Hearing protector performance in impulse noise

Status:

completed 10/1996

Aims:

Directive 86/188/EEC on the protection of workers from risks related to exposure to noise at work and Directive 89/686/EEC (among others) on hearing protector performance require that peak levels of impulse noise shall not exceed 140 dB SPL. However, there were no means of assessing the peak attenuation of hearing protectors. The aim of this project was to find out how the peak attenuation of hearing protectors can be assessed in order to enable the selection of adequate workers' protection for virtually any industrial impulse noise.

Activities/Methods:

The sound parameters of typical industrial sound sources were determined by in-situ-measurements . A suitable laboratory impulse sound source was developed for the project. The Microphone-In-Real-Ear (MIRE) technique was investigated to identify optimum framework conditions (e.g. position of the microphone in the ear-canal). The MIRE technique was tested in view of its suitability in connection with ear-muffs. Attempts were made to develop a MIRE-technique for ear-plugs. The peak attenuation of hearing protectors for impulse sounds was determined by field measurements; the aim was to recommend methods for attenuation assessment. Acoustic head simulators and their suitability for measuring the sound attenuation of hearing protectors were investigated. An investigation was conducted to check whether criteria used to assess the risk of hearing loss due to impulse noise can actually be applied to sound levels occurring under a hearing protector.

Results:

The workplace analyses show that peak levels of 140 dB or more are rarely found. By using the developed method adequate protection can be found for virtually all kinds of industrial impulse noise. An alternative approach is based on physical measurements of the sound pressure in the outer ear canal when an ear-muff is worn (Microphone-In-Real- Ear, MIRE). This technique can also be applied to some ear- plugs. For many plugs, however, it is not a suitable means of assessing attenuation. A second alternative is based on measurements with acoustic head simulators. The present project shows that further development of these simulators is required. The procedure used up to now for the selection of hearing protectors for impulse noise (see EN 458, Annex B) can sometime lead to a wrong estimation of the protection; it could happen that the use of an ear-muff in impulsive, mainly low frequency noise (e.g. explosions) did not offer sufficient protection. To make sure users are reliably protected as requested by the Directive 86/188/EEC requirements for hearing protectors used in impulse noise with peak levels >= 140 dB must be tightened up.

Publications:

Smoorenburg, G. F.: IMPRO - Assessment of hearing protector performance in impulse noise - Final report, Contract No. MAT1-CT92-0018, Standards, Measurements and Testing Programme, EC-DG XII

Further informations:

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