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.
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.
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.
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).
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 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.
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