Part III: Measuring hearing-critical job tasks 147 audiometry and the speech reception test performed in fluctuating noise. These results confirm that the ability to detect acoustic warning signals in the acoustic environment of a train cabin is related to an engineer’s hearing acuity. It also indicates that conventional hearing tests do not cover the whole construct measured with the signal detection test, which supports the idea that a separate test is required for evaluating the hearing-critical task of detecting auditory warning signals. Contrary to the moderate association with the SRT in fluctuating noise, we did not find moderate associations with the SRT in continuous noise. Since the only difference between these tests is that the test in fluctuating noise assesses the ability to make use of fluctuations in the noise, and the test in continuous noise does not, this finding suggests that the ability to detect warning signals in train cabin noise is also related to the ability to process temporal effects in either the signal or the noise. It might also be the case that the train noise used is less stationary than we assumed it to be. The lack of a moderate association with the speech reception test in continuous noise could be interpreted as suggesting that the SRT in continuous noise is not suitable to determine if warning signals can be detected. The results of our study support the recommendation of using a task and job specific test for measuring auditory fitness for job performance (Tufts et al., 2009) since the signal detectability depended on the acoustic characteristics of the background noise. In line with earlier studies (Kurze et al., 2000; Pronello, 2003; Van der Hoek-Snieders et al., 2021), we found that the detectability of a signal could be different when another warning signal or ambient noise spectrum was used. Also, the results suggest that locomotive engineers could better compensate for their hearing loss when detecting theDSD signal than the ATP signal since the engineer’s SNR50 (experiment 2, Table 2) was only slightly higher compared with the normally-hearing individuals (experiment 1, Table 1) for the DSD signal, but much higher for the ATP signal. Hearing loss may have greater relative effect on the detectability of the ATP signal than on the detectability of the DSD signal. This might be explained by the frequency content of the test signals. The DSD signal contains lower frequencies compared with the ATP signal. Since the hearing loss of the included engineers was mostly a high-frequency loss, this effect is probably due to essential high-frequency components in the ATP signal. The construct validity of the signal detection test could be further assessed by future research, for example with hypotheses regarding the signal detection test and objective hearing-critical aspects of the locomotive engineer job.
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