Tone decay test is done for:

(b) Neural deafness(Ref. Anirban Biswas, 5th ed. 56)Tone decay is a function of nerve fatigue.If the nerves are fatigued the patient stops hearing a sound given to him continuously before the completion of 1 minute.A tone decay of more than 30dB is suggestive of retrocochlear or neural deafness