APPLIED ERGONOMICS
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Applied Ergonomics 30 (1999) 167-171
Technical note
On using psychophysical techniques to achieve urgency mapping in auditory warnings * Elizabeth Helliera,t, Judy Edworthyb a Department b
of Psychology, City University, London, ECl V OHB, UK Department of Psychology, University of Plymouth, Plymouth, UK
Received 13 May 1995; accepted 5 February 1997
Abstract It is well established that warning implementation should aim to achieve urgency mapping between the perceived urgency of the warning itself and the situational urgency of the condition that it indicates, This paper describes how Stevens Power Law [Psychological Review, 64, 153-181, 1957], which quantifies the relationship between objective parameters (such as the pitch of a warning) and subjective parameters (such as perceived urgency), can be applied to the design of auditory warnings to facilitate such urgency mapping, Studies that have quantified and predicted the effects of different warning parameters on perceived urgency using an application of Stevens Power Law are reported, © 1996 CYBERG 96, Published by Elsevier Science Ltd, All rights reserved,
Keywords: Warnings; Urgency mapping
1. Introduction: the need for urgency mapping Patterson (1982) highlighted the problems with the design and implementation of non-verbal auditory warnings. For example, he noted that they were too loud, too numerous, indistinguishable from one another, not standardised and not mapped systematically to the situations that they indicated. He went on to make recommendations for improving warning design and implementation, and also proposed a method of constructing warnings so that they could be tailored for specific environments. Since then there has been considerable research effort devoted to establishing how best to achieve his recommendations so that warnings can be tailored ergonomically to the environments that they are heard in and the conditions that they indicate. One recommendation has been considered particularly important for achieving successful warning implementation and has generated a wide body of research. Patterson recommended that t This paper was originally presented at CybErg 1996-the first international cyberspace conference on ergonomics * Corresponding author
non-verbal auditory warnings should be constructed with different levels of urgency. A sense of urgency is only one of many impressions that a listener may pick up from a sound, but it is one that is particularly relevant for the construction of auditory warnings. It is obviously important that auditory warnings convey some sense of immediacy of action or attention that is urgency. This impression of urgency that a listener gets when listening to a particular sound is referred to as perceived urgency. Different sounds communicate different levels of urgency to the listener and so have different levels of perceived urgency. If Patterson's recommendation that warnings be constructed with different levels of perceived urgency is followed, then there are two potential consequences. Firstly, it could be possible to implement a particular warning with three (or more) versions (e.g. low, medium and high urgency). Such a warning could initially sound at a moderate level of urgency to attract the operator's attention, it could then revert to a low level of urgency to allow communication and fault finding, but would change to a high level of urgency to again demand attention after a pre-set time interval, if the fault had not been rectified.
0003-6870/99/$ - see front matter (g 1996 CYBERG 96. Published by Elsevier Science Ltd. All rights reserved. PH: SO 0 0 3 - 6 8 70 ( 9 7) 0 0 0 1 3 - 6
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Such an implementation policy would confer obvious ergonomic benefits in terms of attracting attention without invoking startle, allowing operator communication and ensuring that the warning is dealt with in a timely manner. The second consequence of following Patterson's recommendations with regard to the perceived urgency of warnings is that warnings with different levels of perceived urgency could be matched to different situations. In this way, critical situations could be indicated by warnings that sound more urgent and less critical situations could be indicated by less urgent sounding warnings. This matching between the perceived urgency of the warning and the urgency of the situation that it indicates has been termed urgency mapping (Edworthy, 1994) and is widely considered to be an essential pre-requisite for the successful implementation of auditory warnings. Urgency mapping increases the informativeness of warnings because it ensures that warnings contain information about their level of priority. Urgency mapping should thus allow warnings to be mapped meaningfully to the situations that they indicated and facilitate an appropriate operator response, particularly in instances of simultaneous warning activation. When two or more alarms sound simultaneously, the priority information (the urgency) in the warning sound itself could inform the operator which warning to respond to first. While urgency mapping may seem a commonsense approach to auditory warning implementation, it should be noted that it requires particular knowledge of human responses to sound, and to date such information has not been available. Until recently, there were few attempts to match warning and situational urgency. In fact, much of the reported dissatisfaction with auditory warnings may be a direct consequence of this omission. Recently, however, the potential advantages of urgency mapping for warning design and implementation have excited an applied research effort in the fields of Ergonomics and Applied Psychology devoted to finding out what determines perceived urgency in warning sounds, and how to manipulate it in measurable ways.
e.g. that increasing an acoustic parameter such as pitch or a temporal parameter such as speed (faster warnings have shorter pulse to pulse times) increased the perceived urgency of a warning. Over the years the effects of changes in a variety of other warning parameters such as harmonicity, physical intensity (dB), rhythm and length on perceived urgency have been documented. While this research showed that it was possible to design auditory warnings by manipulating selected acoustic or temporal parameters to vary perceived urgency, there were limitations to the applications of the research. Although the results provided ordinal information, e.g. increasing the pitch of a warning increases its perceived urgency, the degree to which an increase in pitch resulted in an increase in perceived urgency could not be specified. Furthermore, the results did not indicate which of several parameters, e.g. pitch or speed, is the most effective for increasing perceived urgency. Typically, there can be up to eight to nine auditory warnings in a given environment. In order to achieve precise and subtle urgency mappings between these warnings and the situations that they indicate it is important that the above questions are answered. For example, it is necessary to know not only that a specified increase in warning pitch increases perceived urgency, but also by how much it increases perceived urgency. Does the change in pitch increase urgency by 50%? Does it double it? Furthermore, it is important to know how a particular change in perceived urgency that the designer wishes to communicate can be affected by changing a variety of parameters. How much does pitch have to change to double perceived urgency? How much does speed have to change to double perceived urgency? In order to answer these and other questions, and so to facilitate effective urgency mapping, it is necessary to quantify precisely the relationship between changes in the objective parameters of the warning (e.g. pitch or speed) and changes in the subjective perceptions of urgency. One branch of science that specialises in quantifying relationships between objective and subjective parameters is psychophysics. It is from psychophysics that the techniques to facilitate urgency mapping were derived.
2. How is urgency mapping achieved? In order to map the perceived urgency of auditory warnings to the urgency of different situations it is necessary to know the perceived urgency of different sounds and which variables affect it. Only then will it be possible to create warnings that can be mapped to situations that differ in urgency. Early work in this field (e.g. Edworthy et al., 1991) showed that it is possible to manipulate the perceived urgency of auditory warnings by manipulating the acoustic and temporal parameters of the warning, and that systematic variations in these parameters have predictable effects on perceived urgency. It was shown,
3. Psychophysical techniques for urgency mapping Stevens Power Law (Stevens, 1957) describes the relationship between changes in an objective parameter and changes in a subjective parameter according to the equation S = kom.
(1)
In Stevens model, S is the value of the subjective parameter, 0 is the value of the objective parameter, and k and m are the intercept and slope of the line of best fit
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drawn when the logarithmic values of the subjective and objective parameters are plotted as xy coordinates. In an attempt to go further than merely describing the fact that changes in warning parameters produce changes in perceived urgency, Hellier et al. (1989, 1990, 1993) borrowed Stevens' equation from psychophysics, and applied it to describe and quantify the relationship between changes in objective warning parameters and subjective changes in perceived urgency. Stevens' Power Law is a useful description of this function providing that the function is linear when plotted in logarithmic xy coordinates. According to this application, the relationship between changes in an objective warning parameter (e.g. pitch) and changes in subjective perceptions of urgency can be described, thus Perceived urgency = k (Value of warning parameter e.g. Pitch
E
Hz)m
(2)
Applied in this way, Stevens' Power Law can be used to facilitate urgency mapping. Firstly, laboratory experimentation is required to establish how systematic changes in a variety of objective warning parameters affect subjective judgements of perceived urgency. These subjective judgements of perceived urgency are usually taken by requiring subjects to listen to sounds that vary systematically along various warning parameters, and then to judge the urgency of the sounds. Secondly, data from such studies are plotted as logarithmic xy coordinates, and the studies are replicated so that the data are reliable. This work results in separate plots of perceived urgency against warning parameter levels for the different parameters (e.g. pitch, speed, harmonicity, etc). Stevens' Power Law equations could then be derived and used to specify the objective level of each warning parameter that produces a specified perception of urgency. As an example, the power function values for the constant (k) and the exponent (m) for the warning parameter 'length' (total stimulus on time) is shown below (equation 3). Perceived urgency = 1.65 (Warning length (ms»O.49 (3) By substituting different values of perceived urgency into equation 3 it is possible to specify the length of warning (in ms) that is required to produce that particular perception of urgency. Furthermore, in this formulation the slope of the line or exponent, m, of the power function describes the rate of change of the relationship between the objective warning parameter and perceived urgency. The higher the value of m, the greater the relationship between that warning parameter and perceived urgency, because a greater change in perceived urgency results from a specified change in the objective value of the warning parameter. Thus, deriving the power function of a particular warning parameter would reveal how to manipulate that parameter to affect a specified change in perceived urgency, e.g. how much of a change in pitch
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(Hz) is required to double the perceived urgency of a warning. Power functions enable comparisons to be made between warning parameters to establish which warning parameter most affects perceived urgency. For example, a warning parameter with a large exponent would require a small change in itself to effect a unit change in perceived urgency, whereas a warning parameter with a smaller exponent would require a large change in itself to effect the same unit change in perceived urgency. This is useful to know because it can be applied to situations in which changes in urgency by the most economical method are required. As an example, it may be necessary to design an auditory warning with particular acoustic characteristics so that it is not masked in a particular noise environment (like a helicopter cockpit). In this case, it would be desirable to implement urgency mapping with minimal changes to the acoustic structure of the warning to prevent the warning changing so much in acoustic structure that it became masked. The most economical parameter could be used to produce changes in urgency with minimal changes to the acoustic structure of the warning. Alternatively, it may be necessary to produce a set of auditory warnings that sound similar so that they are identifiable as part of the same 'family' (e.g. all the warnings for a particular piece of a type of equipment). In this case again it would be important to manipulate the urgency of the warning without changing the structure of it so much that the warning sounded qualitatively different. Effecting changes in urgency by manipUlating the most economical parameter would afford a change in perceived urgency with minimal change to the structure of the warning. In order to establish which acoustic features are desirable for the environment in which a warning will be heard, designers should consult the relevant standards and guidelines (e.g. Patterson, 1982).
4. Scaling perceived urgency
Having demonstrated how Stevens' Power Law can be applied to the design of warnings to quantify and predict the affects of different warning parameters on perceived urgency, as well as the benefits that such an application could confer for urgency mapping, we can go on to review some of the studies we have done that put this application into practice. In a series of studies, Hellier et al. (1989, 1993) quantified the effects of different warning parameters on perceived urgency using Stevens' Power Law. The warning parameters pitch, speed, repetition rate, inharmonicity and length were all investigated in separate studies. Subjects used a variety of scaling techniques to judge the urgency of a set of sounds that varied in only one warning parameter at a time, while all other aspects of the sounds were held stable. Thus, subjects produced or
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estimated numbers proportional to the perceived urgency of the sounds, The objective values for each warning parameter were those values that were varied in the sounds. For pitch this was the fundamental frequency (Hz); for speed, it was the pulse rate, the number of pulses per unit sound so that a higher pulse rate indicated more pulses per unit sound and thus a faster stimulus; for repetition rate, it was the number of repetitions of a unit of sound; for inharmonicity, it was the number of inharmonic partials between the fundamental frequency and the first harmonic; and for length, it was the total duration of the stimulus in ms. For each warning parameter, the numerical estimates of perceived urgency and the objective values of the changing warning parameter were plotted as logarithmic xy coordinates. From the logarithmic plots of each warning parameter and perceived urgency, power functions that related changes in perceived urgency to objective changes in each of the warning parameters were derived. It was demonstrated that each of the five warning parameters (pitch, speed, repetition rate, inharmonicity and length) were related to subjective judgements of urgency by a different power function. Table 1 shows, for each of the warning parameters, the exponents that specify the power of the relationship between each warning parameter and perceived urgency. The different exponents show that different changes in the objective value of each parameter were required to produce a unit change in perceived urgency. The largest exponent is for speed, which indicates that changes in speed have the most powerful relationship with changes in perceived urgency. Thus, relatively small changes in warning speed are required to produce a unit change in perceived urgency. It is also shown that the smallest exponent is for inharmonicity, and thus changes in inharmonicity have the least powerful relationship with changes in perceived urgency. Thus, much larger changes in warning inharmonicity are required to elicit a unit change in urgency. This can be demonstrated by taking different values of urgency and using the intercepts and exponents of the power functions for each warning parameter to calculate which objective value of four of the parameters must be multiplied to effect a 50% increase, a doubling or a trebling in urgency. For example, to communicate a doubling in urgency, pitch would have to be multiplied by 6, speed by 1.6, repetition by 4 and inharmonicity by 307 (Table 2). While findings such as these are obviously subject to replication, there is some encouraging evidence to support the use of power functions to predict levels of perceived urgency on the basis of objective parameter levels. The exponents and intercepts revealed in these studies were used to establish the values at which each of three warning parameters (pitch, speed and repetition) should be set to elicit equal perceptions of urgency. Three levels of urgency (high, medium and low) that were theoretically equivalent were generated for each of the three parameters, and 27 stimuli were produced from all pos-
Table 1 Power function exponents for five warning parameters Warning parameter
Exponent
Pitch Speed Repetition Inharmonicity Length
0.38 1.35 0.50 0.12 0.49
sible combinations of urgency levels and parameters. The perceived urgency of the 27 stimuli was predicted on the basis of power functions previously revealed. It was predicted that the relative levels of urgency between high, medium and low urgency would be preserved for all warning parameters so that high urgency was judged more urgent than medium urgency, which in turn was judged more urgent than low urgency. It was also predicted that each particular level of urgency would be equal regardless of the warning parameter through which it was communicated. Thus, stimuli with the same combination of levels were predicted to be judged equally urgent, regardless of which parameter was at each level. For example, a stimulus with speed at the highest level of urgency, pitch at the middle level of urgency and repetition at the lowest level of urgency would be predicted to be judged equally urgent to a stimulus with repetition at the highest level of urgency, speed at the middle level of urgency and pitch at the lowest level of urgency. It was predicted that the effects of the parameters on perceived urgency would be additive. A study of these stimuli (Hellier et aI., 1993) revealed that the predicted order of perceived urgency was highly correlated with that obtained from transformed (Engen, 1971) free modulus magnitude estimates of the stimuli (Spearmans rank correlation, 0.901). This supports the predictions that the relative levels of urgency were preserved from previous work in all parameters and that a particular level of urgency would be judged the same regardless of the parameter that communicates it, and that the effects of urgency are additive. The fact that we were able to create stimuli by combining different warning parameters and to predict their perceived urgency on the basis of the power functions previously revealed for each of those parameters supports the use of psychophysical techniques to quantify and predict the effects of design parameters on subjective responses to warnings. By allowing us to specify the effect of various warning parameter levels on perceived urgency, psychophysical techniques can thus facilitate urgency mapping between auditory warnings and the situations that they indicate.
5. The way forward
The work that has been conducted thus far demonstrates that psychophysical techniques can usefully be
E. Hellier, J. Edworthy/Applied Ergonomics 30 (1999) 167-171
Table 2 Relationship between warning parameters and urgency Warning parameter
Pitch Speed Repetition Inharmonicity
Increment to increase urgency 50%
Double
Treble
x 2.8 xl.3 x 2.2 x 28.5
x6 x 1.6 x4 x 307
x x x x
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a large range of objective parameters applicable to both auditory and visual warnings affect changes in a large range of subjective response parameters, so that warnings can be designed to elicit the most desirable and appropriate responses.
17.4
2.2 8.9 8773
applied to the design of auditory warnings to reveal how changes in objective warning parameters affect changes in subjective perceptions of urgency. This information can be used to help designers construct warnings with specific levels of urgency so that urgency mapping between the warning and the situation that it indicates can be achieved. Here we have focused on the application of these techniques to auditory warning design because it was to help operators cope with auditory warnings in high work load environments, e.g. intensive care units and cockpits, that the calls for urgency mapping were first made. It is, however, possible to extend the techniques reported here to the design of visual warning signs and labels, by quantifying the effect of objective warning parameters, e.g. colour, font size and type face to subjective parameters, e.g. urgency. In fact, some work in the area has already been done (Adams et aI., 1995). Eventually, it may be possible to know how variations in
References Adams, A., Edworthy, E., 1995. Quantifying and predicting the effects of basic text display variables on the perceived urgency of warning labels; trade offs involving font size, border, weight and colour. Ergonomics 38(11), 2221-2237. Edworthy, J., Loxley, S., Dennis, D., 1991. Improving auditory warning design: relationship between sound parameters and perceived urgency. Human Factors 33(2), 205-231. Edworthy, J., 1994. Urgency mapping in auditory warning signals. In Stanton, N. (Ed). Human Factors in Alarm Design Taylor & Francis, London, pp. 15-30. Engen, T., 1971. Scaling method. In: Kling, J., Riggs, L. (Eds.), Experimental Psychology. London, Methuen & Co. Ltd. Hellier, E., Edworthy, J., 1989. Quantifying the perceived urgency of auditory warnings. Can. Acoustics 17(4), 2-11. Hellier, E., Edworthy, J., 1990. Altering the perceived urgency of auditory warnings: an experimental study. Contemp. Ergonomics 389-392.
Hellier, EJ., Edworthy, 1., Dennis, I.D., 1993. Improving auditory warning design: quantifying and predicting the effects of different warning parameters on perceived urgency. Human Factors 35(4), 693-706.
Patterson, R., 1982. Guidelines for auditory warning systems on civil aircraft. CAA Paper 82017. Stevens, S., 1957. On the psychophysical law. Psycho!. Rev. 64, 153-181.