Chapter 5 118 filtering were applied in compliance with respectively IEC 61260 Class 1 and IEC 61260 (Couvreur, 1997). The DSD and ATP signals were measured in all trains. If adjustable, the volume setting of the warning signal was set at maximum. The DSD signal was measured in quiet. The ATP signal does not occur in quiet and was therefore measured at the lowest speed at which the signal occurs. Unlike the DSD, the ATP signal decays over time. The ATP recordings were therefore averaged over the first 200 milliseconds after onset. This duration roughly corresponds to the human integration time for tonal signals (Viemeister, 1996). Laboratory measurements Six subjects (one male; five females) took part in the laboratory measurements. The detectability of the DSD and ATP signal was assessed in the ambient noise of six train types. All participants had normal hearing, defined by pure-tone detection thresholds from 250 to 8000 Hz via air conduction below 25 dB HL. Prior to taking part in the study, informed consent was provided. A stepwise two-alternative forced choice adaptive approach was used to determine the Signal-to-Noise Ratio (SNR) at which 50 percent of the warning signals can be detected. We will refer to this outcome measure as the SNR50. The noise level was fixed at a presentation level that corresponds with the real-live noise level at the train’s maximum speed. The signal level varied and started at a level of 30 dB Sound Pressure Level (SPL) above the expected SNR50. After each correct response, the signal level decreased with a step size of 4 dB SPL until the individual failed to detect the signal correctly. Then, the signal level increased again and followed a one-up onedown procedure with a 2 dB stepsize. The test was continued until five reversals were obtained. The test was programmed in Matlab (The Mathworks, 2005). The detection test was performed in a sound-isolated booth in a free field setting.The individual was sitting in a chair in the middle of the booth and was surrounded by six omnidirectional speakers at 0, 45, 80, 180, 280, and 315 degrees, and a subwoofer. All subjects were instructed to push the button when a signal was heard, even if the signal was very soft. All individuals completed the test twelve times, since the SNR50 was determined for the DSD and ATP signal separately in the ambient noises of six train types. The testing order of the noises and warning signals was counterbalanced across subjects.
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