1. Chantal Laroche, Christian Giguère, Rida Al Osman and Yun Zheng
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. 2010 NHCA Conference – Orlando, Florida
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. 2010 NHCA Conference – Orlando, Florida
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. 2010 NHCA Conference – Orlando, Florida
The problem
How many alarm devices needed? Where? Sound power level?
2010 NHCA Conference – Orlando, Florida
February 25-27, 2010

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

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

7. 2010 NHCA Conference – Orlando, Florida
AlarmLocator
The outcome of “AlarmLocator” is a solution of warning devices (D) to meet “Detectsound” targets at all workstations (W) W1 W2 W3 D1 D2 D3
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. 2010 NHCA Conference – Orlando, Florida
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. 2010 NHCA Conference – Orlando, Florida
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. 2010 NHCA Conference – Orlando, Florida
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. 2010 NHCA Conference – Orlando, Florida
Simulation Study
Work Area:
Reverberation times: 0.9s ( Hz), 0.8s ( Hz)
3 workstations (W1 = 86 dBA, W2 = 91 dBA, W3 = 96 dBA)
Low-frequency noise (upper spread of masking)
8.77 m X Y 
14.75 m  W2 W3 W1
2010 NHCA Conference – Orlando, Florida
February 25-27, 2010

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

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

14. 2010 NHCA Conference – Orlando, Florida
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. 2010 NHCA Conference – Orlando, Florida
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)
8.77 m X Y 
14.75 m  D1 W2 W3 W1
X (m)
Y (m)
Z (m)
Sound Power
Level Lw (dB) D1
14.8
5.0
3.0
500 Hz: 116
600 Hz: 114
1000 Hz: 114
1600 Hz: 110
2010 NHCA Conference – Orlando, Florida
February 25-27, 2010

16. 2010 NHCA Conference – Orlando, Florida
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. 2010 NHCA Conference – Orlando, Florida
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 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