Detection and reaction thresholds for reverse alarms in noise with and without passive hearing protection.

OBJECTIVE: To measure masked detection and reaction thresholds for two reverse alarms (tonal and broadband) and compare results to available standards and psychoacoustic criteria for setting alarm levels.

DESIGN: Alarm detection and reaction thresholds were adaptively measured in 80-dBA background noises without hearing protection (Experiment 1), and with a passive earmuff-style hearing protection device (HPD) (Experiment 2).

STUDY SAMPLE: Twenty-four young adults with normal hearing in each experimental group.

RESULTS: Reverse alarms remained audible at levels well-below background noises [thresholds: -11 to -25 dB signal-to-noise ratio (SNR)], with and without the selected HPD. Detection was more influenced by alarm and noise type, while reaction was more susceptible to HPD use. HPD use resulted in lower detection thresholds by up to 2.5 dB compared to unprotected listening but increased reaction thresholds by 5-10 dB depending on the alarm.

CONCLUSIONS: Since noise type appears to have a more limited effect on reaction thresholds, adjusting alarms based on a global dBA method appears preferable to methods based on masked detection thresholds. However, while the >0 dB SNR recommended in ISO 9533 seems adequate for unprotected listening, an additional 5-10 dB may be warranted to elicit the same reaction when the selected HPD is used.