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Design of an Auditory Warning Device (2005)

Organization: Auditory systems lab,
Human Factors engineering and ergonomics center, Virginia Tech
Duration: 2 months

Introduction:
We had to design an auditory warning device for a peculiar environment in a manufacturing plant. This plant has a large airtight, windowless paint cabin in the middle of an assembly line made of concentric turn-tables. A worker does the painting job isolated in this booth filled with toxic (asphysixiant), explosive fumes wearing a self-contained breathing apparatus (SCBA).

No loudspeakers or other non-intrinsically-safe equipment could be added inside the paint booth, due to the explosive potential of the interior atmosphere. In fact, due to plant design, all loudspeakers and strobe lights must be located on the foreman's booth roof and project throughout the room.

Analysis and Design procedure:
After initial site-observation and data collection about the plant environment, we started working on design aspects that included:

  • Distance from the device place
  • Reverberation characteristics of the room
  • Compliance with the OSHA HCP requirements
  • Compliance with the ISO-7731 regulations
  • Shadow zones because of obstructions
  • Worst-case scenario of worker's hearing loss (presbycusis: hearing level)
  • Paint cabin attenuation
  • Masking effects for the pure-tone frequencies

The final design solution was comprised of a complex tone auditory warning signal of the following characteristics:

  • Spectral characteristics: The chosen audio warning signal has two pure-tone frequencies respectively at 500 Hz and at 4000 Hz.
  • Sound Pressure Levels: The 500 Hz tone and the 4000 Hz have Sound Pressure level of 113 dBA and 115 dBA respectively at 5 ft. from the device location.
  • Temporal characteristics: The tones have maximum repetition frequency of 4 Hz.

The Presbycusis graph to show the expected hearing levels used for the worst-case scenario hearing levels

Presbycusis hearing levels

The Presbycusis graph used for the worst-case scenario hearing levels