Part III: Measuring hearing-critical job tasks 125 Signal detectability in a laboratory setting All subjects completed the experiment. There was no missing data. The A-weighted SNR50 for the DSD and ATP signal in the background noises of the six train types are expressed in dB SNR in Table 1. For detecting the DSD signal, the SNR50 ranged from -32.8 to -23.4 dB SNR. For detecting the ATP signal, the SNR50 ranged from -47.4 to -25.0 dB SNR. Except for one train type, the SNR50 was lower for detecting the ATP signal compared to the DSD signal. Table 1. Mean A-weighted signal-to-noise ratio’s at which fifty percent of the warning signals were detected correctly by six normally-hearing individuals DSD signal SNR50 (SD) ATP signal SNR50 (SD) Mat’64 -23.4 (3.0) -44.2 (3.6) Loc1800 -27.7 (1.3) -39.5 (3.0) Sprinter -28.1 (4.8) -47.4 (4.4) ICM -29.2 (2.2) -44.6 (3.2) mDDm -32.8 (1.7) -25.0 (1.3) V-IRM -26.6 (1.6) -44.8 (3.4) ATP indicates automatic train protection; DSD, Dead Man’s Switch; SNR50, the signal-to-noise ratio at which 50 percent of the warning signals was detected correctly. Discussion This study examined the acoustic characteristics of thewarning signals and the ambient noise in Dutch train cabins to evaluate the effectiveness of these warning signals when presented to normally- hearing locomotive engineers. The DSD and ATP signal were shown to have different acoustic characteristics and the ambient noise levels depended on train type and driving speed. Although the DSD and ATP signals both contained more sound energy than the background noise, the model predictions indicated that the detectability of the signals was critical in a number of conditions. The levels of the warning signals were – in accordance with the advice in ISO 7731 – larger than 65 dBA and softer than 118 dBA. A spectral analysis indicated that the measured DSD signals did adhere to the ISO requirements, but the ATP signals did not, because most sound energy was measured above 4000 Hz. The measured intensity of the background noise varied from 53.2 to 77.1 dBA. This is roughly in line with an earlier study that reported maximum noise exposure levels between 70 and 80 dBA for locomotive engineers inNorway (Lie et al., 2013). Higher maximum
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