Audiogram
Encyclopedia
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 100 Hz to 8000 Hz (8 kHz) 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 dB(HL). They are not the same as equal-loudness contour
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...

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 dB
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...

 SPL (sound pressure level).

Audiograms are set out with frequency in hertz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

 (Hz) on the horizontal axis, most commonly on a logarithm
Logarithm
The logarithm of a number is the exponent by which another fixed value, the base, has to be raised to produce that number. For example, the logarithm of 1000 to base 10 is 3, because 1000 is 10 to the power 3: More generally, if x = by, then y is the logarithm of x to base b, and is written...

ic scale, and a linear
Linear
In mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...

 dBHL scale on the vertical axis. Normal hearing is classified as being between -10dB(HL) and 15dB(HL), although 0dB from 250 Hz to 8 kHz 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
Pitch (music)
Pitch is an auditory perceptual property that allows the ordering of sounds on a frequency-related scale.Pitches are compared as "higher" and "lower" in the sense associated with musical melodies,...

) and intensity (loudness
Loudness
Loudness is the quality of a sound that is primarily a psychological correlate of physical strength . More formally, it is defined as "that attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud."Loudness, a subjective measure, is often...

). 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 potentials
Brainstem auditory evoked potentials
Brainstem 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 audiometer
Audiometer
An audiometer is a machine used for evaluating hearing loss. Audiometers are standard equipment at ENT clinics and in audiology centers. They usually consist of an embedded hardware unit connected to a pair of headphones and a test subject feedback button, sometimes controlled by a standard PC...

, 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
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....

 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 Phon
Phon
The phon was proposed in DIN 45631 and ISO 532 B 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 contour
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...

s, but is standardised in an ANSI
Ansi
Ansi is a village in Kaarma Parish, Saare County, on the island of Saaremaa, Estonia....

 standard to a level somewhat higher at 1 kHz 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 1 kHz 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 impairment
Hearing impairment
-Definition:Deafness is the inability for the ear to interpret certain or all frequencies of sound.-Environmental Situations:Deafness can be caused by environmental situations such as noise, trauma, or other ear defections...

 which occurs to some extent with increasing age, but may be exacerbated 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 otosclerosis
Otosclerosis
Otosclerosis is an abnormal growth of bone near the middle ear. It can result in hearing loss.-Clinical description:Otosclerosis can result in conductive and/or sensorineural hearing loss...

 or Ménière's disease
Ménière's disease
Ménière's disease is a disorder of the inner ear that can affect hearing and balance to a varying degree. It is characterized by episodes of vertigo and tinnitus and progressive hearing loss, usually in one ear. It is named after the French physician Prosper Ménière, who, in an article published...

 and these can be diagnosed from the shape of the audiogram.
Otosclerosis
Otosclerosis
Otosclerosis is an abnormal growth of bone near the middle ear. It can result in hearing loss.-Clinical description:Otosclerosis can result in conductive and/or sensorineural hearing loss...

 results in an audiogram with significant loss at all frequencies, often of around 40dB(HL) http://www.gpnotebook.co.uk/simplepage.cfm?ID=-234487807&linkID=3223&cook=yes.

Ménière's disease
Ménière's disease
Ménière's disease is a disorder of the inner ear that can affect hearing and balance to a varying degree. It is characterized by episodes of vertigo and tinnitus and progressive hearing loss, usually in one ear. It is named after the French physician Prosper Ménière, who, in an article published...

 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 4 kHz 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.

Typical examples of audiograms showing conductive, noise induced and age-related hearing loss can be found here.

See also

  • Hearing range
    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
    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
    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
    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
    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
    Psychoacoustics is the scientific study of sound perception. More specifically, it is the branch of science studying the psychological and physiological responses associated with sound...

  • Hearing (sense)
    Hearing (sense)
    Hearing is the ability to perceive sound by detecting vibrations through an organ such as the ear. It is one of the traditional five senses...

  • Audiology
    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
    Audiometry is the testing of hearing ability, involving thresholds and differing frequencies. 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...

  • A-weighting
    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
    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 Administration set the EAV to...


External links

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