Hearing acuity and susceptibility to noise-induced hearing loss.

Above a critical level (CL) of fatiguing tone, temporary threshold shift (TTS) rises rapidly with intensity. Using a fatiguing tone of 1000 Hz, threshold shift following 1 min stimulation at varying intensities was measured at the same frequency in 55 normal subjects and the CL determined for each. In a second and more limited study, TTS was measured at a frequency half an octave above that of the fatiguing tone. A significant inverse correlation was established between hearing level (HL) and CL at the same frequency with a fall of 2.5 dB in the latter for every 10 dB rise in the former. CLs measured half an octave above the fatiguing tone frequency were appreciably lower by 5 to 11 dB. No correlation was apparent between TTS and HTL, implying that TTS and CL are subserved by differing physiological mechanisms. The findings have relevance to the topic of susceptibility to noise-induced hearing loss and the formulation of damage risk criteria.