Part III: Measuring hearing-critical job tasks 117 Materials and methods This study comprises a cross-sectional and observational design. Acoustical measures were carried out to obtain the level and spectrum of the warning signals and the ambient noise in different Dutch locomotive cabins. Setting The measurements took place fromApril 2006 toMarch 2007. Six types of trains were included, specifically: Materieel64 (Mat’64), Locomotief1800 (Loc1800), Sprinter, Motorrijtuig DubbelDeks Materieel (mDDm), InterCity Materieel (ICM), and Verlengd InterRegio Materieel (V-IRM). All trains had electric engines and were exclusively used for conveyance of passengers. For each train type, measurements were performed in two or three different trains with a different locomotive engineer operating within the standard schedule. To avoid possible bias of too low or too high accelerations, the engineers were told that the sound measurements would be used to investigate the audibility of warning signals and not to judge their driving skills or sound exposure. The railroad tracks were selected to be representative for the Netherlands and thus did not contain hills. The train speed at which was measured depended on the railroad tracks that were selected. Since the highest noise levels were expected at maximum speed, at least two measurements were performed at maximum speed in each of the train types. The maximum speed is 120 kilometres per hour (km/h) in the Sprinter and V-IRM, and 130 km/h in the other trains. The measurements were performed under dry weather conditions with a maximumwind speed of 35 km/h. Acoustical measurements The acoustical data were collected by sound recordings on digital tape. The on-site measurement set-up consisted of a calibrated sound level meter (B&K 2260 SLM with calibrator B&K 4230) connected to a portable Digital Audio Tape (DAT)- recorder (Tascam DAP). Prior, during, and after the on-site measurements, the recording system was calibrated and checked with a B&K Sound Calibrator Type 4231. The level of the calibration tone was recorded on the Tascam DAT recorder in the same way as the real measurements were made. This recorded calibrationtone was then used to determine the correct level of the DAT recordings during the off-line analysis. The acoustical data were digitally transferred to a computer that was connected to an Echo Gina 24/96 sound card. A-weighting and octave band
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