Evaluation of audiometers for determining the sound attenuation of individual earplugs

Status:

completed 12/2014

Aims:

When used in practice in plants, hearing protectors frequently fail to attain the sound-attenuation levels measured in the laboratory during type tests. This is particularly the case for earplugs. This is generally due to the earplugs being inserted incorrectly. For the product group of custom moulded earplugs, the reason is often an improper fit, leading to leaks. For this reason, provisions such as the technical rules pursuant to the German Ordinance on noise and vibration protection require function checks to be performed for custom moulded earplugs when they are first supplied and subsequently at regular intervals of two years, in order to assure the protective action.

One means by which this personal function check can be performed is by the use of an audiometer for determining of the sound attenuation. This is achieved by determining the wearer's hearing threshold with and without the wearing of hearing protection. The sound attenuation is the difference between the two thresholds. This method could be used in particular for performance of the regular function checks by the occupational physician.

Personal measurement also enables the sound attenuation actually achieved to be determined. This particularly enables the risk of impaired hearing for persons with existing or emerging hearing loss to be reduced to a minimum.

For this purpose however, it must be known whether the attenuation value determined by the audiometer differs from that determined in the type test (performed in accordance with EN 24869-1). Since these differ in both their sound field and test noise, deviations were to be expected between the two measurement results. The aim of the project was to assess the suitability of audiometric measurement methods for personal sound attenuation and, if necessary, to define correction values for the comparison with attenuation values obtained in type tests.

Activities/Methods:

Comparative measurements were performed in which the two methods for determining the sound attenuation were compared: type testing to EN 24869-1 in a free field, and audiometric measurement of the hearing threshold with headphones. The two measurements were performed immediately after one another in a soundproof chamber on experienced test subjects. The fit of the earplug was not changed between the tests. Different types of earplug were tested: custom moulded earplugs, pre-shaped flanged earplugs, and user-shaped foam earplugs.

A PC-based screening audiometer with a range of sound level intervals (5, 2 and 1 dB) was used. In order to rule out influences from the equipment, discrete measurements were performed with a standalone audiometer separate from the PC. The sound level in the ear canal was also measured on some of the test subjects by means of probe microphones. This enabled the effect of closure of the ear canal by an earplug or an audiometry headphone upon the physiological sound detectable in the ear canal to be determined.

Around 120 sound attenuation characteristics were determined in total on 28 test subjects. 16 products were tested. For seven of these, random samples with over five data records were available. For these products, evaluation was possible not only for the discrete personal data records, but also for the random sample as a whole.

Results:

The direct comparison between the two test methods used on a single test subject generally indicated that the audiometric method exhibited low sound attenuation for the frequencies of 125 and 250 Hz. This is evident from the mean value over the entire random sample. The effect is independent of the sound level interval during measurement of the hearing threshold (5 or 1 dB).

The effect could be attributable to the influence of physiological sounds in the ear canal upon the hearing thresholds. The objective measurements with a probe microphone revealed a clear increase in the level of the physiological sounds in the ear canal when the audiometric headphones were worn. This may result in the hearing threshold shifting towards higher levels when hearing protection is not worn. If an earplug is also inserted, the sound level in the ear canal remains virtually unchanged. During measurement in accordance with EN 24869-1, the hearing threshold is not influenced when the ear canal is completely open.

Based upon the data currently available, no recommendation can be made at this stage for an audiometric method to be used as a standard method for determining the personal sound attenuation. In particular, it is not clear at this stage whether correction values are needed for the low frequencies of 125 and 250 Hz. This is to be determined by the study of further audiometric instruments – particularly with different types of headphones – during which the influence of the occluded volume upon the sound attenuation values is to be determined.

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