System Design for Auditory Perception

Thu, 05/13/2010 - 9:21am

Farm Blog LogoA recent article in the Boston Globe sheds light on a dangerous, often overlooked, and in this case deadly, problem. To summarize, the Boston Globe reported that "alarm fatigue'' was found to be a contributing factor in the death of a heart patient at Massachusetts General Hospital. Safety officials believe this case is not an isolated incident, but a national problem in which device alarms are often ignored, unheard, accidentally turned off, or purposely disabled due to the annoying effect they have on staff.

    Unfortunately, auditory fatigue, or auditory desensitization, occurs in many working environments where auditory perceptual needs go unmet. Poor auditory environments challenge our ability to understand a situation, make appropriate decisions, and respond in a timely manner. Fortunately these challenges can and should be addressed through appropriate auditory system design. A brief introduction to the auditory system will enable us to better understand our perceptual needs.

The Auditory System
Sounds play a profound and pervasive role in our experience of reality, yet most of our experiences with sound go largely unnoticed. We respond to sounds quickly, intuitively, and instinctively, and still, few spaces fully consider the implications of our auditory environment. Why? The answer may lie in the simple fact that much of our experience with sound is pre-attentive, and our understanding of auditory environments primarily unconscious. Clearly opportunities have been missed.

Structure of our Auditory Environment
While there are many similarities between our perception of visual and auditory information, there are a few fundamental differences. One key difference is that auditory events are dynamic-they're experienced over time (Stevens, 1938). Researchers refer to this fact as auditory streaming. An auditory stream is the perceptual unit of sound, much like the object is the perceptual unit of vision (Bregman, 1990). For example, an auditory stream could be the low hum of a heating fan or a quiet group of people conversing in the background. Our ability to separate auditory streams into appropriate regions or groups is referred to as auditory scene analysis (Bregman, 1990).

Given an auditory scene, auditory objects can be integrated to form a perceptual stream in one of two ways: sequentially or simultaneously. Sequential integration means that auditory events are connected over time, while simultaneous integration means that auditory events share spectral qualities, such as frequency, at the same moment in time (Darwin, 1997).

Auditory Relationships
Researchers in the perceptual sciences have often used the Gestalt principles of visual grouping as a heuristic launching pad from which to conduct studies about preattentive auditory grouping (Aksentijevic, A., Elliot, M.A., & Barber, P.J., 2001). For example, auditory streams with similar frequencies are said to have similarity because they have physically similar wavelengths. An important strength of our auditory system is its ability to segregate streams with speed and relative accuracy. This has important implications for the design of any hospital environment, where misinterpreting auditory data can cost time and cause serious health risks.

Sequential Grouping
It's fairly easy to segregate auditory streams through spectral qualities; however, it's much more challenging to address concerns such as auditory fatigue or desensitization. The solution, as we'll see, lies in sequential grouping.

Sequential grouping allows us to experience the dynamic nature of sound by locating auditory sources and identifying auditory streams over time (Cusack, Deeks, Aikman, & Carlyon, 2004). One of the keys to identifying the source of an auditory stream is our physiological interaural time differences, or ITDs. ITDs refer to our neurological ability to understand the time difference between sounds entering the ear closer to the event, versus sounds entering the ear farther away from the event, and calculate a sense of spatial location (Stevens, 1938; Wickens, 2004). Unfortunately interference of auditory events and special auditory illusions, such as binding, greatly inhibit our ability to understand the source of streams. It is important that alarms are shut off manually, and at the source, to ensure that they are not overlooked or forgotten during interference.

Design Implications
Emergency rooms are rich auditory environments inundated with complex sounds and interferences. The context of our auditory environment greatly affects our perception of spectral and temporal auditory properties, and as a result, our ability to group and understand our auditory scene. Below I list several solutions to improve scene analysis and decision making. The list is far from comprehensive, and I welcome readers to add to it.

  • Monitors with similar alarm frequencies should be spread out so that their auditory streams are clearly segregated, and so that different alarms are correctly identified during an emergency.
  • Monitoring should also be decentralized and ubiquitous so that important auditory events, such as onset time, are perceived as distinct and better understood.
  • Auditory systems must be supported by other sensory input, such as visualization, so that information is received effectively by everyone. Our visual perception is a much stronger system for understanding more complex information.
  • The challenge of auditory fatigue must be addressed by a larger, overarching system that has input from many different sources, and more importantly, has the intelligence to distribute this information appropriately and effectively.
  • Dyanamic displays that allow users to understand the general auditory scene, such as the source of different monitoring devices, will greatly reduce their cognitive workload and enable them to respond more appropriately and efficiently.


Aksentijevic, A., Elliot, M.A., & Barber, P.J. (2001). "Dynamics of Perceptual Grouping: Similarities in the Organization of Visual and Auditory Groups." Visual Cognition, 8(5), 349-358.

Cusack, R., Deeks, J., Aikman, G., & Carlyon, R.P. (2004). "Effects of location, frequency region, and time course of selective attention on auditory scene analysis." Journal of Experimental Psychology: Human Perception and Performance, 30(4), 643-656. Retrieved from

Darwin, C.J. (1997). "Auditory Grouping." Trends in Cognitive Sciences, 1(9), 327-333. Retrieved from

Stevens, S.S., & Davis, H.D. (1938). Hearing: Its Psychology and Physiology. New York: John Wiley & Sons, Inc.

Wickens, C.D., Lee, J.D., Liu, Y., Becker, S.E. (2004). An Introduction to Human Factors Engineering. (2nd ed.). Upper Saddle River, New Jersey: Pearson Prentice Hall.



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