DETECTION OF REVERSE ALARMS IN NOISY WORKPLACES

Transcription

1 DETECTION OF REVERSE ALARMS IN NOISY WORKPLACES Chantal Laroche, Véronique Vaillancourt, Christian Giguère, Nicolas Ellaham, Christelle Gagnon and Patricia Laflamme Hearing Research Laboratory, University of Ottawa, 451 Smyth Road, Ottawa, Ontario, Canada K1H 8M5 Hugues Nélisse and Jérome Boutin Institut de recherche Robert-Sauvé en santé et en sécurité du travail, 505 Boulevard de Maisonneuve Ouest, Montréal, Québec, Canada, H3A 3C2 In many countries, the use of reverse alarms is mandatory on most heavy vehicles. Nevertheless, a high number of accidents involving vehicles in reverse motion still occur and various questions related to the use of reverse alarms in the workplace are regularly raised. These include fundamental knowledge on the ability to detect, localize and react to single or multiple reverse alarms in the field, as well as practical issues such as the optimal sound level and proper mounting of alarm devices in the field. This paper focuses on alarm detection and presents new laboratory data on 24 subjects tested in 12 noises that vary spectrally and temporally to represent a wide range of workplace noise environments. Across all 12 noises, average masked detection thresholds range from to db SNR for a tonal alarm and from to db SNR for a broadband alarm. Results are compared to criteria set by various standards on warning sound perception, such as ISO 7731 and ISO Overall, the project is a step forward in optimizing the use of reverse alarms while maintaining adequate safety for workers and minimizing noise nuisance for workers and nearby residents. 1. Introduction Audible backup alarms installed on mobile equipment are used to promptly alert nearby workers or passers-by of safety risks associated with reverse vehicular operation and to focus the warning in the danger zone right behind the moving equipment. Standards such as SAE J994 1 and ISO provide guidance on the technical characteristics of backup alarms and the sound level adjustment procedures to be used in the field, while ISO sets out general audibility criteria and measurement requirements to ensure the proper recognition of auditory danger signals. Although the use of reverse alarms is mandatory on most heavy vehicles in many countries, accidents and fatalities involving vehicles operating in reverse are reported every year Moreover, various issues related to the use of reverse alarms are regularly raised such as the optimal setting of fixed- ICSV22, Florence (Italy) July

2 level alarms, the behaviour and effectiveness of self-adjusting alarms, the optimal mounting of alarm devices on the vehicles, and the spatial perception of alarms. Methods to optimize the level of warning sounds are typically based on the concept of masked threshold. 3, The latter is the signal level which is just detectable in the presence of an interfering masker (e.g. the workplace noise) and is often expressed as a signal-to-noise ratio (SNR). Warning sounds need to be adjusted at a certain level above the masked threshold to ensure they attract attention and are recognizable. In practice, a level of 10 to 15 db above the masked threshold has been proposed An upper limit is also warranted to prevent overly loud warning signals, typically 25 db above the masked threshold for each frequency component of the warning signal Many studies have addressed detection and audibility of reverse alarms (see Vaillancourt et al. 17 for review). One recent study performed in our laboratory 17 compared the performance of a broadband alarm (BBS) to that of a conventional tonal alarm from a worker-safety standpoint, and included measures of sound detection in 24 young adults with normal hearing, with and without the use of hearing protection. Masked thresholds for each alarm were measured in four workplace noises (limestone plant, quicklime plant, and two sawmills) characterized by different spectra. Across all testing conditions, average A-weighted detection thresholds, expressed in SNR, varied from -13 to -24 db SNR, indicating that alarms can remain audible when adjusted at levels significantly below that of the background noise. Slight advantages were found with the tonal alarm for sound detection (up to 7 db lower thresholds; average of 2dB unprotected and 4 db with hearing protection) in the limited set of noises used in the laboratory study. The focus of the current study is to extend the work performed in our previous laboratory study 17 by including a wider choice of 12 workplace noises that vary spectrally and temporally. Comparing results to criteria set by various standards on warning sound perception, such as ISO and ISO , is a first step in fulfilling our main objective of identifying the optimal SNR at which alarms should be set in order to ensure good alarm audibility by workers in the vicinity of heavy vehicles, without being excessively loud. Overall, the project is a step forward in optimizing the use of reverse alarms while maintaining adequate safety for workers and minimizing noise nuisance for workers and nearby residents. 2. Methods A total of 24 (11 men; 13 women) young adults (average = 34.0 years; standard deviation = 2.8 years) with normal hearing (hearing thresholds 25 db HL from 250 to 8000 Hz) took part in laboratory measurements of alarm detection thresholds in a series of 12 spectrally and temporally different workplace noises set at 80 dba. The signals to be detected consisted of a tonal alarm and a broadband alarm. The frequency content of the two alarms is illustrated in Figure 1. Sound pressure levels (SPL), measured at approximately 1m in front of the alarms in a semi-anechoic chamber are shown as a function of frequency. The conventional tonal alarm contains a single primary tone around 1250 Hz in addition to even and odd harmonics weaker by 30 db or more. For the broadband alarm, the energy is distributed over a larger frequency span, most of the energy being found in the Hz range. Noise spectra are illustrated in Figure 2, using a third-octave band analysis, whereas the temporal characteristics of the 12 noises are displayed in Figure 3 for each 25- ms time segment. An adaptive method was used for all threshold measurements. Using a tablet PC and software specifically designed for this study, subjects were required to adjust the level of each alarm up and down using 2-dB steps until it was barely audible. Each trial consisted of the subject adjusting the two alarms to threshold level, one at a time, in a given noise. Testing was performed three times in each of the 12 noises to measure the test-retest reliability of the threshold estimation. A total of 72 thresholds were therefore measured (12 noise conditions x 2 alarms x 3 repetitions). Initial ICSV22, Florence, Italy, July

3 presentation levels were varied across alarms and trials, but always remained at suprathreshold levels. From this initial level, participants were required to adjust the alarm level until it became inaudible, then up to barely audible in an ascending excursion. The testing order of the noises and alarms was counterbalanced across subjects. Figure 1. Alarm spectra. Figure 2. Noise spectra using a 1/3 octave-band analysis. ICSV22, Florence, Italy, July

4 3. Results Figure 3. Noise temporal characteristics over 25-ms time segments. Since threshold measurement was repeated for each alarm in each noise condition, individual thresholds were averaged across all three measurements. Masked thresholds for the two alarms in each of the 12 background noises are expressed as the average SNR at threshold across all participants in Figure 4 and in Table 1. Other metrics such as the between-subject standard deviation and the within-subject standard deviation are also summarized in Table 1. Over all testing conditions, average A-weighted detection thresholds ranged from to db SNR (average = -16.9; s.d. = 3.6) for the tonal alarm and from to db SNR (average = -17.6; s.d. = 3.1) for the broadband alarm. In general, the tonal alarm seems easier to detect in the high frequency noises (e.g. drill, chisel and riveting noises), whereas the broadband alarm seems easier to detect in low frequency noises (e.g. bulldozer, freeway). Although statistical analyses have not yet been performed on the data, the spread of results across subjects (the between-subject standard deviation) and over repeated measurements with the same subjects (the within-subject standard deviation) is slightly greater with the tonal alarm than with the broadband alarm. The between-subject standard deviation ranged from 2.4 to 5.4 db for the tonal alarm and from 1.8 to 4.7 db for the broadband alarm, while the within-subject standard deviation ranged from 2.2 to 3.4 db for the tonal alarm and from 1.7 to 2.7 db for the broadband alarm. ICSV22, Florence, Italy, July

5 Figure 4. Average masked detection thresholds (db SNR) across the 24 participants with normal hearing. 4. Discussion and conclusion The current study was an extension of the work performed by Vaillancourt et al. 17 on a restricted set of 4 background noises. Average masked detection thresholds are consistent across both studies, ranging from -13 to -24 db SNR across conditions in Vaillancourt et al. 17 and from -13 to -26 db SNR in the current study. Results of both studies indicate that alarms remain audible when adjusted at levels significantly below that of the background noise, a finding which should be taken into consideration in setting alarm levels so that they may be clearly audible and recognizable, without being excessively high. Two standards are often referenced when adjusting acoustic warning devices: ISO and ISO Specific to audible reverse alarms, ISO states that differences between sound pressure levels measured while the vehicle is operating at maximum governor engine speed (high idle) with the alarm off and those measured while the vehicle is in low idle (neutral) with the alarm on must exceed 0 db at 7 measurement points behind the vehicle. With average alarm detection thresholds ranging from -13 to -26 db SNR across both studies, backup alarms should be clearly audible when adjusted based on ISO recommendations (SNR 0 db), yielding alarm levels 13 to 26 db above masked thresholds. Indeed, according to various authors 12,18 auditory warning signals should be adjusted to levels db above the masked detection threshold for optimal use. It should be noted that in the current study, alarm levels are being compared to overall workplace noise levels rather than that of the heavy vehicle engine noise. As per ISO , alarm levels are evaluated using a single background noise scenario with the vehicle engine running at full speed (high idle). Realistically, however, a vehicle can operate at various idling speeds and other engine regimes, with other noise sources in proximity of workers also contributing to the overall noisy background; hence, lower or higher alarm levels may be required depending on the situation. ICSV22, Florence, Italy, July

6 Table 1. Average masked detection thresholds (db SNR) across the 24 participants and variability metrics. Noise Limestone Quarry Quicklime Quarry Sawmill 1 Sawmill 2 Drill Bulldozer Chisel Industrial Knife Riveting Freeway Construction City Traffic Alarm Average (db SNR) Between-subject standard deviation (db) Within-subject standard deviation (db) Tonal BBS Tonal BBS Tonal BBS Tonal BBS Tonal BBS Tonal BBS Tonal BBS Tonal BBS Tonal BBS Tonal BBS Tonal BBS Tonal BBS ISO more broadly addresses auditory danger signals and its scope is not limited to reverse alarms. To ensure good signal audibility according to this standard, alarms levels should not be less than 65 dba in the reception area and one of the following criteria must be met: 1) the A-weighted SPL of the alarm must exceed the A-weighted SPL of the ambient noise by at least 15 db (Method A), 2) the sound-pressure level of the signal in one or more octave-bands shall exceed the effective masked threshold by at least 10 db in the octave-band under consideration (Method B), or 3) the sound-pressure level of the signal in one or more 1/3 octave-bands shall exceed the effective masked threshold by 13 db in the 1/3 octave-band under consideration (Method C). As per ISO , the ambient noise is defined to include all sounds in the reception area that are not produced by the warning device. Calculations taking into account the spectral content of the two alarms and the 12 noises used in this study were performed to check our data against the various criteria set forth in ISO Based on average masked thresholds obtained across both studies, it can be determined that using Method A would result in excessively high alarm levels. Indeed, alarms set at 15 dba above background noise would result in levels 28 to 41 db above masked thresholds, thereby exceeding the prescribed upper limit proposed by some authors to prevent overly loud warning signals (typically 25 db above the masked threshold for each frequency component of the warning signal ). Methods B and C seem more appropriate to maintain good audibility without setting alarms at excessively loud levels. For each of the alarm and noise combinations used across both studies, setting the alarm to meet the Method B criterion resulted in alarm levels across the 12 noises of about 19 db above thresholds for the tonal alarm (14.1 to 26.4 db) and 21 db above masked thresholds for the broadband alarm (15.5 to 31.3 db) on average across the 12 noises, while using Method C re- ICSV22, Florence, Italy, July

7 sulted in average alarm levels of about 18 db above thresholds for the tonal alarm (14.8 to 27.1 db) and 23 db above masked thresholds for the broadband alarm (15.0 to 34.9 db), thereby yielding more optimal alarm levels than with Method A. By including in this study additional background noises, varying in spectral content and temporal characteristics, conclusions similar to those from a previous study were reached when comparing the data to standards commonly used to guide the adjustment of reverse alarm levels. Using Method A described in ISO may result in excessive adjustment level values for reverse alarms. Despite their more complex acoustic analysis using frequency bands, Methods B and C are therefore encouraged, if additional verifications are also carried out in compliance with ISO However, as stated above, the definition of background noise in ISO should not be limited to a single background noise scenario with the vehicle running at high idle, but should rather include all noise sources at the reception point(s). Statistical analyses, an in-depth analysis of the effects of the noise spectra and temporal characteristics on detection thresholds, and more detailed comparisons with the available standards are warranted. This work is a first step in trying to identify the optimal SNR at which alarms should be set and is part of a greater mandate of optimizing the use of reverse alarms while maintaining adequate safety for workers. ACKNOWLEDGEMENT The work was funded by the Institut de Recherche Robert-Sauvé en Santé et Sécurité du Travail. REFERENCES SAE. Alarm Backup Electric Laboratory Performance Testing. Society of Automotive Engineering, SAE J994 (2009). ISO. Earth-moving machinery -- Machine-mounted forward and reverse audible warning alarm -- Sound test method. International Standards Organization, ISO 9533 (2010). ISO. Danger signals for work places Auditory danger signals. International Standards Organization, ISO 7731 (2003). Blouin, S. Bilan de connaissances sur les dispositifs de détection de personnes lors des manoeuvres de recul des véhicules dans les chantiers de construction. Montréal, Canada: Études et recherches / Rapport B-067 / IRSST (2005) Laroche, C., and Denis, S. Bilan des connaissances sur la signalisation acoustique et les chariots élévateurs. Internal Report IRSST (2000). Laroche, C., Hétu, R., and L Espérance, A. Des alarmes de recul qui tuent! Travail et Santé. 7(1) (1991). Laroche, C., Ross, M-J., Lefebvre, L., and Larocque, R. Détermination des caractéristiques optimales des alarmes de recul. Montréal, Canada: Études et recherches / Rapport R-117 / IRSST (1995) Murray, W., Mills, J., and Moore, P. Reversing accidents in UK. Transport fleets UK: Transport and Logistics Research Unit, University of Huddersfield (1998) NIOSH. The Worker Health Chartbook NIOSH Publication (2004). ICSV22, Florence, Italy, July

8 10 Purswell, J.P., and Purswell, J.L. The effectiveness of audible backup alarms as indicated by OSHA accident investigation records. In: Bittner AC, Champney PC, Morrissey SJ, editors. Advances in Occupational Ergonomics and Safety, ISO Press, Amsterdam, the Netherlands, , (2001). 11 Robinson, G.S., and Casali, J.G. Speech communications and signal detection in noise. In Berger, E.H., Royster, L.H., Royster, J.D., Driscoll, D.P., and Layne, M. (eds.): The Noise Manual, American Industrial Hygiene Association, Fairfax, VA, , (2000). 12 Zheng, Y., Giguère, C., Laroche, C., Sabourin, C., Gagné, A., and Elyea, M. A psychoacoustic model for specifying the level and spectrum of acoustic warning signals in the workplace. J. Occup. Environ. Hyg., 4, 87-98, (2007). 13 Patterson, R.D., and Milroy, R. The appropriate sound level for auditory warnings on civil aircraft. J. Acoust. Soc. Am., 67, S58, (1980). 14 Wilkins, P., and Martin, A.M. Hearing protection and warning sounds in industry- a review. Appl. Acoust., 21, , (1978). 15 Laroche. C., Tran Quoc, H., Hétu, R., and McDuff, S. Detectsound: A computerized model for predicting the detectability of warning signals in noisy workplaces. Appl. Acoust., 32, , (1991). 16 Coleman, C.J., Graves, R.J., Collier, S.G., Golding, D., Nicholl, A.G., Simpson, G.C., Sweetland, K.F., and Talbot, C.F. Communications in noisy environments. Final Report on CEC Contract 7206/00/8/09. Edinburgh: Institute of Occupational Medicine, (1984). 17 Vaillancourt, V., Nélisse, H., Laroche, C., Giguère, C., Boutin, J., and Laferrière, P. Comparison of sound propagation and perception of three types of backup alarms with regards to worker safety. Noise & Health, 15(67), , (2013). 18 Tran Quoc, H., and Hétu, R. La planification de la signalisation acoustique en milieu industriel: critères de conception des avertisseurs sonores de danger. Canadian Acoustics, 24(2), 3-17, (1996). ICSV22, Florence, Italy, July