Attenuation of high-level impulses by earmuffs

Abstract

Attenuation of high-level acoustic impulses (noise reduction) by various types of earmuffs was measured using a laboratory source of type A impulses and an artificial test fixture compatible with the ISO 4869-3 standard. The measurements were made for impulses of peak sound-pressure levels (SPLs) from 150 to 170 dB. The rise time and A duration of the impulses depended on their SPL and were within a range of 12-400 mus (rise time) and 0.4-1.1 ms (A duration). The results showed that earmuff peak level attenuation increases by about 10 dB when the impulse's rise time and the A duration are reduced. The results also demonstrated that the signals under the earmuff cup have a longer rise and A duration than the original impulses recorded outside the earmuff. Results of the measurements were used to check the validity of various hearing damage risk criteria that specify the maximum permissible exposure to impulse noise. The present data lead to the conclusion that procedures in which hearing damage risk is assessed only from signal attenuation, without taking into consideration changes in the signal waveform under the earmuff, tend to underestimate the risk of hearing damage.

Spectral analysis of hearing protector impulsive insertion loss

“…This is in agreement with previous findings that earmuffs have a linear response to shock waves below approximately 150–160 dB, and do not appreciably demonstrate nonlinear effects until over 170 dB (Zera and Mlynski, 2007). Further evidence for the linear, level-independent earmuff response is provided by the steady-state insertion loss and REAT measurements, which agree nicely with the ISIL data.…”

“…However, the measured IPIL increases as a function of impulse peak test level. Similar results have been observed in previous impulse measurements of passive earmuff peak attenuation (for example, Ylikoski et al, 1987, 1995; Zera and Mlynski, 2007; and unpublished results from our own laboratory). As in these earlier studies, the increase in IPIL with increasing test level may be attributed to a corresponding decrease in both the impulse’s A duration and its low-frequency energy content.…”

“…They found that attenuation was greatest for small-caliber weapons such as pistols and least for large-caliber weapons such as howitzers and cannons. Zera and Mlynski (2007) also studied the peak reduction of impulses by earmuffs, using a laboratory shock tube impulse source and an ATF for measurements. Their measurements were made at peak levels from 150 to 170 dB, with impulses at the higher levels having a shorter A duration.…”

Measurement of impulse peak insertion loss for four hearing protection devices in field conditions

“…Above 140 dB SPL, the attenuation exhibits a small increase by a few tenths of a decibel per decibel increase in peak level (Allen & Berger, 1990;Murphy, 2003). For a hearing protector, with a nonlinear orifice and for earmuffs, the attenuation can increase as much as a 0.5 dB/dB at levels of 140 to 170 dB peak SPL (Dancer et al, 1999;Zera & Mlynski, 2007;Berger & Hamery, 2008).…”

In-depth survey report: design and construction of an acoustic shock tube for generating high-level impulses to test hearing protection devices.

“…Abstract Background: The noise reduction performances of Hearing Protection Devices (HPDs) have normally been evaluated at or near the threshold of hearing because the performance of the device was assumed to be constant across the typical range of occupational noise exposure levels. With the advent of new technologies in hearing protection and associated electronics, the noise reduction performance for some HPDs was found to increase with the increase in the sound levels (Parmentier et al 2002;Zera and Mlynski, 2007;Berger and Hamery 2008;Murphy et al, 2012). These devices are typically described as amplitude-sensitive, non-linear, sound-restoration or level-limiting hearing protectors.…”