Development of a measurement method regarding audibility of warning signals for engine driver/shunters impaired by hearing protection

Status:

completed 05/2014

Aims:

With the implementation of the new ordinance on noise and vibration protection, the values for prevention have been decreased. Engine drivers/shunters work partly under strong and threshold exceeding noise. The importance of audibility of warning signals causes, that hearing protection is not always used by every engine driver/shunter. There is a mathematical method to sort the hearing protection in different groups with different tolerances. In addition to the mathematic procedure, it is necessary to perform an individual hearing test according to BGI/GUV-I 5147 to guarantee the adequate perception of the signals. This investigation shall be added to the existing procedure which is very time consuming and complex. The purpose of this investigation is to make sure, that warning signals are not impaired by hearing protection. The hearing test shall be done in a room with normal acoustical conditions where the situation on the rail and loco shall be adopted as realistically as possible.

Activities/Methods:

To perform the hearing test under most realistic conditions in a normal room, all relevant acoustic signals (warning and interfering signals) need to be recorded on the rail with head-based measurement equipment. The test signals are evaluated binaurally by the engine driver/shunter. A 2AFC (two alternative forced choice) method is used for the panel test. The developed software detects the masked threshold of the test person by classification of their answers. At the end of the investigation, a panel of engine drivers/shunters shall perform the test according to BGI/GUV-I 5147 as well as the new hearing test. Therefore, both results can be matched and the new method can be validated.

Results:

The new testing program shows differences the hearing exercise according to BGI/GUV-I:

• Test environment: The hearing exercise according to BGI/GUV-I 5147 is done on the engine and thus in a normal, familiar environment, whereas the hearing protection test with RaLa-GS is performed in a testing room, which is a specific occupational situation, comparable with e.g. the consultation of the company physician.
• Interferences: The background noise used in the hearing exercises according to BGI/GUV-I 5147 – diesel engine constantly at 2/3 of the nominal rotation speed – is time-invariant, which does not reflect the real situation. In contrast, several different interfering sounds are used in the RaLa-GS hearing protection test that may change during the performance time according to reality and that may also be in different intensities.
• Testing scope: Due to the available options for testing scenarios, background noise and warning signals the scope is significantly larger with the program-controlled hearing protection test than it is with the hearing test as described in BGI/GUV-I 5147. Consequently, the two options for testing scenarios tailored to the special needs of shunters respectively engine drivers as implemented in the program RaLa-GS are facilitated, these options could in fact serve as standard tests. Furthermore, a hearing exercise can be individually configurated in detail with regard to railcar type, case of operation and warning signal. With the general possibility to record, feed and use further warning signal/interference-combinations, high flexibility is given and future adaptations to other testing tasks within the warning signal issue are feasible.
• Time and effort: Time and organisational efforts are significantly reduced in the program-controlled hearing protection test compared to the hearing test as in BGI/GUV-I 5147. It is possible to integrate the hearing protection test into the routine screening by the company’s physician.

The variability of the interference noise, as enabled by the testing program, and consequently the close approach of the acoustic environment to reality do require a considerably higher degree of attention and concentration over a longer period of time than it would be necessary in the hearing exercise according to BGI/GUV-I 5147. There is a greater chance than in the hearing test of BGI/GUV-I 5147 that a hearing protection system will not pass the audio test due to the slightly higher requirements on the proband’s skills. It might happen that some probands will have to repeat the audio test after a short break if they cannot manifest the required concentration instantly in the new and unfamiliar hearing situation.

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