Audiogram
- For the record label named "Audiogram", see Audiogram (label)
An audiogram is a graphical representation of how well a certain person can perceive different sound frequencies.
An audiogram is a normalised conversion of hearing thresholds from dBSPL to dBHL, where dB is decibel, SPL is sound pressure level and HL is hearing level. Audiograms are set out with frequency in Hz on the horizontal axis, most commonly on a logarithmic scale, and a linear dBHL scale on the vertical axis. Normal hearing is classified as being between -10dBHL and 15dBHL, although 0dB from 250Hz to 8kHz is deemed to be 'average' normal hearing.
Hearing thresholds of humans 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.
File:Landrange.jpg File:Landmarinerange.jpg
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 deaf 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 10kHz to 70kHz. They do not hear the lower frequencies that we can; therefore they communicate using high frequency noises that are inaudible for 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 then the high frequency sounds (Lawlor).
Marine Mammals
Marine mammals are mammals that primarily inhabit marine environments. There are many different types and all vary in terms of form and function. As the environment in which they live completely contrasts the habitat of land mammals, there are many differences in evolution. Due to this, the internal functioning of marine mammals varies to that of the land mammals. These differences extend as far as the 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. If such mammals are submerged underwater, the traditional methods of auditory transmission cannot be sustained. Instead the sound is transmitted via bone conduction at all angles around the skull. Due to this process however there is a loss of localization ability, partially due to an impedance mismatch between the water and cranial tissue.
In the dolphin population and in other toothed whales however, this process does not occur. The auditory organs are isolated from the skull, preventing mass bone conduction from taking place.
It has been found through numerous experiments, that dolphins and similar species of marine mammal have two potential ways of hearing; one for low frequency signals and one for echolocation signals. The first is similar to that of land mammals. Bottlenose dolphins have an external meatus, as found in humans, and despite often being filled with cellular debris, particularly in adult dolphins, tests have shown that it acts as an auditory pathway. Electrophysiological tests have found minimum audible threshold levels, when sounds are played below 20-30 Hz, just above this anatomical area which is made up of the ear canal leading to the tympanic membrane.
Dolphins use the process of echolocation to determine the position of objects, such as prey. Studies have shown that echolocation signals are transmitted through a process similar to bone conduction. It is thought that the sounds travel from the water to the dolphin, via the mandibular fat channels, found in the lower jaw. As this area has a similar impedance to that of the water, transmission is much faster.
Studies have found there to be two different types of cochlea in the dolphin population. Type one has been found in the Amazon River dolphin and harbour porpoises. These types of dolphin communicate with high frequency signals that are better suited to traveling shorter distances. It has been found that the harbour porpoise emits click sounds in two frequency bands, one at 2 kHz and one between 120-140 kHz. The cochlea in these dolphins is specialised to accommodate high frequency sounds and it becomes is narrower at the apex.
The second type of cochlea is found in offshore and open water species of dolphin, such as the bottlenose dolphin. The sounds produced by Bottlenose dolphins are low frequency and range between 0.25 to 150 kHz. The higher frequencies in this range make up echolocation and the majority of the low frequencies are for social interaction as the signals travel much further distances. (Hoelzel 2002)
These frequency ranges relates to the thresholds obtained from studies of dolphins.
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 clicks and whistles are used in sociable herds as identification and communication devices. The auditory system of marine mammals is adapted to enable these forms of communication. This is shown through the evolution of the positioning of the auditory processes in dolphins, and the specialised cochlea’s found in dolphins dependent on their habitat.
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 (2003). "Frequency Range of Human Hearing". The Physics Factbook. Retrieved 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
- 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]
