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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 Decibel (dB) and the X axis represents frequency measured in hertz (Hz). The threshold of hearing is plotted relative to a standardised curve that represents 'normal' hearing, in dB(HL) (hearing level). 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).
The frequencies displayed on the audiogram are octaves, which represent a doubling in frequency (e.g., 250 Hz, 500 Hz, 1000 Hz, wtc). Commonly tested "inter-octave" frequencies (e.g., 3000 Hz) may also be displayed. The intensities displayed on the audiogram appear as linear 10 dBHL steps. However, decibels are a logarithmic scale, so that successive 10 dB increments represent greater increases in loudness.
For humans, normal hearing is between −10 dB(HL) and 15 dB(HL), (2025). 9780133491463, Pearson. ISBN 9780133491463 although 0 dB from 250 Hz to 8 kHz is deemed to be 'average' normal hearing.
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 (). 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 Child, 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. Noise and Hearing Loss Prevention: Frequently Asked Questions. NIOSH Safety and Health Topic.
Different symbols indicate which ear the response is from and what type of response it is. Results of air conduction audiometry (in which the signals are presented to the ear through headphones, which create vibrations in the air) are reported using circles for the right ear and Xs for the left ear. Results of bone conduction audiometry (in which signals are presented using a vibrator which creates vibrations in the temporal bones of the head in order to bypass the outer and middle ear and test the inner ear and auditory nerve alone) are reporting using brackets. The open edge of the bracket indicates the ear tested, with < or representing a left bone conduction threshold. When colors are used on an audiogram, red indicates the right ear and blue indicates the left ear.
In adults, normal hearing is typically defined as thresholds of 25 dB HL or better (lower). Thresholds of 30 dB HL and above indicate hearing loss.
The configuration of thresholds on an audiogram can often help determine the cause(s) of the hearing loss. For example, aging typically leads to hearing thresholds which get poorer as test frequencies get higher. Noise induced hearing loss is typically characterized by a "notch" in the audiogram, with the poorest threshold occurring between 3000 and 6000 Hz (most often 4000 Hz) and better thresholds at lower and higher frequencies.
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). pure tone audiometry in otosclerosis from General Practice Notebook. Retrieved 2012 A deficiency particularly around 2 kHz (termed a Carhart notch in the audiogram) is characteristic of either otosclerosis or a congenital ossicular anomaly. Ménière's disease results in a severe loss at low frequencies. pure tone audiometry in Meniere's disease from General Practice Notebook. Retrieved 2012
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