Hearing Range - Determination of Hearing Thresholds

Determination of Hearing Thresholds

Audiograms in humans 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 '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 (Sound Pressure Level) 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 older people.

Hearing thresholds of humans unable to cooperate fully in audiometric testing, 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) and intensity (loudness). 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 the toy man in the 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. 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.

The information on different mammals' hearing was obtained primarily by behavioural hearing tests.