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

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


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


The problem

How many alarm devices needed? Where? Sound power level?


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 ]


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:


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)


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


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)


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


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


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


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


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


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


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).


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.

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Tools for optimizing the installation of warning sounds in noisy workplaces

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