Tone decay test is done for

THRESHOLD TONE DECAY TEST It is a measure of nerve fatigue and is used to detect retrocochlear lesions. Normally, a person can hear a tone continuously for 60 s. In nerve fatigue, he stops hearing earlier. The threshold tone decay test is simple and is performed in the following manner: A tone of 4000 Hz is presented at 5 dB above the patient's threshold of hearing, continuously for a period of 60 s. If patient stops hearing earlier, intensity is increased by another 5 dB. The procedure is continued till patient can hear the tone continuously for 60 s, or no level exists above the threshold where tone is audible for full 60 s. The result is expressed as number of dB of decay. A decay more than 25 dB is diagnostic of a retrocochlear lesion. Ref:- Dhingra; pg num:-26