Designing urgency into auditory warnings using pitch, speed and loudness

Haas, E. C., & Edworthy, J. (1996). Designing urgency into auditory warnings using pitch, speed and loudness. Computing & Control Engineering Journal,7(4), 193-198.

The perception of urgency in an audio alarm is affected by several factors and parameters. In Patterson’s Pulse method for the creation of “customizable” audio alarms, the pulse can be manipulated in terms of fundamental frequency, the degree of inharmonicity, the pulse’s amplitude envelope,and so in terms of burst rhythm, speed, melodic pattern, pitch contour and length.

This study intended to investigate the effect of pulse fundamental frequency, pulse level and inter-pulse interval on the perceived urgency of warning signals; to investigate the effect of these variables on the response time to warning signals; and to examine the relationship between perceived urgency and response time to warning signals.

The stimuli

27 auditory signals were used, each made of a train of four pulses. Each pulse had a duration of 350 ms, including an onset time of 25 ms and an offset of 25 ms. The total signal time was a minimum of 1.4 s (at 0 ms inter-pulse interval) and a maximum of 3.4 s (at the 500 ms inter-pulse interval).

Inter-pulse interval is the time elapsed from the end of the offset (decay) of one pulse to the beginning of the onset of the next.
The times were 0,250 and 500 ms. Pulse onset was the time from the waveform’s increase from zero amplitude until it reached maximum output. Pulse offset was the reverse.

The three fundamental frequencies used were 200, 500 and 800 Hz.
The signal with a fundamental frequency of 200 Hz had components of 200, 400, 600, 800 and 1000 Hz.
The signal with a fundamental frequency of 500 Hz had components of 500, 1000, 1500, 2000 and 2500 Hz.
The signal with a fundamental frequency of 800 Hz had components of 800, 1600,2400,3200 and 4000 Hz.
Signals with a fundamental frequency of 1000 Hz and greater were avoided because the high-frequency components normally make the signals sound aversive.

The pulse levels were set at 5,25 and 40 dB LIN SPL above the ambient noise level.

Each signal was presented twice, in random order, and the participant verbally rated the perceived urgency. In another task, the participant had to react to the signals with the dominant hand as soon has s/he detected it.

Results Perceived Urgency

All three variables affected the perception of perceived urgency.

Signals with a fundamental frequency of 200 Hz sounded significantly less urgent than those containing 500 Hz or 800 Hz fundamental frequencies.  Participants rated signals with an inter-pulse interval of 0 ms as sounding significantly more urgent than those containing intervals of 250 ms and 500 ms. As for loudness, participants rated signals 40 BLW above ambient as significantly more urgent than those 25 dB LIN above ambient.

These results suggest that a strong perceptual difference is elicited by temporal differences in signal construction.

Results Reaction Time

For the reaction time results, the effective variables were pulse level and fundamental frequency.
Signals with a fundamental frequency of 800 Hz had a significantly shorter response time than those with a fundamental of 200 Hz. Response times for signals 240 dB LIN above ambient were the shortest, followed by signals 25 dB above ambient, and those at 5 dE3 LIN above ambient.
Response time to signals decreased differently as fundamental frequency and pulse level increased, and as inter-pulse interval decreased.

Pulse level and fundamental frequency are the signal design characteristics which consistently affected all measures of perceived urgency and response time.

One topic for future research could involve defining the point at which fundamental frequency no longer produces differences in perceived urgency.

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