Development of a testing method for the perceptibility of acoustic warning signals with hearing protection in public road

Status:

completed 07/2021

Aims:

Activities on public roads, e.g. the maintenance of green areas, street and sewer cleaning or construction work, may require the wearing of hearing protection for reasons of occupational safety. However, this must not reduce the perceptibility of warning signals (e.g. car horn, siren, reversing signals).

Thus, according to DGUV Information 212-673, a procedure for an individual hearing test is described. This procedure is very time-consuming and material-intensive (two vehicles are needed) and is associated with noise pollution for the surrounding area.

Therefore, an easy-to-perform test procedure was to be developed for the hearing of warning signals with hearing protection in public road traffic, which enables the performance of an individual hearing test with suitable warning signals and working noises in normal office spaces. The method to be developed has to reproduce the acoustic situation of workplaces in public road traffic spaces.

In order to be able to take into account the different hearing protection devices used for activities in public roads areas, it must be possible to present the test sounds both via loudspeakers (free-field method for testing earmuffs or full-protection equipment, as prescribed e.g. for working with power scythes) and via headphones (direct sound field method for testing earplugs and earmolds).

Activities/Methods:

The stereophonic test sounds required for the testing method were recorded in real operation situations on construction sites, in a road maintenance depot and in the public road space around Munich with the aid of a head-related recording system. From the recorded sound material, two warning signals and four masking background noises are used in the test program which can be considered representative for most of the activities requiring hearing protection in public road areas. The test system consists of a laptop computer with two loudspeakers for the free-field method, a pair of audiometric headphones for the direct sound field method and the input device for the subject’s answer. A suitable test room for the free-field method can either be a small office room with an area of at least 12 m², equipped with sound-absorbing ceiling or carpet floor, or a larger (≥ 25 m²) furnished room without particular acoustic properties. The background noise level inside the test room should not exceed an average of Leq = 45 dB(A).

With the direct sound field method, thanks to the sound insulation of the audiometric headphones, a higher background noise level of Leq = 55 dB(A) on average is possible, and, beyond this, no special requirements are placed on the test room. The test program StraLa-GS (Straßenlärm-Gehörschutz-Test – traffic noise hearing protection test) was written in the programming language MAX featuring a graphic programming interface.

In preliminary tests, the masked thresholds were first determined for a large number of warning signal/background noise combinations using an adjustment method with four test subjects. These masked threshold data were stored in the test program in order to be able to adjust the respective warning signal level to the appropriate value range during the listening test. The finalised test software was used within the Müller-BBM GmbH for comparison purposes by a group of trained test persons involved in the preliminary tests during the program developing stage and by a group of untrained test subjects who had not performed any audio tests before. With regard to their experience with listening tests the group of inexperienced subjects can be regarded as representative for the test program’s target group.

Results:

The computer-assisted test procedure developed within the framework of the FP 433 research project is a much simpler alternative to the procedure described in DGUV Information 212-673 for the individual hearing test for insured persons engaged in activities requiring hearing protection in public road areas. The time and organisational effort required for the program-controlled hearing protection test is significantly reduced compared to the hearing test according to the above-mentioned DGUV Information. It would be possible to integrate the hearing protection test, for example, into a recurring aptitude test or also into the occupational health check-up.

The variability of the background noise made possible by the StraLa-GS test programme and the close approximation of the acoustic environment to reality, however, require a high level of attention and concentration from the test subjects in the unfamiliar environment over the duration of the test programme, which is approx. 10 minutes. For some of the test subjects who cannot immediately summon up the necessary concentration due to the hearing situation which is new and unfamiliar to them, it may be necessary to repeat the hearing protection test – at the earliest after a break of approx. 10 minutes.

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