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Failure of an individual to maintain the perception of a steady sound, presented initially at a just-audible level, under prolonged stimulation. TTD is measured by the increase in sound level required to maintain the sound at a just-audible level for a specified period of time, e. g. 30 or 60 s.
The stimulus level is varied continuously via a dial and the stimulus level is measured by the tester at the end. The threshold is the mean of the just audible and just inaudible levels.
The most commonly used assessment of hearing is the determination of the threshold of audibility, i. e. the level of sound required to be just audible. This level can vary for an individual over a range of up to 5 Decibels from day to day and from determination to determination, but it provides an additional and useful tool in monitoring the potential ill effects of exposure to noise. Before carrying out a hearing test, it is important to obtain information about the person’s past medical history, not only concerning the ears but also other conditions which may have a bearing on possible hearing loss detected by an audiometric test.