tools for optimizing the installation of warning sounds in noisy workplaces
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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
2010 NHCA Conference – Orlando, FloridaFebruary 25-27, 2010
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, FloridaFebruary 25-27, 2010
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, FloridaFebruary 25-27, 2010
The problem
How many alarm devices needed? Where? Sound power level?
2010 NHCA Conference – Orlando, FloridaFebruary 25-27, 2010
General Framework
WORKERS
NumberND
Coordinates(Xi, Yi, Zi)
Power level Lw
WARNING DEVICES
NoiseLp
WORKSTATIONS
Warning signal
target levels
AlarmLocator Detectsound
Room layout, Reverberation time, Workstation coordinates
Hearingthresholds
HPD attenuation
WORK ENVIRONMENTFrequenc
y selectivit
y
(Xk, Yk, Zk)
[TLlow , TLup ]
2010 NHCA Conference – Orlando, FloridaFebruary 25-27, 2010
DetectsoundThe outcome of “Detectsound” is a design window for warning sound levels at each workstation W
60
70
80
90
100
110
125 200 315 500 800 1250 2000 3150
Frequency (Hz)
dB
SP
L
Design window
Background noise
TLup = THR + 25 dB
TLlow = THR + 12 dB
TLmax = 105 dB SPL
Window:
2010 NHCA Conference – Orlando, FloridaFebruary 25-27, 2010
AlarmLocator
The outcome of “AlarmLocator” is a solution of warning devices (D) to meet “Detectsound” targets at all workstations (W)
W1
W2
W3
D1
D2D3
Solutions:
Number of devices
Location on walls
Sound Power Level
Simulations:
Mirror image method (early reverberation)
Classical room acoustics (late reverberation)
2010 NHCA Conference – Orlando, FloridaFebruary 25-27, 2010
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, FloridaFebruary 25-27, 2010
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, FloridaFebruary 25-27, 2010
Simulation Study
Goals:
1. Investigate the effects of hearing protectors on the warning sound design window (TLlow, TLup) for individual workers at specific workstations.
2. 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, FloridaFebruary 25-27, 2010
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)
Noise
60
70
80
90
100
31.5 63 125 250 500 1000 2000 4000 8000
Frequency (Hz)
Lev
el (
dB
SP
L)
8.77
m
X
Y
14.75 m
W2
W3
W1
2010 NHCA Conference – Orlando, FloridaFebruary 25-27, 2010
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)
0
1
2
3
4
5500 1000 2000 3000 4000
Frequency (Hz)
Bro
aden
ing
fac
tor
0
10
20
30
40
50
60
70
80500 1000 2000 3000 4000 6000
Frequency (Hz)
Hea
rin
g L
evel
(d
B H
L)
HEARING THRESHOLDS FREQUENCY SELECTIVITY
2010 NHCA Conference – Orlando, FloridaFebruary 25-27, 2010
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
MINIMUM ATTENUATION0
5
10
15
20
25
30
35
125 250 500 1k 2k 3k 4k 6.3k 8k
Frequency (Hz)
Att
en
ua
tio
n (
dB
)
C
B
A
2010 NHCA Conference – Orlando, FloridaFebruary 25-27, 2010
Simulation StudyCommon design window for 3 workers at W1 (86 dBA)
Poste 1
60
70
80
90
100
110
125 200 315 500 800 1250 2000 3150
Fréquence (Hz)
Niv
eau
(d
B S
PL
)
Frequency (Hz)
W1
Le
vel
dB
SP
L)
Poste 1
60
70
80
90
100
110
125 200 315 500 800 1250 2000 3150
Fréquence (Hz)
Niv
eau
(d
B S
PL
)
Frequency (Hz)
W1
Le
vel
dB
SP
L)
Frequency (Hz)
W1
Le
vel
dB
SP
L)
At high frequencies, warning sounds cannot simultaneously meet requirements for Indiv1 and Indiv3. No design window above 2500 Hz.
Common design window for 3 workers at W3 (96 dBA)
Poste 3
60
70
80
90
100
110
125 200 315 500 800 1250 2000 3150
Fréquence (Hz)
Niv
eau
(d
B S
PL
)
Frequency (Hz)
Le
vel
dB
SP
L)
W3Poste 3
60
70
80
90
100
110
125 200 315 500 800 1250 2000 3150
Fréquence (Hz)
Niv
eau
(d
B S
PL
)
Frequency (Hz)
Le
vel
dB
SP
L)
W3
Design window limited by 105 dB SPL maximum at low frequencies and by the conflicting requirements for Indiv1 and Indiv3 at high frequencies.
Class C Class A
2010 NHCA Conference – Orlando, FloridaFebruary 25-27, 2010
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, FloridaFebruary 25-27, 2010
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, FloridaFebruary 25-27, 2010
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.