The equal energy hypothesis (EEH) is the basis of our current noise exposure criteria. Application of the EEH in many industrial environments severely underestimates hearing loss. For the past two funding cycles we have been studying, in an animal model, the effects that the statistical properties of a complex noise exposure, as embodied in the single number metric, the kurtosis, have on hearing. The kurtosis [(t)], defined as the ratio of the fourth-order central moment to the squared second-order moment of the amplitude distribution, has been shown to be important in identifying and quantifying the hazard to hearing from complex industrial noise environments. The primary aim of the earlier work was to develop an approach to measuring noise that would predict the hearing loss that will result from long-term industrial exposures. This proposal, which is a continuation of this effort, has a single specific aim: To develop an understanding of how the variables that define a complex noise affect hearing loss for a fixed value of the kurtosis. In our previous work we varied parameters such as (i) the peak and (ii) interval histograms and (iii) duration of the impact noise transients in order to produce, at a constant energy, a broad range of (t) over which the EEH and hearing trauma could be studied. An extension of the work, and one that is necessary to establish guidelines for using (t) in noise assessment and measurement practice, is to understand how the above three `primary variables'[(i), (ii), and (iii)] affect hearing trauma at a constant (t). Considering that for a given spectral distribution of energy, there is virtually an infinite number of different noise exposures that can have the same (t), the question that we are asking is: In order to estimate hearing loss from prolonged exposure, to what extent do we need to know something about the `primary variables'that define the complex noise or is the kurtosis a sufficiently encompassing metric? All planned exposures will be of equal energy and will be structured to model some of the essential features of an idealized workweek lasting three weeks. [Long-term exposures often yield results that differ from those found from shorter acute exposures.] Exposures will be nonGaussian, interrupted and intermittent. For three values of the kurtosis, (t) = 10, 25 and 50, we will establish the relation among: (i) the three primary variables (peak, interval and impact duration histograms) that reflect the basic statistical properties of a complex noise;(ii) the kurtosis;and (iii) the noise-induced hearing loss as established by hearing thresholds and sensory cell loss. These results will complement the results generated over the previous two funding cycles by evaluating the sensitivity of (t) to the primary variables that define a complex noise. Such data is a necessary prerequisite to the application of (t) to industrial hearing conservation/noise measurement protocols. This data will also contribute to our continuing evaluation, application and understanding of the EEH. Project Narrative: The equal energy hypothesis (EEH) is the basis of our current noise exposure criteria. Application of the EEH in many industrial environments severely underestimates hearing loss. The proposed research is aimed at developing a new noise measurement and evaluation strategy. That will predict the potential of an industrial noise to produce hearing loss.
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