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Hearing range usually describes the range of frequencies that can be heard by an animal or human, though it can also refer to the range of levels. In humans the audible range of frequencies is usually 20 to 20,000 Hz, although there is considerable variation between individuals, especially at high frequencies, where a gradual decline with age is considered normal. Sensitivity also varies with frequency, as shown by equal-loudness contours. Routine investigation for hearing loss usually involves an audiogram which shows threshold levels relative to a standardised norm.
Audiograms in humans are produced using an audiometer, which presents different frequencies to the subject, usually over calibrated headphones, at specified levels. The levels are but weighted with frequency relative to a standard graph known as the minimum audibility curve, which is intended to represent "normal" hearing. The threshold of hearing is set at around 0 phon on the equal-loudness contours, but is standardised in an ANSI standard to 1 kHz. There are several of the minimal audibility curve, defined in different international standards; different audiometers thus give rise differences in audiograms. The ASA-1951 standard, for example, used a level of 16.5 dB SPL (sound pressure level) at 1 kHz, whereas the later ANSI-1969/ISO-1963 standard uses 6.5 dB SPL.A 10 dB correction is allowed for older people.
Behavioural hearing tests or physiological tests can be used to find hearing thresholds of humans and animals. For humans, the test involves tones being presented at a specific frequencies (pitch) and intensities (loudness). When the subject hears the sound, he or she indicates it by raising a hand or pressing a button. The lowest intensity they can hear is recorded.
The test varies for children; their response to the sound can be a indicated by a turn of the head or using a toy. The child learns what to do upon hearing the sound, such as placing a toy man in a boat. A similar technique can be used when testing animals, where food is used as a reward for responding to the sound.
Physiological tests do not need the patient to consciously respond. For example, when performing the brainstem auditory evoked potentials, brainstem responses are measured when a sound is played into their ear.
The information on different mammals' hearing was obtained primarily by behavioural hearing tests.
In a human, sound waves funnel into the ear via the external ear canal and hit the eardrum (tympanic membrane). Consequently, the compression and rarefaction of the wave set this thin membrane in motion, causing the middle ear bones (the ossicles: malleus, incus and stapes) to move. The number of sound pressure level vibrations (sonic waves) per second denotes the frequency. Infrasonic (below hearing), sonic (aural), and ultrasonic (above hearing) frequencies are measured in Hertz (Hz); one Hertz is one cycle wave (or singular pressure wave in audionics) per second. Humans have a maximum aural range that begins as low as 12 Hz under ideal laboratory conditions, to 20 kHz[note 1] in most children and some adults. The range shrinks during life, usually beginning at around age of 8 with the upper frequency limit lowering. Inaudible sound waves can be detected (felt) by humans through physical body vibration in the range of 4 to 16 Hz. There is a difference in sensitivity of hearing between the sexes, with women typically having a higher sensitivity to higher frequencies than men.[better source needed] The vibrations of the ossicular chain displace the basilar fluid in the cochlea, causing the hairs within it, called stereocilia, to vibrate. Hairs line the cochlea from base to apex, and the part stimulated and the intensity of stimulation gives an indication of the nature of the sound. Information gathered from the hair cells is sent via the auditory nerve for processing in the brain.
So-called "Nelson" dog whistles generate sounds at frequencies higher than those audible to humans but well within the range of a dog's hearing.
When compressing a digital signal, an acoustic engineer can safely assume that any frequency beyond approximately 20 kHz will not have any effect on the perceived sound of the finished product, and thus use a low pass filter to cut everything outside this range. The sound can then be sampled at the standard CD sample rate of 44.1 kHz (or 48 kHz in DAT), set somewhat higher than the calculated Nyquist-Shannon rate of 40 kHz to allow for the cut-off slope of a reasonable low pass filter.
When additional compression of sound is required, higher frequencies are usually cut off first, because regular adults' hearing in those areas is often even less than 20 kHz. This is due to loss of hearing in the high-frequency range, due to either hearing damage (e.g. from listening to loud music) or aging. For instance, the commonly used MP3 coding often cuts sounds above 18 kHz, or when compressing as high as 128 kbit/s, at 16 kHz.
The hearing ability of a dog is dependent on breed and age, though the range of hearing is usually around 40 Hz to 60 kHz, which is much greater than that of humans. As with humans, some dog breeds' hearing ranges narrow with age, such as the German shepherd and miniature poodle. When dogs hear a sound, they will move their ears towards it in order to maximise reception. In order to achieve this, the ears of a dog are controlled by at least 18 muscles, which allows the ears to tilt and rotate. Ear shape also allows for the sound to be more accurately heard. Many breeds often have upright and curved ears, which direct and amplify the sounds.
As dogs hear much higher frequency sounds than humans, they have a different acoustic perception of the world. Sounds that seem loud to humans often emit high frequency tones that can scare away dogs. Ultrasonic signals are used in training whistles, as a dog will respond much better to such levels. In the wild, dogs use their hearing capabilities to hunt and locate food. Domestic breeds are often used to guard property due to their increased hearing ability.
Bats require very sensitive hearing to compensate for their lack of visual stimuli. Their hearing range varies by species; at the lowest it can be 10 Hz for some species and for other species the highest reaches up to 200 kHz. Bats that can detect 200 kHz cannot hear very well below 10 kHz. In any case, the most sensitive range of bat hearing is narrower: about 15 kHz to 90 kHz.
Bats navigate around objects and locate their prey using echolocation. A bat will produce a very loud, short sound and assess the echo when it bounces back. The type of insect and how big it is can be determined by the quality of the echo and time it takes for the echo to rebound; there are two types; constant frequency (CF), and frequency modulated (FM) calls that descend in pitch Each type reveals different information; CF is used to detect an object, and FM is used to assess its distance. FM and CM are two different types of echo which inform the bat on the size and distance of the prey. The pulses of sound produced by the bat last only a few thousandths of a second; silences between the calls give time to listen for the information coming back in the form of an echo. Evidence suggests that bats use the change in pitch of sound produced via the Doppler effect to assess their flight speed in relation to objects around them. The information regarding size, shape and texture is built up to form a picture of their surroundings and the location of their prey. Using these factors a bat can successfully track change in movements and therefore hunt down their prey.
Mice have large ears in comparison to their bodies. They hear higher frequencies than humans; their frequency range is 1 to 70 kHz. They do not hear the lower frequencies that humans can; they communicate using high frequency noises some of which are inaudible by humans. The distress call of a young mouse can be produced at 40 kHz. The mice use their ability to produce sounds out of predators' frequency ranges: they can alert other mice of danger without also alerting the predator to their presence. The squeaks that humans can hear are lower in frequency and are used by the mouse to make longer distance calls, as low frequency sounds can travel farther than high frequency sounds.
As aquatic environments have very different physical properties than land environments, there are differences in how marine mammals hear compared to land mammals. The differences in auditory systems have led to extensive research on aquatic mammals, specifically on dolphins.
The auditory system of a land mammal typically works via the transfer of sound waves through the ear canals. Ear canals in seals, sea lions, and walruses are similar to those of land mammals and may function the same way. In whales and dolphins, it is not entirely clear how sound is propagated to the ear, but some studies strongly suggest that sound is channeled to the ear by tissues in the area of the lower jaw. One group of whales, the Odontocetes (toothed whales), use echolocation to determine the position of objects such as prey. The toothed whales are also unusual in that the ears are separated from the skull and placed well apart, which assists them with localizing sounds, an important element for echolocation.
Studies have found there to be two different types of cochlea in the dolphin population. Type I has been found in the Amazon river dolphin and harbour porpoises. These types of dolphin use extremely high frequency signals for echolocation. Harbour porpoise emits sounds at two bands, one at 2 kHz and one above 110 kHz. The cochlea in these dolphins is specialised to accommodate extreme high frequency sounds and is extremely narrow at the base of the cochlea.
Type II cochlea are found primarily in offshore and open water species of whales, such as the bottlenose dolphin. The sounds produced by bottlenose dolphins are lower in frequency and range typically between 75 to 150,000 Hz. The higher frequencies in this range are also used for echolocation and the lower frequencies are commonly associated with social interaction as the signals travel much further distances.
Marine mammals use vocalisations in many different ways. Dolphins communicate via clicks and whistles, and whales use low frequency moans or pulse signals. Each signal varies in terms of frequency and different signals are used to communicate different aspects. In dolphins, echolocation is used in order to detect and characterize objects and whistles are used in sociable herds as identification and communication devices.