Frédéric Laville

Measurement of hearing protection devices performance in the workplace during full-shift working operations.

OBJECTIVESnThe effectiveness of hearing protection devices (HPDs), when used in workplace conditions, has been shown over the years to be usually lower than the labeled values obtained under well-controlled laboratory conditions. Causes for such discrepancies have been listed and discussed by many authors. This study is an attempt to understand the issues in greater details and quantify some of these factors by looking at the performance of hearing protectors as a function of time during full work shift conditions.nnnMETHODSnA non-invasive field microphone in the real ear (F-MIRE)-based method has been developed for measuring the effectiveness of different HPDs as a function of time in the workplace. Details of the test procedures, the equipment used, and the post-processing operations are presented and discussed. The methodology was developed in such a way that a complete time and frequency representation are possible. The system was used on a total of 24 workers in eight different companies. Work shifts of up to 9-h long were recorded. Various types of earmuffs and one type of molded earplugs were tested.nnnRESULTSnAttenuation data reported as a function of time showed, for most workers tested, considerable fluctuations over entire work shift periods. Parts of these fluctuations are attributed to variations in the low-frequency content in the noise (in particular for earmuffs) as well as poor insertion and/or fitting of earplugs. Lower performances than laboratory-based ones were once again observed for most cases tested but also, important left and right ear differences were obtained for many individuals. When reported as a function of frequency, the attenuation results suggested that the few approximations used to relate the measurements to subjective real-ear-attenuation-at-threshold (REAT) data were realistic.nnnCONCLUSIONSnThe use of individualized attenuation data and performance ratings for HPDs as well as a good knowledge of the ambient noise in the workplace are key ingredients when evaluating the performance of hearing protectors in field conditions.

Systematic evaluation of the relationship between physical and psychoacoustical measurements of hearing protectors' attenuation

The most commonly used methods to measure hearing protectors attenuation can be divided into two categories: psychoacoustical (subjective) and physical (objective) methods. In order to better understand the relationship between these methods, this article presents various factors relating attenuation values obtained with these methods through a series of tests. Experiments on human subjects were carried out where the subjects were instrumented on both ears with miniature microphones outside and underneath the protector. The subjects were then asked to go through a series of hearing threshold measurements (psychoacoustical method) followed by microphone sound recordings using high-level diffuse field broadband noises (physical method). The proposed test protocol allowed obtaining various factors relating the test methods as well as attenuation values and ratings for different protection conditions (open ear, earmuffs, earplugs, and dual protection). Results are presented for three models of passive earmuffs, three models of earplugs and all their combinations as dual hearing protectors. The validity and the relative importance of various terms used to correct the physical attenuation values when comparing with psychoacoustical attenuation values are examined.

Using auditory steady-state responses for measuring hearing protector occlusion effect

Introduction: The currently available methods for measuring the occlusion effect (OE) of hearing protection devices (HPDs) have limitations. Objective microphonic measurements do not assess bone-conducted sounds directly transmitted to the cochlea. Psychophysical measurements at threshold are biased due to the low-frequency masking effects from test participants’ physiological noise and the variability of measurements based on subjective responses. An auditory steady-state responses (ASSRs) procedure is used as a technique that might overcome these limitations. Participants and Methods: Pure-tone stimuli (250 and 500u2009Hz), with amplitude modulated at 40u2009Hz, were presented to twelve adults with normal hearing through a bone vibrator at three levels in 10-dB steps. The following two conditions were assessed: the unoccluded ear canal and occluded ear canal. ASSR amplitude data as a function of the stimulation level were linearized using least-square regressions. The ASSR-based “physiological” OE was then calculated as the average difference between the two measurements. Results: A significant statistical difference was found between the average threshold-based psychophysical OE and the average ASSR-based OE. Conclusion: This study successfully ascertained that it is possible to objectively measure the OE of HPD using ASSRs collected on the same participant both with and without protectors.

Using auditory steady-state responses for measuring hearing protector attenuation

Introduction: Present methods of measuring the attenuation of hearing protection devices (HPDs) have limitations. Objective measurements such as field microphone in real-ear do not assess bone-conducted sound. Psychophysical measurements such as real-ear attenuation at threshold (REAT) are biased due to the low frequency masking effects from test subjects’ physiological noise and the variability of measurements based on subjective responses. An auditory steady-state responses (ASSRs) procedure is explored as a technique which might overcome these limitations. Subjects and Methods: Pure tone stimuli (500 and 1000u2009Hz), amplitude modulated at 40u2009Hz, are presented to 10 normal-hearing adults through headphones at three levels in 10u2009dB steps. Two conditions were assessed: unoccluded ear canal and occluded ear canal. ASSR amplitude data as a function of the stimulation level are linearized using least-square regressions. The “physiological attenuation” is then calculated as the average difference between the two measurements. The technical feasibility of measuring earplug attenuation is demonstrated for the group average attenuation across subjects. Results: No significant statistical difference is found between the average REAT attenuation and the average ASSR-based attenuation. Conclusion: Feasibility is not yet demonstrated for individual subjects since differences between the estimates occurred for some subjects.