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 contourAn equal-loudness contour is a measure of sound pressure , over the frequency spectrum, for which a listener perceives a constant loudness when presented with pure steady tones. The unit of measurement for loudness levels is the phon, and is arrived at by reference to equal-loudness contours...
s, 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
dBThe decibel is a logarithmic unit of measurement that expresses the magnitude of a physical quantity relative to a specified or implied reference level. Since it expresses a ratio of two quantities with the same unit, it is a dimensionless unit...
SPL (sound pressure level).
Audiograms are set out with frequency in
hertzThe hertz is a unit of frequency. It is defined as the number of complete cycles per second. It is the basic unit of frequency in the International System of Units , and is used worldwide in both general-purpose and scientific contexts...
(Hz) on the horizontal axis, most commonly on a
logarithmIn mathematics, the logarithm of a number to a given base is the power or exponent to which the base must be raised in order to produce the number....
ic scale, and a
linearThe word linear comes from the Latin word linearis, which means created by lines.In mathematics, a linear map or function f is a function which satisfies the following two properties......
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 (
pitchPitch represents the perceived fundamental frequency of a sound. It is one of the three major auditory attributes of sounds along with loudness and timbre. When the actual fundamental frequency can be precisely determined through physical measurement, it may differ from the perceived pitch because...
) and intensity (
loudnessLoudness is the quality of a sound that is the primary psychological correlate of physical strength .Loudness, a subjective measure, is often confused with objective measures of sound pressure such as decibels or sound intensity. Filters such as A-weighting attempt to adjust sound measurements to...
). 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 a toy man in a 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 potentialsBrainstem Auditory evoked potentials are very small electrical voltage potentials which are recorded in response to an auditory stimulus from electrodes placed on the scalp. They reflect neuronal activity in the auditory nerve, cochlear nucleus, superior olive and inferior colliculus of the...
the patient’s brainstem responses are being measured when a sound is played into their ear.
How often hearing should be tested depends mainly on noise exposure. People who are regularly exposed to hazardous noise should have their hearing tested once a year. People with healthy hearing and who are not exposed to much noise should have their hearing tested once every three years.
Measurement
Audiograms are produced using a piece of test equipment called an
audiometerAn audiometer is a machine used for evaluating hearing loss. The invention of this machine is generally credited to Dr. Harvey Fletcher of Brigham Young University. Audiometers are standard equipment at ENT clinics and in audiology centers...
, 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 curveMinimum audibility curve is a standardised graph of the threshold of hearing versus frequency for an average human, and is used as the reference level when measuring hearing loss with an audiometer as shown on an audiogram....
which 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
PhonThe phon was proposed as a unit of perceived loudness level LN for pure tones by S. S. Stevens.-Definition:The purpose of the phon scale is to compensate for the effect of frequency on the perceived loudness of tones...
or the threshold of hearing on the
equal-loudness contourAn equal-loudness contour is a measure of sound pressure , over the frequency spectrum, for which a listener perceives a constant loudness when presented with pure steady tones. The unit of measurement for loudness levels is the phon, and is arrived at by reference to equal-loudness contours...
s, but is standardised in an ANSI standard 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 impairmentA hearing impairment or deafness is a full or partial decrease in the ability to detect or understand sounds. Caused by a wide range of biological and environmental factors, loss of hearing can happen to any organism that perceives sound...
which 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
otosclerosisOtosclerosis is an abnormal growth of bone near the middle ear. It can result in hearing loss.-Clinical description:Chronic conductive hearing loss is the finding in almost all cases of otosclerosis...
or
Meniere's diseaseMénière's disease is a disorder of the inner ear that can affect hearing and balance. It is characterized by episodes of dizziness and tinnitus and progressive hearing loss, usually in one ear. It is caused by lymphatic channel dilation, affecting the drainage of endolymph...
and these can be diagnosed from the shape of the audiogram.
OtosclerosisOtosclerosis is an abnormal growth of bone near the middle ear. It can result in hearing loss.-Clinical description:Chronic conductive hearing loss is the finding in almost all cases of otosclerosis...
results 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 diseaseMénière's disease is a disorder of the inner ear that can affect hearing and balance. It is characterized by episodes of dizziness and tinnitus and progressive hearing loss, usually in one ear. It is caused by lymphatic channel dilation, affecting the drainage of endolymph...
results in a severe loss at low frequencies
http://www.gpnotebook.co.uk/simplepage.cfm?ID=-281739263&linkID=3221&cook=yes.
Noise induced deafness or sensorineural loss 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.
See also
- 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...
- Equal-loudness contour
An equal-loudness contour is a measure of sound pressure , over the frequency spectrum, for which a listener perceives a constant loudness when presented with pure steady tones. The unit of measurement for loudness levels is the phon, and is arrived at by reference to equal-loudness contours...
- Minimum audibility curve
Minimum audibility curve is a standardised graph of the threshold of hearing versus frequency for an average human, and is used as the reference level when measuring hearing loss with an audiometer as shown on an audiogram....
- Articulation index
The Articulation Index is a tool used by audiologists to predict the amount of speech that is audible to a patient with a specific hearing loss. The AI reading for a given patient can range from zero to 1.0, representing the proportion of the average speech signal that is audible...
- Pure tone audiometry
Pure tone audiometry is the key hearing test used to identify hearing threshold levels of an individual, enabling determination of the degree, type and configuration of a hearing loss. Thus, providing the basis for diagnosis and management. PTA is a subjective, behavioural measurement of hearing...
- Psychoacoustics
Psychoacoustics is the study of subjective human perception of sounds. Alternatively it can be described as the study of the psychological correlates of the physical parameters of acoustics.- Background :...
- Hearing (sense)
Hearing is one of the traditional five senses. It is the ability to perceive sound by detecting vibrations via an organ such as the ear...
- Audiology
Audiology is the branch of science that studies hearing, balance, and related disorders. Its practitioners, who treat those with hearing loss and proactively prevent related damage are audiologists. Employing various testing strategies Audiology (from Latin , "to hear"; and from Greek , -logia) is...
- Audiometry
Audiometry is the testing of hearing ability. Typically, audiometric tests determine a subject's hearing levels with the help of an audiometer, but may also measure ability to discriminate between different sound intensities, recognize pitch, or distinguished speech from background noise. ...
- A-weighting
A Weighting curve is a graph of a set of factors, that are used to 'weight' measured values of a variable according to their importance in relation to some outcome. The most commonly known example is frequency weighting in sound level measurement where a specific set of weighting curves known as A,...
- Exposure action value
- Exposure Action Value – Noise :An Exposure Action Value or Action Value is a limit set on occupational exposure to noise where beyond those values, employers must take steps to monitor the exposure levels. These levels are measured in decibels. The American Occupational Safety and Health...
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