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Mobile Equipment Warning
Signal Detection in Noise
Chantal Laroche, Ph.D.
Audiology-SLP Program
University of Ottawa
AIHce, June 5th
New Orleans
PROBLEM

Each year, serious accidents occur in noisy
workplaces because a warning sound is not
heard.
 The bad perception of reverse alarms on mobile
vehicles (e.g. dump trucks, forklifts) is one
element which can explain some of the
accidents.
 Potential causes: high levels of ambiant noise,
noise-induced hearing loss, lack of attention,
placement and acoustic features of alarms,
hearing protectors
SOME STATISTICS

Fatal accidents involving workers struck by a
machine in motion:
- 1980-1992: 667 fatal accidents
(USA)
(on construction sites; Sweeney et al., 1999)
- 1975-2000: > 25 fatal accidents (Quebec)
(in reverse mode; Laroche, 2001)
SOME STATISTICS

Serious or fatal accidents involving
forklifts:
– 1984-1992: 14 deaths
(Illinois)
– 1991-1992: 24 deaths
(OSHA)
– 1994-1997: 316 serious accidents
(CSST, Quebec)
– 1974-1999: 9 deaths (CSST, Quebec)
Examples of accidents
involving mobile vehicles
Employee
Type of
vehicle
Cause of
accident
Comments
flag person
(construction site)
dump
truck
reverse alarm
not detected
alarm: 90dBA;
noise from truck: 87dBA
quality control
attendant
(construction site)
dump
truck
reverse alarm
not detected
alarm in front of the
2 back axles and directed towards the left
alarm : 80-85dBA
noise : 105-107dBA
worker
(road repair)
dump
truck
Absence of
reverse alarm?
Noise from a
mechanical
digger
Background noise > truck noise
Examples of accidents
involving forklifts
Employee
Cause of accident
Recommendations
Seagoing personnel
- High level of noise
- Low visibility (dead
angle)
- Pedestrians and
vehicles
-Low visibility
-Bad judgment
- Noise control
- Mirors
- Delimit pedestrian
corridors
Student,
Maintenance
No specific
recommendantion
POTENTIAL CAUSES

High level of ambiant noise in the workplace
 High prevalence of noise-induced hearing loss
(NIHL)
 Worker’s attentional demand
 Inadequate placement of reverse alarms on
vehicles
 Deficient acoustic features of existing reverse
alarms
 Hearing protectors
High levels of ambiant noise
 On
construction sites:
(Suter, 1999)
745 000 (out of 5 million) workers exposed to
Lp > 85 dBA
Inadequate signal to noise ratio
PREVALENCE OF NIHL

On construction sites:
18 to 33% of workers have
hearing thresholds
greater than an average of 25 dB
at 1, 2, and 3 kHz
(Ohlin, 1999)
Worker ’s attention demand
and perception

Wilcox (1994) has discussed myths associated
with the perception of reverse alarms and the
attention demand on forklift trucks:
– Pedestrians get used to reverse alarms
(habituation phenomenon)
– Many forklifts, each equipped with an
alarm, would create confusion
Worker’s attention demand
and perception
– Accidents are caused by a task that
demands too much of the victim’s
attention
– Reverse alarms are annoying
Inadequate placement of
reverse alarms

SAE-J994 standard on reverse alarms:
« The alarm should be tested in free field, 4 feet
above a horizontal reflecting plane, with the
microphone 4 feet from the alarm’s horn along
its 0 degree axis ».

This standard does not insure that every
worker will perceive the alarm at any position
behind the vehicle.
Deficient acoustic features

What is found on the market?
Reverse alarms with a pure tone (10001300 Hz) or modulated tones between
two approximate frequencies (e.g. 12721310 Hz)
Axis positions behind the
heavy vehicles
Positions of the alarm
on a grader
Variations in
sound pressure levels
at the rear of a grader
Sound wave reflections
Hearing protection

Sense of insecurity frequently reported:
Difficulty to hear and understand
speech communication
and warning signals
Recommendations

In order to reduce the number of fatal
accidents involving pedestrians at the rear of
vehicles:
– Modifications in the design and positioning
of reverse alarms
• Avoid pure tones
• Optimize the position
– Better ergonomic design of mobile
equipment
– Addition of pedestrian corridors, when
possible
Recommendations (cont’d)
– Explore ways to limit the habituation
phenomenon
• Obstacle detectors
• Use of electronic mirrors
There is a need for improvement in the reliability
of these devices
– Noise reduction in the workplace
• Would reduce the prevalence of NIHL
• Would reduce the masking effect of the
background noise