Mobile Equipment Warning Signal Detection in Noise

1
Mobile Equipment Warning Signal Detection in Noise

• Chantal Laroche, Ph.D.
• Audiology-SLP Program
• University of Ottawa

AIHce, June 5th New Orleans
2
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

3
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)

4
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)

5
Examples of accidents involving mobile vehicles
 
 
6
Examples of accidents involving forklifts
7
POTENTIAL CAUSES

• High level of ambiant noise in the workplace
• High prevalence of noise-induced hearing loss
  (NIHL)
• Workers attentional demand
• Inadequate placement of reverse alarms on
  vehicles
• Deficient acoustic features of existing reverse
  alarms
• Hearing protectors

8
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

9
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)

10
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

11
Workers attention demand and perception

• Accidents are caused by a task that demands too
  much of the victims attention
• Reverse alarms are annoying

12
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 alarms horn along
  its 0 degree axis .
• This standard does not insure that every worker
  will perceive the alarm at any position behind
  the vehicle.

13
Deficient acoustic features

• What is found on the market?
• Reverse alarms with a pure tone (1000-1300 Hz)
  or modulated tones between two approximate
  frequencies (e.g. 1272-1310 Hz)

14
Axis positions behind the heavy vehicles
15
Positions of the alarm on a grader
16
Variations in sound pressure levels at the rear
of a grader
17
Sound wave reflections
18
Hearing protection

• Sense of insecurity frequently reported
• Difficulty to hear and understand
• speech communication
• and warning signals

19
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

20
Recommendations (contd)

• 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