Alarm/Alert Auditory Displays - Basic Principles

Research is continuously on-going to determine the optimum acoustical characteristics for multiple auditory displays in complex systems. However, there are basic principles that can be applied:

• Limit the number of alarms and alerts. Optimally, limit to 6 immediate action signals and 2 precursor, attention signals.
• For 100% dectectability, signals must be 15 dB above above ambient, background noise.
• For signals requiring rapid response, level should be approximately 15 above ambient, background noise.
• Signals must differ in frequency from the frequencies of background noise.
• Signals must differ sufficiently that one signal does not mask another.
• Signals should be between 500 and 5000 Hz and composed of at least 4 prominent frequency components, each within the range of 1000 to 4000 Hz.
  • Signals should be composed of regular, harmonic, frequency components instead of inharmonic components (perceived tonal sequence vs. noise).
  • Rapid (100 msec) glides in the signal's fundamental frequency command attention and convey urgency.
• To gain attention, modulate the signal, using intermittent "beeps" or rise and fall in pitch (1 to 3 cycles per second).
• Use temporal patterns to increase signal detectability and discriminability.
• Signals that must be heard from more 1000 feet away should be below 1000 Hz, as lower frequencies travel farther.
• Signals that must be heard through partitions or around obstacles should be below 500 Hz, as lower frequencies can travel through barriers.
• Use redundant visual indicators.
• Always test in the operational environment to ensure effectiveness.