Audiogram

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 (dB) and the X axis represents frequency measured in hertz (Hz).^[1] 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).

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" frequenices (e.g., 3000 Hz) may also be displayed. The intensities displayed on the audiogram appear as linear 10 dBHL steps. However, decibels are a logarithimic 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),^[2][3] 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 (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.^[4]