Hearing

Hearing, auditory perception, or audition^{[note 1]} is the ability to perceive sound by detecting vibrations,^[1] changes in the pressure of the surrounding medium through time, through an organ such as the ear. Sound may be heard through solid, liquid, or gaseous matter.^[2] 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.^[3]

Hearing mechanism [edit]

There are three main components of the ear: the outer ear, the middle ear and the inner ear.

The outer ear includes the visible part of the ear (or the pinna), the auditory canal and the eardrum. The eardrum is made of an airtight flap of skin. Sounds travel in waves, and when these waves arrive at the eardrum, they cause it to vibrate, following the waveform of the sound. The eardrum simplifies incoming air pressure waves to a single change with a certain amplitude. This allows for the differentiation of sound.

The middle ear consists of a small air filled chamber that is located behind the eardrum. Within this chamber are three smallest bones in the body, known collectively as the ossicles. They aid in the transmission and amplification of the vibrations from the ear drum to the inner ear.

The inner ear contains the cochlea, which is a spiral shaped, fluid filled tube that is considered the organ of auditory transduction. It is divided lengthwise by the basilar membrane, a structure that vibrates when waves from the middle ear propagate through the cochlear fluid–membrane system. The basilar membrane is tonotopic, so that each frequency has a characteristic place of resonance along it. Characteristic frequencies are high at the basal entrance to the cochlea, and low at the apex. Basilar membrane motion causes the movement of the hair cells, specialized auditory receptors located within the basilar membrane. ^[4] The space–time pattern of vibrations in the basilar membrane is converted to a spatial–temporal pattern of firings on the auditory nerve, which transmits information about the sound to the brainstem.^[5]

Hearing tests [edit]

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 [edit]

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^{[citation needed]} 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 loss [edit]

There are defined degrees of hearing loss:^[6]

Mild hearing loss

• People who suffer from mild hearing loss have difficulties keeping up with conversations, especially in noisy surroundings. The most quiet sounds that people who suffer from mild hearing loss can hear with their better ear are between 25 and 40 dB SPL.

Moderate hearing loss

• People who suffer from moderate hearing loss have difficulty keeping up with conversations when they are not using a hearing aid. On average, the most quiet sounds heard by people with moderate hearing loss with their better ear are between 40 and 70 dB SPL.

Severe hearing loss

• People who suffer from severe hearing loss depend on powerful hearing aid. However, they often rely on lip-reading even when they are using hearing aids. The most quiet sounds heard by people with severe hearing loss with their better ear are between 70 and 95 dB SPL.

Profound hearing loss

• People who suffer from profound hearing loss are very hard of hearing and they mostly rely on lip-reading and sign language. The most quiet sounds heard by people with profound hearing loss with their better ear are from 95 dB SPL or more.

Hearing aids [edit]

Hearing aids are electronic devices that enable a person with hearing loss to receive sounds at certain amplitudes. This technological development has led to the benefit of improving the sense of hearing of a person, but the usage of these devices is significantly low. Psychologically, the first time that a person realizes that he/she needs help from a professional or needs an audiologist is when they feel that their hearing is severely poor. Initially, people don't like to believe that they are becoming deaf; hence it negatively affects their approach towards the use of hearing aids. Familiarity with the devices and consultation with professionals do help people feel good about using the hearing aids.^[7]

Hearing underwater [edit]

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.^[8] Aquatic animals such as fish, however, have a more specialized hearing apparatus that is effective underwater.^[9]

Hearing in animals [edit]

Not all sounds are normally audible to all animals. Each species has a range of normal hearing for both amplitude and 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.^[10] 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. Some fishes have the ability of hearing due from the well-developed parts of their swim bladders. The "aid to the deaf" of fishes appears in some species such as carp and herring.^[11]

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

Mathematics [edit]

The basilar membrane of the inner ear spreads out different frequencies: high frequencies produce a large vibration at the end near the middle ear (the "base"), and low frequencies a large vibration at the distant end (the "apex"). Thus the ear performs a sort of frequency analysis, roughly similar to a Fourier transform.^[12][13] However, the nerve pulses delivered to the brain contain both rate-versus-place and fine temporal structure information, so the similarity is not strong.

See also [edit]

References [edit]

Further reading [edit]

• Enrique A. Lopez-Poveda, Alan R. Palmer & Ray Meddis (Eds.) (2010). The Neurophysiological Bases of Auditory Perception (pp. 99–110). New York: Springer. ISBN 978-1-4419-5685-9
• Article about hearing on LabSpace

External links [edit]

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