Effects of hearing protection on detection and reaction thresholds for reverse alarms C. Laroche, C. Giguère, V. Vaillancourt, K. Roy, H. Nélisse, N. Ellaham 2017 NHCA Annual Conference Feb 23-25, San Antonio, Texas Hearing Research Laboratory
Background Effects of HPDs The effects of hearing protection devices (HPDs) on communication depends on many factors, e.g.: The nature of the task: sound detection, localization, identification speech production and recognition The characteristics of the device: amount of attenuation type (earplug vs earmuff, passive vs electronic devices, etc.) The hearing profile of the individual: type and degree of hearing loss This presentation focuses on the effects of passive HPDs on the perception of vehicle reverse (back-up) alarms by normal-hearing individuals.
Background Reverse Alarms Audible backup alarms are commonly used Accidents still happen! Non-uniform sound propagation Poor spatial localization
Background Reverse Alarms Conventional alarm ( beep-beep ) Single primary tone around 1250 Hz + weaker harmonics Broadband alarm ( pschtt-pschtt ) Energy spread over larger frequency span, with most energy around 700-4000 Hz 1250 Hz 700-4000 Hz
Background Previous study on reverse alarms (tonal vs broadband alarms) Objective measurements of sound field behind vehicules (sound propagation) Broadband: more uniform sound field Tonal: variations up to 20 db over short distances (~1m) Noise & Health vol. 15:67 (2013)
Background Previous study on reverse alarms (tonal vs broadband alarms) Laboratory psychoacoustic measures, with and without passive HPDs Detection Perceived loudness Perceived urgency Sound localization 24 subjects (normal hearing) 4 workplace noises (81 to 89 dba) Noise & Health vol. 15:67 (2013)
Background Previous study on reverse alarms (tonal vs broadband alarms) Masked detection thresholds Better detection thresholds with passive HPDs than without Earmuffs Noise 1 Noise 2 Noise 3 Noise 4 Similar trends observed with earplugs Noise & Health vol. 15:67 (2013)
Background Previous study on reverse alarms (tonal vs broadband alarms) Perceived urgency Reduced growth-of-urgency with increasing SNR with passive HPDs Earmuffs Noise 1 Noise 2 Noise 3 Noise 4 Similar trends observed with earplugs HPDs appeared to improve masked detection thresholds but to reduce urgency at supra-threshold levels
Objectives 1) Measure effects of passive HPDs on reverse alarm perception at both threshold and supra-threshold levels 2) Compare to standardized specifications for alarm level adjustment ISO 9533 Audible reverse alarms on mobile equipment SNR > 0 db at 7 measurement points behind the vehicle ISO 7731 Auditory danger signals (3 methods) Method C: alarm level to exceed calculated masked threshold by at least 13 db in one or more 1/3 OB bands Psychoacoustic studies: Alarm level 12 to 25 db above masked threshold Overall goal: identify optimal SNR at which alarms should be NHCA 2017, San Antonio, Texas (February 23-25) set to ensure safety without being excessively loud
Methods Participants: 24 individuals with normal hearing Psychoacoustic measures, with and without earmuffs: Masked detection thresholds: level at which the alarm is just heard in noise Masked reaction thresholds: level that elicits immediate attention or retreat to safety in noise Stimuli: Tonal and broadband alarms 6 different workplace noises at 80 dba Device: Peltor Optime 95 (NRR = 21 db) Frequency (Hz) 125 250 500 1000 2000 4000 8000 Mean att. (db) 12.4 15 26.2 35.2 35.2 33.3 37.5 Std. Dev. (db) 2.6 1.8 2.5 3.2 2.5 2 3.2
Noise spectra and temporal characteristics
Results Average detection and reaction thresholds in 6 workplace noises Tonal Broadband Reaction Detection Significant effect of noise type on detection and reaction thresholds Size of noise type effect ~ 2-3 times greater in detection
Results Overall average detection and reaction thresholds Reaction Detection
Results Effects of HPDs on thresholds 8.1 db 4.8 db Reaction -1.6 db -1.5 db Detection
Results Reaction thresholds minus detection thresholds 19.9 db 28.6 db Reaction 19.0 db 13.5 db Detection
Comparison to standardized specifications for alarm adjustments without HPDs
Comparison to standardized specifications for alarm adjustments with HPDs
Conclusions Reverse alarms remain audible at levels well below lower background noise, with or without passive HPDs. Small benefit (1-2 db) of HPDs for detection THRs, but detrimental effect on reaction THRs for tonal (8 db) and broadband (5 db) alarms. Reaction THRs 13-19 db above detection without HPDs Results are consistent with standardized specifications for alarm adjustments Reaction THRs 20-29 db above detection with HPDs Adjustments according to standardized specifications results in alarm levels often too low and sometimes too high compared to measured reaction thresholds Lower reaction/detection threshold difference for broadband alarm than tonal alarm.
Conclusions Further work: Effect of amount of attenuation and potential benefits of level-dependent HPDs on alarm detection and reaction Combined effect of hearing loss and HPD use Effect of HPDs on localization of alarm/vehicle Real-world applications: Population-wide familiarization with broadband alarm Importance of public and worker education Reactions to broadband alarm Positive experiences at the University of Victoria and in Whistler: less invasive sound than tonal alarms Ongoing survey by WorkSafe BC NHCA 2017, San Antonio, Texas (February 23-25)