Page 39 - Winter 2020
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 Kurtosis: A New Tool for Noise Analysis
Wei Qiu, William J. Murphy, and Alice Suter
Hearing loss due to high-level noise exposure remains a significant occupational health hazard that continues to increase in prevalence in industrial and military work environments despite government-mandated hearing conservation programs. The underlying assumption in current noise standards is that hearing loss over an 8-hour A-weighted equivalent continuous exposure level (often abbreviated as LAeq,8h) can be predicted by the equal energy hypothesis. This method assumes equivalent effects on hearing for a 3 dB increase or decrease in exposure intensity with a halving or doubling of exposure duration, respectively. In other words, equal amounts of hearing loss are expected regardless of how the noise exposure levels have occurred over time. The equal energy hypothesis is the basis of most noise standards and guidelines in the United States and internationally. Although this approach is generally considered appropriate for steady-state noise, it is not adequate for complex noise (Hamernik and Qiu, 2001).
Some Background
Consensus has been lacking on the use of simple energy averaging to predict the effects of noise on hearing. In the United States, some government agencies use a modification consisting of a 5 dB trading relationship, whereas others use the internationally accepted 3 dB rule. Use of the 3 dB rule has been recommended by the National Institute of Occupational Safety and Health (NIOSH) since 1998, which recommendation has been validated based on additional, more recent research (Suter, 2017).
Another issue with using a simple energy metric is the inability of sound energy averaging to account for the increased hazard of noise with impulsive components. Although intermittences in noise exposures may have been considered helpful to hearing in the past, this no longer seems to be the case with complex noise exposures, which are found frequently in manufacturing industries.
Because the additional hazards from impulsive noise were already recognized, the earliest version of the International Standards Organization (ISO) 1999 standard (1971) suggested a 10 dB adjustment to the average exposure level when impulsive noise is superimposed on a background of continuous noise. At a 1981 meeting of noise experts in Southampton, UK, some participants proposed keeping the 10 dB adjustment, with others wanting to change it to 5 dB, and a third group proposing just using simple energy averaging (Personal Observation, Suter, 1981). The resulting report concluded that hearing conservation programs should be initiated at a 5 dB lower level as a precautionary measure whenever there are impulsive noise conditions (von Gierke et al., 1981). Consequently, the 1990 version of the standard contained a note suggesting a 5 dB correction but even that disappeared without explanation in later iterations of the ISO 1999 standard (2013). Since then, more evidence has emerged regarding the hazard to hearing from complex noise environments relative to continuous noise environments.
Complex or Non-Gaussian Noise
A steady-state, continuous noise exposure typically has a normal or Gaussian amplitude distribution (see background in Figure 1). However, the temporal pattern of noise exposures often varies significantly in work environments. A complex noise environment may be described as Gaussian background noise punctuated by a series of high-level transient noises resulting in a non-Gaussian distribution (as shown in Figure 1). These transients can be brief, high-level noise bursts, impulses, or impacts with varying interpeak intervals, peak levels, and peak durations. Industrial workers are often exposed to complex noise environments. Examples include jobs involving maintenance work, metalworking, and power tools, such as impact wrenches and nail guns.
Over the past several decades, a number of studies using animal models have shown that exposure to complex noise produces more hearing damage than an equivalent energy
Volume 16, issue 4 | Winter 2020 • Acoustics Today 39

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