Page 46 - Winter 2020

KURTOSIS METRIC IN NOISE ANALYSIS
The effective range of kurtosis in hearing loss evaluation also needs to be determined because the effectiveness of kurtosis is related to exposure level. If the average energy level of the noise exposure is low (e.g., less than 70 dB), it will not contribute to hearing loss no matter how high the value of kurtosis is. At the other extreme, if the peak level of an impulse noise exceeds 140 dB SPL, the mechanisms of hearing damage include both mechanical (potentially immediate effects) and metabolic (longer term effects) strains. The use of kurtosis would be questionable because there are no data about its effectiveness in this area.
Development of Noise Measurement Equipment
The application of kurtosis in noise analysis requires determining the appropriate methods for incorporating kurtosis in a noise measurement. Subsequently, the firmware of noise measurement devices (sound level meters or dosimeters) and existing standards for noise measurement and equipment specifications (e.g., ISO 9612 and IEC 60804) will need to be updated to include kurtosis.
Kurtosis in Theory
Interpretations of the kurtosis statistic for the wide range of noise distributions encountered by acousticians have not been established. Three amplitude distributions may specifically apply to acoustics and kurtosis: (1) pure tones have a bimodal distribution with peaks at the maximum/ minimum pressures; (2) white noise has a normal, Gaussian distribution; and (3) impulsive noise can be realized using a randomly occurring sequence of exponentially decaying noise (see Figure 1). Complex noise may contain pure tones, white noise, and impulsive noise. Quantifying the pure tones, white noise, and impulsive noise may be useful to develop a more realistic noise exposure model (Zechmann, 2019).
Conclusions
Noise-induced hearing loss continues to be a major occupational health hazard. Despite indications that complex noise environments with significant impulsive components are more hazardous to hearing than steady- state noise, simple energy-averaging techniques are still used to measure the effects on hearing. Meanwhile, animal research and, more recently, large-scale studies of workers’ hearing have confirmed that noise kurtosis is critical to the estimation of noise-induced auditory effects. It is now clear
that hearing loss increases as kurtosis increases, and the resulting losses appear to be independent of the detailed temporal structure of the noise for fixed values of kurtosis and energy. These findings have important implications for the protection of noise-exposed populations. A better understanding of the role of the kurtosis metric should lead to a new and more accurate method of noise-exposure measurement and hearing risk assessment.
Disclaimer
Thefindingsandconclusionsinthisreportarethoseofthe authors and do not necessarily represent the official position of the National Institute for Occupational Safety and Health or the Centers for Disease Control and Prevention.
References
Davis, R. I., Qiu, W., and Hamernik, R. P. (2009). Role of the kurtosis statistic in evaluating complex noise exposures for the protection of hearing. Ear and Hearing 30, 626-634. https://doi.org/10.1097/AUD.0b013e3181b527a8.
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Erdreich, J. (1986). A distribution based definition of impulse noise. The Journal of the Acoustical Society of America 79, 990-998. https://doi.org/10.1121/1.393698.
Fuente, A., Qiu, W., Zhang, M., Xie, H., Kardous, C. A., Campo, P., and Morata, T. C. (2018). Use of the kurtosis statistic in an evaluation of the effects of noise and solvent exposures on the hearing thresholds of workers: An exploratory study. The Journal of the Acoustical Society of America 143, 1704-1710. https://doi.org/10.1121/1.5028368.
Goley, G. S., Song, W. J., and Kim, J. H. (2011). Kurtosis corrected sound pressure level as a noise metric for risk assessment of occupational noises. The Journal of the Acoustical Society of America 129(3),1475-1481. https://doi.org/10.1121/1.3533691.
Hamernik, R. P., and Qiu, W. (2001). Energy-independent factors influencing noise-induced hearing loss in the chinchilla model. The Journal of the Acoustical Society of America 110, 3163-3168. https://doi.org/10.1121/1.1414707.
Hamernik, R. P., Qiu, W., and Davis, R. (2003). The effects of the amplitude distribution of equal energy exposures on noise-induced hearing loss: The kurtosis metric. The Journal of the Acoustical Society of America 114, 386-395. https://doi.org/10.1121/1.1582446.
Hamernik, R. P., Qiu, W., and Davis, R. (2007). Hearing loss from interrupted, intermittent, and time varying non-Gaussian noise exposure: The applicability of the equal energy hypothesis. The Journal of the Acoustical Society of America 122, 22457. https://doi.org/10.1121/1.2775160.
International Organization for Standardization (ISO) 1999, (1971). Assessment of Occupational Noise Exposure for Hearing Conservation Purposes, ISO, Geneva, Switzerland.
46 Acoustics Today • Winter 2020