An audiogram is a graph that shows the audible threshold for standardized frequencies as measured by an audiometer. The Y axis represents intensity measured in decibels and the X axis represents frequency measured in hertz. The threshold of hearing is plotted relative to a standardised curve that represents 'normal' hearing, in dB(HL). 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).
Hearing thresholds of humans and other mammals can be found with behavioural hearing tests or physiological tests used in audiometry. For adults, a behavioural hearing test involves a tester who presents tones at specific frequencies (pitches) and intensities (loudnesses). When the testee hears the sound he or she responds (e.g., by raising a hand or pressing a button. The tester records the lowest intensity sound the testee can hear.
With children, an audiologist makes a game out of the hearing test by replacing the feedback device with activity-related toys such as blocks or pegs. This is referred to as conditioned play audiometry. Visual reinforcement audiometry is also used with children. When the child hears the sound, he or she looks in the direction the sound came from and are reinforced with a light and/or animated toy. 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, or otoacoustic emissions which are generated by a healthy inner ear either spontaneously or evoked by an outside stimulus. In the US, the NIOSH recommends that people who are regularly exposed to hazardous noise have their hearing tested once a year, or every three years otherwise.
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 curve 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 or the threshold of hearing on the equal-loudness contours, but is standardised in an ANSI standard to a level somewhat higher at 1 kHz. 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 16.5 dB(SPL) at 1 kHz whereas the later ANSI-1969/ISO-1963 standard uses 6.5 dB(SPL), and it is common to allow a 10 dB correction for the older standard.
Audiograms and diagnosing types of hearing loss
Most commonly, "conventional" audiometry (utilizing audiograms up to 8 kHz) is used to measure hearing status. For research purposes, or early diagnosis of age-related hearing loss, ultra-high frequency audiograms (up to 20 kHz), requiring special audiometer calibration and headphones, can be measured.
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 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 CMV or Ménière's disease and these can be diagnosed from the shape of the audiogram.
Otosclerosis results in an audiogram with significant loss at all frequencies, often of around 40 dB(HL). A deficiency particularly around 2 kHz (termed a Carhart notch in the audiogram) is characteristic of either otosclerosis or a congenital ossicular anomaly.
Different symbols indicate which ear the response is from and what type of response it is. Red circles are the right ear using headphones and left x's are the left ear using headphones. Brackets < or > denote that a bone conductor was used, not headphones, and this just tests hearing from the middle and inner ear. The direction that the bracket is opening tells you the ear it is testing. These two types of testing help determine if the hearing loss is conductive or sensorineural.
Audiograms are unable to measure hidden hearing loss, which is the inability to distinguish between sounds in loud environments such as restaurants. Hidden hearing loss is caused by synaptopathy in the cochlea, as opposed to sensorineural hearing loss caused by hair cell dysfunction. Audiograms are designed to "estimate the softest sounds the patient can detect" and are not reflective of the situations someone with hidden hearing loss would find themselves in.
- Hearing range
- Equal-loudness contour
- Minimum audibility curve
- Articulation index
- Pure tone audiometry
- Hearing (sense)
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The most commonly employed measure of auditory status is conventional audiometry (0.5-8 kHz).
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For research purposes, or early diagnosis of presbycusis, ultra-high frequency audiograms can be measured. In such cases the test frequencies can go as high as 20 kHz and require special audiometer calibration and headphones.
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