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Hearing range

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 20Hz to 20,000Hz (20kHz) (Hz is the standardised term for 'cycles per second'), 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 show 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.

Determination of hearing thresholds

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 1kHz 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 16.5dB SPL at 1kHz whereas the later ANSI-1969/ISO-1963 standard uses 6.5dB SPL, and it is common to allow a 10dB correction for the older standard.

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 they 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.

Land mammals

These graphs show 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.

Humans

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 inner ear bones (the ossicles; malleus, incus and stapes) to move. Sound waves can also be detected through vibration. The number of vibrations per second is called the frequency. Frequency is measured in hertz (Hz); one hertz is one vibration. Specifically in a human, we have a 20- 20,000 Hz frequency range, and an intensity range of 120dB (Elert n.d). Interestingly, 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 cochlear, causing the hairs within it to vibrate. Hairs line the cochlear 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.

Dogs

The hearing ability of a dog is dependent on its breed and age. However, the range of hearing is approximately 40 to 60 000 Hz, which is much greater than that of humans. As with humans, some dog breeds become deafer with age, such as the German Shepard and Miniature Poodle. When dogs hear a sound, they will move their ears towards it, in order to gain maximised 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 to humans, they have a different perception of the world in comparison to humans. Sounds that seem loud to humans often emit high frequency tones that can scare away dogs and 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

Bats require very sensitive hearing to compensate for their lack of visual stimuli, particularly in a hunting situation, and for navigation. 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 return. A bat uses high frequency sounds, inaudible for most human beings (typically 20kHz- 200kHz whereas a human can only hear up to approximately 20kHz). This ensures that the bat has a better echo to go by, as the sound waves are shorter and therefore more specific. The ‘squeaks or squeals’ produced are very loud; 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 movement 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

Mice have large ears in comparison to their bodies; if we compare the relative size of our ears and mice ears we can see a large difference. Mice hear higher frequencies then humans; their frequency range is 1kHz to 70kHz or 90kHz. 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 40kHz. 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

Marine mammals are mammals that inhabit the oceans, bays, and some rivers. As aquatic environments have very different physical properties than that of land mammals, there are many differences in some aspects of how marine mammals hear compared to land mammals. These differences how sound is received particularly auditory system, leading to an extensive amount of research being carried out for this select group of mammals, most 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. On 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.

See also

References

  • Bennu, D. A. N (2001) The Night is Alive With the Sound of Echoes [online] Available from: http://research.amnh.org/users/nyneve/bats.html [28th feb 2007]
  • Richardson, P [n.d] The Secret Life of Bats [online] Available from: http://www.fathom.com/course/21701775/session3.html [28th Feb 2007]
  • D'Ambrose, Chris Frequency Range of Human Hearing. The Physics Factbook. (2003). Retrieved on 28 Feb 2007..
  • Gotfrit, M (1995) Range of human hearing [online] Available from http://www.sfu.ca/sca/Manuals/ZAAPf/r/range.html Zen Audio Project [28th Feb 2007]
  • Hoelzel, A Rus (2002) Marine mammal biology: an evolutionary approach, Oxford: Blackwell Science Ltd
  • Katz, J (2002)5th ed. Clinical Audiology Lippen-Cott Williams and Wilkins

Ketten, D.R. and D. Wartzok (1990) Three-dimensional reconstructions of the dolphin ear. In: Sensory Abilities of Cetaceans: Field and Laboratory Evidence, J. Thomas and R. Kastelein (eds.), Plenum Press, Proc. NATO ASI Ser. A, Life Sci., vol. 196 pp. 81-105. http://www.whoi.edu/csi/research/publications.html

Ketten, D.R. (2000) Cetacean Ears. In: Hearing by Whales and Dolphins. W. Au, R. Fay, and A. Popper (eds.), SHAR Series for Auditory Research, Springer-Verlag, pp. 43-108. http://www.whoi.edu/csi/research/publications.html

  • Lawlor M [n.d] A home for a mouse [online] Societies and animals forum. Available from: http://www.psyeta.org/hia/vol8/lawlor.html [25th Feb 2007]
  • Natural History Museum of Los Angeles County [2006] Hearing: Canine ears are much keener than ours, [online] Los Angeles, Natural History Museum of Los Angeles County. Available from: http://www.nhm.org/exhibitions/dogs/formfunction/hearing.html [26th Feb 2007]
  • Richardson W J (1998) Marine mammals and noise London: Academic
  • Rubel, E. Popper, A. Fay, R (1998) Development of the Auditory System New York: Springer-Verlag inc.
  • Timothy Condon (2003) Frequency Range of Dog Hearing [online] The Physics Factbook. Available from: http://hypertextbook.com/facts/2003/TimCondon.shtml [1st March 2007]

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