Real-world attenuation of custom-moulded earplugs: Results from industrial in situ F-MIRE measurements

Abstract

Abstract:The attenuation provided by hearing protection devices (HPDs) is traditionally determined under laboratory conditions by means of a standardised method titled REAT (Real Ear Attenuation at Threshold). This performance is often overestimated compared to what has been measured so far in industrial reality. Concerning the earmuff, the formable earplug and preformed earplug type of HPD, there are numerous data related to the discrepancy between, on one hand, the laboratory measured attenuation which is labelled by the manufacturers and, on the other hand, the in situ estimated attenuation. But, few exist concerning custom-moulded earplug (CMEP) type protection devices. This paper presents the practical application of measurements intended to estimate the attenuation of this type of protection device while it is being worn by users during execution of their task. Three manufacturers of hearing protectors took part in this initiative; this participation involved the manufacturing of custom-moulded earplugs with a miniature microphone inserted. The measurements were carried out on 63 employees in nine industrial sites with diverse activities. The estimate of this in situ attenuation, in this study called level reduction (LR), was obtained from the difference between the noise exposure level measured close to the ear of the employee and the residual noise level measured under the earplug. The CMEP level reduction estimated in situ highlighted significant variability characteristics. The results of the measurements confirm the overestimation of the values indicated by the manufacturers. From 3 to 5 dB at high frequencies, the discrepancy can reach 8-10 dB at medium and low frequencies. The study also confirms that CMEPs which have been tested here, like other earplugs types, are unsuited to attenuate low frequencies.

Systematic Evaluation of the Relationship between Physical and Psychoacoustical Measurements of Hearing Protectors’ Attenuation

“…As with other studies, it is concluded that the MIRE approach leads to lower standard deviation than with the REAT method. Other comparisons between MIRE-based and REAT attenuation values obtained in laboratory or field environments have been published recently (10)(11)(12)(13)(14) .They all showed good correlations between the two measurements methods but also how can F-MIRE be used and implemented with success to measure the attenuation and assess the efficiency of HPDs in the field. However, correction factors used to relate the MIRE/F-MIRE to REAT are generally 7 approximations derived from ensemble averages or taken from literature data and there are generally little details on how these corrections are affected by various factors such as the positioning of the microphones, the physical characteristics of the individuals, the sound field, etc.…”

“…On the other hand, the REAT can also be expressed in terms of the noise reduction using equation (11):…”

Suitability of commercial systems for earplug individual fit testing

“…A drawback of this method is that it does not account for the bone conduction path. A correction has been proposed by Voix and Laville [5] and this has proved to be very accurate either for earmuff attenuation field measurements (see [6]) or for custom-molded earplug attenuation field measurements (see [7,8]). With regard to earplugs, another drawback is that measurement requires positioning a tube in the ear canal through the earplug.…”

Selection of Earmuffs and Other Personal Protective Equipment Used in Combination

“…The sound attenuation of hearing protectors is specified by manufacturers in the user manual. Quite often, there is a difference between the assumed effectiveness of hearing protection resulting from the use of hearing protectors based on the sound attenuation values depicted in the user manual and the effectiveness of protection occurring under real conditions [2,3,4,5,6,7,8,9,10]. The reason for these differences lies in the fact that the tests conducted to determine sound attenuation of hearing protectors are performed under controlled laboratory conditions, usually by a group of trained subjects, on brand new samples of hearing protectors [11,12,13].…”