Complex noise exposures: an energy analysis.

Industrial noise environments usually present a complex stimulus to the exposed individual. These environments often contain mixtures of multiply reflected impact noises and a relatively Gaussian broadband noise. Noise exposure standards do not consider the possibility of interactions between the two classes of noise that can exacerbate the amount of hearing trauma. This paper presents the results of a large series of experiments designed to document the hazard posed to hearing from complex noise exposures. Twenty-three groups of chinchillas with 5 to 11 animals per group (total N = 135) were exposed for 5 days to either octave bands of noise, impacts alone, or combinations of impact and octave bands of noise. Evoked potential measures of hearing thresholds and cochleograms were used to quantify the noise-induced trauma. The results show that, for sound exposure levels (SEL) which produce less than approximately 10 dB PTS (permanent threshold shift) or 5% total sensory cell loss, equal-energy exposures tend to produce equivalent effects on hearing. However, there is a range of at least 10 dB in the SEL parameter where hearing loss from equal-energy exposures at a particular SEL can be exacerbated by increasing the repetition rate of the impacts or by the addition of a Gaussian low-level noise. The exacerbation of trauma from the addition of a Gaussian continuous noise is dependent upon the spectrum of that noise.