The Effectiveness of Hearing Protection Devices: A Systematic Review and Meta-Analysis

To prevent intensive noise exposure in advance and be safely controlled during such exposure, hearing protection devices (HPDs) have been widely used by workers. The present study evaluates the effectiveness of these HPDs, partitioned into three different outcomes, such as sound attenuation, sound localization, and speech perception. Seven electronic journal databases were used to search for published articles from 2000 to 2021. Based on inclusion criteria, 20 articles were chosen and then analyzed. For a systematic review and meta-analysis, standardized mean differences (SMDs) and effect size were calculated using a random-effect model. The funnel plot and Egger’s regression analysis were conducted to assess the risk of bias. From the overall results of the included 20 articles, we found that the HPD function performed significantly well for their users (SMDs: 0.457, 95% confidence interval (CI): 0.034–0.881, p < 0.05). Specifically, a subgroup analysis showed a meaningful difference in sound attenuation (SMDs: 1.080, 95% CI: 0.167–1.993, p < 0.05) when to wear and not to wear HPDs, but indicated no significance between the groups for sound localization (SMDs: 0.177, 95% CI: 0.540–0.894, p = 0.628) and speech perception (SMDs: 0.366, 95% CI: −0.100–1.086, p = 0.103). The HPDs work well for their originally designated purposes without interfering to find the location of the sound sources and for talking between the workers. Taking into account various factors, such as the characteristics of the users, selection of appropriate types, and fitting methods for wearing in different circumstances, seems to be necessary for a reliable systematic analysis in terms of offering the most useful information to the workers.

Nonlinear behavior of Helmholtz resonator with a compliant wall for low frequency, broadband noise control

Low frequency sound attenuation is often pursued using Helmholtz resonators (HRs). The introduction of a compliant wall around the acoustic cavity results in a two-degree-of-freedom (2DOF) system capable of more broadband sound absorption. In this study, we report the amplitude-dependent dynamic response of a compliant walled HR and investigate the effectiveness of wall compliance to improve the absorption of sound in linear and nonlinear regimes. The acoustic-structure interactions between the conventional Helmholtz resonator and the compliant wall result in non-intuitive responses when acted on by nonlinear amplitudes of excitation pressure. This paper formulates and studies a reduced order model to characterize the nonlinear dynamic response of the 2DOF HR with a compliant wall compared to that of a conventional rigid HR. Validated by experimental evidence, the modeling framework facilitates an investigation of strategies to achieve broadband sound attenuation, including by selection of wall material, wall thickness, geometry of the HR, and other parameters readily tuned by system design. The results open up new avenues for the development of efficient acoustic resonators exploiting the deflection of a compliant wall for suppression of extreme noise amplitudes.

A comparison of standardized methods for sound attenuation test of hearing protectors

In order to prevent hearing impairment due to occupational noise, it is essential to wear hearing protectors such as earplugs and earmuffs, especially in an extremely noisy environment. The method of measuring their sound attenuation is defined by international standards such as ISO and IEC, and standards such as ANSI, BS, AS/NZS, JIS et.. Although most standards recommend subjective methods where the thresholds of hearing shall be measured once with open ears and once with the hearing protector in place for each subject, measurement and evaluation methods are not unified internationally. In Japan, in April 2020, the old product standard was abolished in consideration of international consistency, and a new method standard was established with the ISO standard as the corresponding international standard for the first time in about 40 years. In this study, we compared the measurement methods and evaluation methods according to the standards for sound attenuation tests of hearing protectors.