Risk of hearing damage: “Significance of temporal characteristics in the exposure signal”

The international standards use the equivalent A-weighted level LAeq as the main parameter for assessment of risk of hearing damage. However, daily noise sources may differ in their physical properties and even though two noise sources can produce the same LAeq, they may not have the same influence on hearing. Many studies have shown that this parameter fails as a predictor of noise-induced hearing loss especially in the assessment of impulse noise. Currently, there is no common agreement neither on the right method to assess impulse noise nor which parameters are the most relevant to describe the impulses and the effects on hearing. This project focuses on the influence of temporal characteristics of impulsive signals with respect to hearing damage. Thus, the project will look into relevant parameters describing temporal characteristics of impulse noise, and how these parameters describe physiological changes in our hearing (as caused by over-exposure). Temporary (and only temporary) changes are induced and monitored by determining the temporal threshold shifts (TTS), and changes in the distortion product otoacoustic emissions (DPOAE). Analysis of impulsive signals and literature reviews point to kurtosis and crest-factor as possible descriptors for impulsive noise that have shown correlation to hearing damage induced in animals. An additional measure developed by the US. military (AHAAH model) based on auditory models aiming at expressing the amplitude of basilar membrane vibration as a measure of risk was also investigated. An experiment has been carried out with the aim of investigating the effects of different time characteristic of the exposure stimulus. The chosen stimuli where constructed from binaural recordings from a machine workshop. Different sound sources form stationary to completely impulsive were combined to give different level of kurtosis, crest-factor and risk-rating according to the AHAAH model. Two exposure stimuli were used: one corresponding to a continuous sound exposure normalized to an exposure level of 80 dB; and a combination of an impulsive and continuous noise normalized to an exposure level of 75 dB. The frequency content was kept constant for both stimuli. The stimuli and level normalization was chosen to investigate differences in the effects of impulsive and continuous noise, and to assess the influence of the 5 dB penalty for noise of impulsive character (penalty suggested by standards and part of the the Danish legislation that affects all sound exposures with peak-levels higher that 114 dB, A- or C-weighted occuring more than once per minute). A group of subjects participated in two experimental sessions, where in each session one of the two exposure stimuli was presented. DPOAE were monitored before and at regular time intervals after each exposure. Hearing thresholds were also monitored before and approximately two hours after each exposure. The group results showed that the continuous exposure had a greater impact on the DPOAE levels, with a maximum DPOAE shift of approximately 5 dB in the frequency range from 2 to 3.15 kHz during the first 10 minutes of the recovery. No evident DPOAE shift was observed after the impulsive exposure. The results from this experiment suggest that for the exposures used, the risk of hearing loss from low-level impulses may be predicted with the equal-energy principle outlined in the current standards and legislation. The 5 dB penalty suggested for impulsive noise seems to overestimate the risk from the low-level impulses used in this experiment. (Supported by FTP)