For more detail on human hearing see Audiogram, Equal loudness contours and Hearing impairment.
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 said to be 20 Hz (cycles per second) to 20 kHz (20,000 Hz), although there is considerable variation between individuals, especially at the high frequency end, where a gradual decline with age is considered normal. Sensitivity also varies a lot with frequency, as shown by equal-loudness contours, which are normally only measured for research purposes, or detailed investigation. Routine investigation for hearing loss usually involves an audiogram which shows threshold levels relative to a standardised norm.
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Audiograms in humans are produced using a piece of test equipment called an audiometer, and this allows different frequencies to be presented to the subject, usually over calibrated headphones, at any specified level. The levels are, however, not absolute, but weighted with frequency relative to a standard graph known as the minimum audibility curve which is intended to represent 'normal' hearing. This is not the best threshold found for all subjects, under ideal test conditions, which is represented by around 0 Phon or the threshold of hearing on the equal-loudness contours, but is standardised in an ANSI standard to a level somewhat higher at 1 kHz.[1] There are several definitions of the minimal audibility curve, defined in different international standards, and they differ significantly, giving rise to differences in audiograms according to the audiometer used. 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, and it is common to allow a 10 dB correction for older people.
Hearing thresholds of humans unable to cooperate fully in audiometric testing, and other mammals can be found by using behavioural hearing tests or physiological tests. An audiogram can be obtained using a behavioural hearing test called Audiometry. For humans the test involves different tones being presented at a specific frequency (pitch) and intensity (loudness). When the person hears the sound they raise their hand or press a button so that the tester knows that they have heard it. The lowest intensity sound they can hear is recorded. The test varies for children; their response to the sound can be a head turn or using a toy. The child learns what they can do when they hear the sound, for example they are taught that when they heard the sound they can put the toy man in the boat. A similar technique can be used when testing some animals but instead of a toy food can be used as a reward for responding to the sound. Physiological tests do not need the patient to respond (Katz 2002). For example when performing the brainstem auditory evoked potentials the patient’s brainstem responses are being measured when a sound is played into their ear.
The information on different mammals' hearing was obtained primarily by behavioural hearing tests.
The following sections describe the frequency range of a specific mammal's hearing in comparison to other mammals. Low pitch sounds are low in frequency; the high pitch sounds are high in frequency.
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. Specifically, humans have a maximum aural range that begins as low as 12 Hz under ideal laboratory conditions,[2] to 20,000 Hz in most children and some adults, but the range shrinks during life, usually beginning at around the age of 8 with the higher frequencies fading. 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 (Gotfrit 1995). 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 "silent" dog whistles exploit this phenomenon by producing 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[1].
The hearing ability of a dog is dependent on its breed and age. However, the range of hearing is approximately 40 Hz to 60,000 Hz,[3] which is much greater than that of humans. As with humans, some dog breeds become more deaf with age,[4] 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. This 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,[4] 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 as guard dogs due to their increased hearing ability (Condon 2003).
Bats require very sensitive hearing to compensate for their lack of visual stimuli, particularly in a hunting situation, and for navigation. Their hearing range is between 20 Hz and 120,000 Hz. They locate their prey by means of 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 (Bennu 2001). Each type reveals different information for the bat; CF is used to detect an object, and FM is used to provide information regarding the nature of the object and its distance. 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. There is also evidence to suggest that bats use the change in pitch of sound produced (the Doppler effect) to assess their flight speed in relation to objects around them (Richardson n.d). 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. Mice hear higher frequencies than humans; their frequency range is 1 kHz to 70 kHz or 90 kHz. They do not hear the lower frequencies that we 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 and hear sounds out of our and other predators' frequency ranges to their advantage. They can alert other mice of danger without also alerting the predator to their presence. The squeaks that we can hear a mouse make are lower in frequency and are used by the mouse to make longer distance calls, as the low frequency sound can travel further than the high frequency sounds (Lawlor).
Marine mammals are mammals that inhabit the oceans, bays, and some rivers. 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 various kinds of dolphins.
The auditory system of a land mammal typically works via the transfer of sound waves through the ear canals. Ear canals in the pinnipeds or 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 or toothed whales, use the process of 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 (Ketten and Wartzok 1990) 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. It has been found that the 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 0.25 to 150 kHz. 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 vocalizations 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.
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