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Figure 5. Prevalence of age-adjusted high-frequency hearing loss (AHFNIHL) as a function of cumulative noise exposure for workers exposed to Gaussian versus complex (non- Gaussian) noise. A: unadjusted dose-response curves. B: kurtosis-adjusted dose-response curves. Adapted from Zhao et al., 2010.
exposure level calculations to estimate the risk of hearing loss from any type of noise environment. Kurtosis can be used to adjust the cumulative noise exposure to yield a consistent estimate of hearing loss from exposures across a variety of noise environments using a single metric.
Recently, Zhang and colleagues (2020) conducted large- scale (N = 2,333) epidemiological studies in Chinese industries. To evaluate the effects of noise level, duration, and kurtosis on hearing loss, the entire cohort was divided into four noise-exposure levels, two exposure
durations, and four kurtosis categories. The details of these divisions are presented in Figure 6.
Figure 6 shows the effects of noise level and kurtosis on the actual measured noise-induced threshold shift for these two exposure durations: less than 10 years and greater than 10 years. The results from this database of workers exposed to various industrial noises are in general agreement with previous experiments using the chinchilla model.
Human data, however, show some peculiarities.
(1) For exposure durations less than or equal to 10 years, the relationship between hearing loss and
kurtosis value is clear. For a fixed noise level, noise- induced hearing loss increased as the kurtosis value of the noise increased (as shown in Figure 6A). In the first decade of exposure to high-level noise, complex noise with a kurtosis greater than 10 was more hazardous than steady-state Gaussian noise.
(2)As the exposure duration increased beyond 10 years, the difference in hearing loss between the Gaussian (K1), low (K2), and medium (K3) kurtosis groups (β ≤ 75) tended to fade away (as shown in Figure 6B). However, the hearing loss in the high (K4) kurtosis group (β > 75) was still significantly larger than that of other groups. This suggests that the presence of certain types of impulse or impact noise characterized by high kurtosis values can cause hearing damage faster and continue over a long exposure time.
Consistent with ISO 1999, hearing loss occurs more quickly during the first 10 years of exposure to complex noise environments, and these data demonstrate the necessity of attention to workers from the beginning of their noise exposures.
Some Considerations Regarding the Kurtosis Application
Kurtosis Calculations
For a sample of n values, the kurtosis is calculated as
β = (1)
where xi is the ith value and x̅ is the sample mean. If, based on existing data, one accepts the proposition that kurtosis should be routinely measured in all industrial noise exposures, then one must try to find the best way to measure it. Because the kurtosis value is dependent on the length of the window over
Winter 2020 • Acoustics Today 43