Directive 86/188/EEC on the protection of workers from the risks related to exposure to noise at work and directive 89/686/EEC on hearing protector performance require that the peak level of impulsive noises that workers are exposed to should not exceed 200 Pa (140 dB SPL). There is, however, no means of assessing the peak attenuation of hearing protectors. The standard method of assessing hearing protector performance (ISO 4869-1) is based on low-level sounds near the human threshold of hearing and cannot be used to measure the attenuation of high level impulse noise. It cannot be used either to measure the attenuation of continuous noise afforded by level-dependent hearing protectors.
The aim of the project was to develop new methods of assessing hearing protector attenuation for noise levels and types of noises, especially impulse noise, against which protection is sought in practice. In particular, the aim was to develop methods for measuring:
the attenuation of hearing protectors, and common and custom moulded ear plugs, using the Microphone-In-Real-Ear (MIRE) technique;
the attenuation of hearing protectors using Acoustic Test Fixtures (ATF's).
The main results of the project are:
It has been shown that the peak noise attenuation of hearing protectors can be estimated on the basis of the H, M, and L parameters that according to EC regulations have to be specified for each hearing protector. The accuracy of the estimate is limited to a standard error of about 6 dB. However, by using this method adequate protection can be found for virtually all industrial impulsive noises.
An alternative approach is based on physical measurements of the sound pressure in the outer ear canal when a hearing protector is worn (Microphone-In-Real-Ear, MIRE). For ear muffs, this approach, where the acoustic probe is inserted 4 to 10 mm into the ear canal, yields very accurate results. It allows measurements to be made on level-dependent ear muffs. The MIRE method can also be used for some ear plugs, albeit with less accuracy and at the cost of affecting the integrity of the ear plug. For many ear plugs, however, it is not a suitable means of assessing the noise attenuation.
Another alternative approach is based on acoustic head simulators. However, the present project has shown that further development of these simulators is required in order to be able to adequately assess hearing protector performance, including inter-subject variability.
The results of the project have been transmitted to CEN TC 159.
The Microphone-In-Real-Ear (MIRE) technique and the use of Acoustic Test Fixtures (ATF's) were applied to the field assessment of hearing protector performance for industrial impulsive noises and for shooting noise and blasts. An analysis of the field data and the results of the ATF measurements was carried out in order to validate the methods and to determine the simplest adequate means of quantifying the attenuation of linear and level-dependent hearing protectors in impulsive noise with regard to the risk of hearing loss.
Funding SchemeCSC - Cost-sharing contracts
53757 Sankt Augustin
M5 4WT Salford