Acoustical test method for level-dependent earmuffs

Status:

completed 12/1999

Aims:

The EC-directive 89/686/EEC for personal protective equipment requires that level-dependent earmuffs with an electronic sound restoration circuit be subjected to type testing. In particular, the external sound pressure level up to which such level-dependent hearing protectors provide sufficient protection must be determined. The usual test procedure applied for type testing of conventional earmuffs or earplugs is not suitable for this purpose. The informative Annex of prEN 352-4 "Hearing protectors - Safety requirements and testing, Part 4: Level-dependent earmuffs" describes a test procedure for level-dependent earmuffs which was drawn up with the participation of BIA and which was developed and tested within the project.

Activities/Methods:

A sound field which satisfies the requirements for maximum sound pressure levels, diffusivity and spatial homogeneity at its reference point (= head of the test subject) was set up in the reverberation room at BIA by suitable grouping of the loudspeakers. For EC type testing of level-dependent earmuffs, four specimens are used worn by eight test subjects in total, with the electronics switched on and set to maximum volume. The MIRE-technique in accordance with ISO/CD 11904-2 (1997) is applied for the measurements. For each measurement, the external sound pressure level of defined test noises (H, M and L noise) is increased until the sound pressure level under the hearing protector effective to the ear reaches or exceeds 85 dB(A). With the measured sound pressure levels and as a mean over the results measured on 16 ears, so called "criterion levels" are determined for each noise type. The test procedure proposed in ISO TR 4869-4 using the artificial test fixture (ATF) was also applied in place of test subjects for the purpose of comparison.

Results:

The test procedure described in Annex A of prEN 352-4 was found to be suitable for routine testing within the context of the EC type test. It enables conclusions to be drawn regarding the scope of deployment of level-dependent earmuffs: the so called "criterion levels" specify the external sound pressure level up to which the hearing protectors may be worn without exposing the hearing to a risk of hazard. Use of the artificial test fixture as described in ISO TR 4869-4 presents a risk of over-estimation of the protective function. The results of the project represent a basis for future standardization work in CEN/TC 159 and for the exchange of experience between the European test bodies responsible for hearing protection.

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