in the body, and fatigue too quickly to be useful in the capacity of protecting against continuous noise.
 * The reflex is too slow to protect against impact noises.

Hearing underwater
Hearing threshold and the ability to localize sound sources are reduced underwater, in which the speed of sound is faster than in air. Underwater hearing is by bone conduction, and localization of sound appears to depend on differences in amplitude detected by bone conduction.

Hearing in animals
Not all sounds are normally audible to all animals. Each species has a range of normal hearing for both loudness (amplitude) and pitch (frequency). Many animals use sound to communicate with each other, and hearing in these species is particularly important for survival and reproduction. In species that use sound as a primary means of communication, hearing is typically most acute for the range of pitches produced in calls and speech.

Frequencies capable of being heard by humans are called audio or sonic. The range is typically considered to be between 20 Hz and 20,000 Hz. Frequencies higher than audio are referred to as ultrasonic, while frequencies below audio are referred to as infrasonic. Some bats use ultrasound for echolocation while in flight. Dogs are able to hear ultrasound, which is the principle of 'silent' dog whistles. Snakes sense infrasound through their bellies, and whales, giraffes, dolphins and elephants use it for communication.

Certain animals have more sensitive hearing than humans which enables them to hear sounds too faint to be detected by humans.

Mathematics
The eardrum of an ear simplifies incoming air pressure waves to a single channel of amplitude. The basilar membrane of the inner ear separates out different frequencies: high frequencies produce a large vibration at the end near the middle ear, and low frequencies a large vibration at the distant end. Thus the ear performs a frequency analysis, roughly similar to a Fourier transform. However, the nerve pulses delivered to the brain contain both place and rate information, so the similarity is not strong.