Temporary threshold shifts from exposures to equal equivalent continuous A-weighted sound pressure level

According to existing methods for the assessment of hearing damage, signals with the same A-weighted equivalent level should pose the same hazard to the auditory system. As a measure of hazard, it is assumed that Temporary Thresholds Shifts (TTS) reflect the onset of alterations to the hearing system that, if continued, will eventually lead to a permanent damage. Under this premise, results from a TTS experiment using three different band-pass signals derived from broad-band noise are presented. TTS was determined for 9 subjects exposed to three 2-octave bands of noise: Low (125 to 500 Hz), mid (0.5 to 2 kHz), and high (2 to 8 kHz). The band-pass signals were adjusted to give the same equivalent A-weighted sound exposure according to ISO-1999 [1] and were presented to one ear of the subjects in three separate experimental sessions. The results showed that there is a great difference in the subjects response to the exposures. The highest TTS with the longest recovery was observed after the high-band exposure. The mid-band exposure also resulted in high levels of TTS, but with shorter recovery. The low-band exposure did not show any statistically significant TTS. These results contradict the assumptions made using the A-weighting curve for the assessment of hearing damage. By modifying exposure ratings to compensate for the build-up of energy at mid and high-frequencies (above 1 kHz) due to the presence of the listener in the sound field and for the levels below an effect threshold that does not induce changes in hearing (equivalent quiet levels), ratings of the sound exposure that reflect the observed temporary changes in auditory function can be obtained.