Part III: Measuring hearing-critical job tasks 137 Figure 1. Octave-band spectra in dBA of the ambient noise and auditory warning signals (DSD and ATP signal) present in two Dutch train types (Mat64 and V-IRM). In addition, the A-weighted equivalent sound pressure levels are shown. ATP indicates automatic train protection; DSD, dead man’s switch; LAeq, the A-weighted sound pressure level. Every combination of a train type with a warning signal (DSD or ATP signal) is called a driving condition. The combination of two warning signals and noise environments of each of the six train cabins resulted in twelve possible driving conditions. The signal detection test uses a stepwise two-alternative forced-choice adaptive approach to determine the detection threshold, defined as the lowest signal-tonoise ratio (SNR) at which 50 percent of the warning signals can be heard. This test outcome will be referred to as SNR50. The noise level was fixed at a presentation level corresponding to the real-world noise level at the train’s maximum speed. The signal level varied and started at a signal level of 30 dBA higher than required to achieve the expected SNR50. After each correct response, the signal level decreased with a step size of 4 dB until the participant failed to detect the signal correctly. Then the signal level increased again and followed a one-up one-down procedure with a 2 dB step size. The test was continued until five reversals were obtained. It took approximately a minute to complete one driving condition and twelve minutes to complete the full test for all twelve driving conditions. The test was programmed in MATLAB (MathWorks, 2005).
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