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Tools for optimizing the installation of warning sounds in noisy workplaces

Tools for optimizing the installation of warning sounds in noisy workplaces. Chantal Laroche, Christian Giguère, Rida Al Osman and Yun Zheng. 2010 NHCA Conference February 25-27, 2010. Background. Safety in the workplace: Noise is a key hazardous factor and can cause hearing loss

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Tools for optimizing the installation of warning sounds in noisy workplaces

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  1. Tools for optimizing the installation of warning sounds in noisy workplaces Chantal Laroche, Christian Giguère, Rida Al Osman and Yun Zheng 2010 NHCA Conference February 25-27, 2010

  2. Background • Safety in the workplace: • Noise is a key hazardous factor and can cause hearing loss • Acoustic warning signals are crucial to alert workers and reduce the risk of accidents • Safety is dependent on alarm recognition and communication ability in the presence of background noise • Hearing protectors: • Minimize the adverse effects of noise in the workplace … BUT • Can compromise the audibility of warning signals 2010 NHCA Conference – Orlando, Florida February 25-27, 2010

  3. Background • Current practices for installing warning devices: • ISO 7731: “Danger signals for public and work areas” • Devices typically installed on walls or ceiling at a certain distance from workstations • Installation is poorly regulated and submitted to intuition • Factors that must be taken into consideration: • Audibility in the workplace • Sound propagation from the device to the various workstations (direct sound path and reflected sound waves) • Noise field (level, spectrum, type) • Warning signal design (frequency components, level) • Number, location and sound power level of warning devices • Effects of hearing status (hearing thresholds, frequency selectivity) and hearing protectors 2010 NHCA Conference – Orlando, Florida February 25-27, 2010

  4. The problem How many alarm devices needed? Where? Sound power level? 2010 NHCA Conference – Orlando, Florida February 25-27, 2010

  5. General Framework WORK ENVIRONMENT WORKERS Room layout, Reverberation time, Hearing thresholds Frequency selectivity (Xk, Yk, Zk) Workstation coordinates Warning signal target levels [TLlow,TLup ] Detectsound AlarmLocator Number ND Coordinates HPD Power level Lw Noise Lp attenuation (Xi, Yi, Zi) WARNING DEVICES WORKSTATIONS 2010 NHCA Conference – Orlando, Florida February 25-27, 2010

  6. Background noise Detectsound The outcome of “Detectsound” is a design window for warning sound levels at each workstation W Window: TLup = THR+25 dB TLlow = THR+12 dB TLmax = 105 dB SPL 2010 NHCA Conference – Orlando, Florida February 25-27, 2010

  7. W2 D1 W1 W3 D2 D3 AlarmLocator The outcome of “AlarmLocator” is a solution of warning devices (D) to meet “Detectsound” targets at all workstations (W) Simulations: Mirror image method (early reverberation) Classical room acoustics (late reverberation) Solutions: Number of devices Location on walls Sound Power Level 2010 NHCA Conference – Orlando, Florida February 25-27, 2010

  8. Validation • Workshop Area (8.77m  14.75m  6.62m) in Building M-37 at NRC (Ottawa). • Experimental set-up: • 3 workstations (W1-W3) • 2 noise sources (N1-N2) • 2 noise types (continuous, impact) • 3 alarm frequencies (500, 1000, 2000 Hz) • 5 subjects • Open ear + HPD 2010 NHCA Conference – Orlando, Florida February 25-27, 2010

  9. Validation • Psychoacoustic validation of “Detectsound”: • Masked THR prediction error (0.0  1.4 dB) • Preferred level for a 3-tone alarm (18.3 dB  3.1 dB above THR) • Detectsound design window (12 to 25 dB above THR). • Acoustic validation of “AlarmLocator”: • 3 source positions, 3 workstations, 3 frequency bands (n=27) • Omnidirectional source B&K 4295 (known power level) • Workstation SPL prediction error (0.1 dB  0.9 dB) 2010 NHCA Conference – Orlando, Florida February 25-27, 2010

  10. Simulation Study • Goals: • Investigate the effects of hearing protectors on the warning sound design window (TLlow, TLup) for individual workers at specific workstations. • Investigate warning sound design constraints when workers with different hearing status share a common work area. Interaction of hearing loss and hearing protectors on the perception of warning sounds 2010 NHCA Conference – Orlando, Florida February 25-27, 2010

  11. X  Y  W1 W3 8.77 m  W2  14.75 m Simulation Study • Work Area: • Reverberation times: 0.9s (250-1000 Hz), 0.8s (2000-4000 Hz) • 3 workstations (W1 = 86 dBA, W2 = 91 dBA, W3 = 96 dBA) • Low-frequency noise (upper spread of masking) 2010 NHCA Conference – Orlando, Florida February 25-27, 2010

  12. Simulation Study • Workers (Hearing Status): • Indiv1(mild HL): Male 40 yr (20 yr @ W1= 86 dBA) • Indiv2 (moderate HL): Male 50 yr (30 yr @ W2= 91 dBA) • Indiv3 (mod. severe HL): Male 55 yr (35 yr @ W3= 96 dBA) HEARING THRESHOLDS FREQUENCY SELECTIVITY 2010 NHCA Conference – Orlando, Florida February 25-27, 2010

  13. MINIMUM ATTENUATION C B A Simulation Study • Hearing Protectors (CSA Z94.2-02; EN 458-2005): Selection: Class C: Leq≤ 90 dBA Class B:90 < Leq ≤ 95 dBA Class A:95 < Leq< 105 dBA Protected levels: Overprotection:<70 dBA Acceptable: 70  75 dBA Optimal: 75  80 dBA Acceptable: 80  85 dBA Insufficient: > 85 dBA 2010 NHCA Conference – Orlando, Florida February 25-27, 2010

  14. Simulation Study Common design window for 3 workers at W1 (86 dBA) Common design window for 3 workers at W3 (96 dBA) Class C Class A At high frequencies, warning sounds cannot simultaneously meet requirements for Indiv1 and Indiv3. No design window above 2500 Hz. Design window limited by 105 dB SPL maximum at low frequencies and by the conflicting requirements for Indiv1 and Indiv3 at high frequencies. 2010 NHCA Conference – Orlando, Florida February 25-27, 2010

  15. X  Y  W1 W3 8.77 m  D1 W2  14.75 m Simulation Study • Installation of warning devices: • Meet common design windows at the 3 workstations • Four warning signal components (500, 600, 1000, 1600 Hz) • AlarmLocator (ND = 1) 2010 NHCA Conference – Orlando, Florida February 25-27, 2010

  16. Simulation Study • Results: • Warning sound design window is fairly insensitive to attenuation of hearing protectors for workers with normal hearing or mild hearing loss. • Design window is highly sensitive to attenuation achieved at high frequencies (>2000 Hz) for workers with moderate or greater hearing losses. Accurate warning sound solutions require accurate estimates of field attenuation. • Design of warning sounds in a workplace can become a challenge when workers with different hearing status share a common work area. • Warning sounds in the frequency range from 500 to 1600 Hz is recommended (in agreement with ISO 7731). 2010 NHCA Conference – Orlando, Florida February 25-27, 2010

  17. Conclusions • Detectsound provides valid estimates of the optimal design window for warning sounds based on a psychoacoustical analysis of the relevant parameters at each workstation. • AlarmLocator provides possible solutions for the number and placement of warning devices based on a simulation of the sound propagation in the work area. • In general, warning sound frequency components in the range 500-1600 Hz are recommended for workers with hearing loss or wearing hearing protectors (ISO 7731). • Care must be taken not to overgeneralize recommendations to special situations, such as high-frequency noise environments, low-frequency hearing loss or unusual attenuation profiles. 2010 NHCA Conference – Orlando, Florida February 25-27, 2010

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