An audiogram is a standard way of representing a person's hearing loss [http://www.phonak.co.uk/ccuk/consumer/hearing/audiogram.htm] . Most audiograms cover the limited range 100Hz to 8000Hz (8kHz) which is most important for clear understanding of speech, and they plot the threshold of hearing relative to a standardised curve that represents 'normal' hearing, in dBHL. They are not the same as
equal-loudness contours, which are a set of curves representing equal loudness at different levels, as well as at the threshold of hearing, in absolute terms measured in dB SPL (sound pressure level).
Audiograms are set out with frequency in
hertz(Hz) on the horizontal axis, most commonly on a logarithmic scale, and a lineardBHL 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
mammalscan 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 potentialsthe patient’s brainstem responses are being measured when a sound is played into their ear.
Audiograms 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 curvewhich is intended to represent a 'normal' hearing. This is not the best threshold found for all subjects, under ideal test conditions, which is represented by around 0 Phonor the threshold of hearingon the equal-loudness contours, but is standardised in an ANSIstandard to a level somewhat higher at 1kHz [http://books.google.co.uk/books?id=DPZ4hvf2gG0C&pg=PA74&lpg=PA74&dq=audiograms+referrence+levels&source=web&ots=b3Q22J-Yo0&sig=sQyzhk_ECTPocPLldvcnlItJzdk&hl=en#PPA56,M1] . 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.
Audiograms and diagnosing types of hearing loss
Ideally the audiogram would show a straight line, but in practice everyone is slightly different, and small variations are considered normal. Larger variations, especially below the norm, may indicate
hearing impairmentwhich occurs to some extent with increasing age, but may be exascerbated by prolonged exposure to fairly high noise levels such as by living close to an airport or busy road, work related exposure to high noise, or brief exposure to very high sound levels such as gunshot or music in either a loud band or clubs and pubs. Hearing impairment may also be the result of certain diseases such as otosclerosisor Meniere's diseaseand these can be diagnosed from the shape of the audiogram. Otosclerosisresults in an audiogram with significant loss at all frequencies, often of around 40dB [http://www.gpnotebook.co.uk/simplepage.cfm?ID=-234487807&linkID=3223&cook=yes] . Meniere's diseaseresults in a severe loss at low frequencies [http://www.gpnotebook.co.uk/simplepage.cfm?ID=-281739263&linkID=3221&cook=yes] . Noise induced deafnessor sensorineuralloss results in loss at high frequencies, especially around 4kHz and above, depending on the nature of the exposure to loud noise [http://www.gpnotebook.co.uk/simplepage.cfm?ID=-275120127&linkID=3222&cook=yes] .
Minimum audibility curve
Pure tone audiometry
Exposure action value
* 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]
* Katz, J (2002)5th ed. Clinical Audiology Lippen-Cott Williams and Wilkins
* Rubel, E. Popper, A. Fay, R (1998) Development of the Auditory System New York: Springer-Verlag inc.
* [http://www.schooltrain.info/deaf_studies/audiology2/levels.htm Levels of hearing loss]
* [http://www.gpnotebook.co.uk/simplepage.cfm?ID=-288358399&linkID=3220&cook=yes Pure tone audiometry in glue-ear]
* [http://www.gpnotebook.co.uk/simplepage.cfm?ID=-281739263&linkID=3221&cook=yes Audiogram of Meniere's disease]
* [http://www.gpnotebook.co.uk/simplepage.cfm?ID=-275120127&linkID=3222&cook=yes Audiogram in nerve-deafness]
* [http://www.gpnotebook.co.uk/simplepage.cfm?ID=-234487807&linkID=3223&cook=yes Audiogram in Otosclerosis]
* [http://www.gpnotebook.co.uk/simplepage.cfm?ID=-227868671&linkID=3224&cook=yes Audiogram in Chronic otitis media]
* [http://www.gpnotebook.co.uk/simplepage.cfm?ID=845873165 Audiogram - GP notebook]
* [http://psychology.utoledo.edu/default.asp?id=202 Behavioral Audiograms of Mammals (in Table Form)]
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