Environment International, Vol. 16, pp. 447-458, 1990 0160-4120/90 $3.00 +.00 Printed in the U.S.A. All fights reserved. Copyright @1990 Pergamon Press plc

RECENT ADVANCES IN THE STUDY OF HUMAN PERFORMANCE IN NOISE

Dylan Jones School of Psychology, University of Wales, College of Cardiff, Cardiff CF1 2YG, UK

El 88-220 (Received 3 October 1988; accepted 5 January 1990)

Recent research associated with three major types of effect of noise on human performance is discussed: 1) Loud noise influences information processing by inducing verbal strategies which may persist after the noise has been turned off, and if verbal strategies are already in use, noise encourages their more fervent adoption. 2) Loud noise also elevates the person's state of arousal. Its effects are qualitatively similar to the effects of fatigue due to mental work and, in some ways, to the effects of wakefulness over the day. 3) Irrelevant speech interferes with reading and memory. The effects appear to be independent of intensity and, for memory at least, independent of the meaning of the speech. Speech appears to have privileged access to (and hence opportuni- ties to disrupt) high-level processing mechanisms essential to efficiency in children's reading and to performance in the modern office environment. Arising from an analysis of these findings, a number of emerging research issues are discussed.

INTRODUCTION

This paper reviews recent research on the effects of noise and performance. Rather than having any claim to be comprehensive, the review is selective, focussing on major areas of development. Inevitably, this means that some careful and imaginative work will be left out. But the strategy of selecting the primary areas of growth can be defended if only because the last five years or so have seen a marked change in emphasis of the research area, coupled with considerable conceptual progress. The three areas which are to be reviewed are: the effects of strate- gies, those performance effects which stem from changes in arousal, and the effects of irrelevant speech on the processing of language.

Much of what seemed to be opaque about the effects of continuous broad-band noise and its ef- fects on performance in the late fifties and sixties is now better understood. As noted by Broadbent (1984), this is largely because simple, unidimensio- nal models of performance in noise, which suggested that the response to noise was a mechanical and involuntary one, have given way to models based on changes in performance strategy. The review outlines advances in the understanding of strategic effects which have helped to reconcile the contradictory and confusing trends reported by Gulian (1973) and Loeb (1980). These advances have also been coupled with the realisation that strategy change fails to account for all the findings. Noise brings about changes in the general state of the person, and this view has been

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recently reinforced by a series of studies showing an interaction between noise and the time of day at which performance is measured.

During this period also, the scope of the study of performance has been extended to include the effects of meaningful sound, and especially speech, on the performance of a range of cognitive activities. Em- phasis on this topic is particularly timely in view of the shift away from heavy industry toward an office- based economy (at least in technologically advanced countries), relying as it does largely on language processing. Much of the recent research has been undertaken in the laboratory by observing the effect of irrelevant (or unattended) speech on processing verbal information presented visually. The review will be concerned primarily with advances which have been made in delimiting the types of task sus- ceptible to disruption and in establishing the pa- rameters of speech responsible for the effect.

THE IMPORTANCE OF STRATEGIES

Advances in mainstream cognitive psychology have helped to change our view of how human perfor- mance may be understood. Stemming, in particular, from the study of human memory, the notion of the individual's active governance and management of processing resources has gradually gained credence within the research community. An important feature of this approach is the recognition that, even for relatively simple laboratory tasks, a variety of con- trol processes may be deployed. These control pro- cesses govern both the flow and form of information within the brain. For example, in learning a list of words, a person may place emphasis on remembering the items in their original order or, may remember them by organising the words and collecting them into groups of similar meaning. Each is a legitimate means of remembering the material, but also each will have slightly different consequences on the form in which the material is recalled.

Before describing the outcome of this research it seems appropriate to sound a note of caution. Al- though it has been clear for some time that a range of diverse findings could be explained by reference to the notion of strategy, it is now becoming evident such an approach would not necessarily be useful in predicting the outcome of a study. Like the "inverted- U" before it, the post hoe explanatory power of strat- egies is overwhelmingly greater than its predictive power. Quite minor and incidental features of the experiment (and even the effect of previous experi- ments) have been invoked as possible ways in which strategies have been influenced. There seems to be

no way of knowing in advance which of these many sources of bias might be at play and what form their influence would take. Despite this difficulty, prog- ress has been made in four areas:

Strategic effects in non-verbal tasks

There has been confirmation that strategic effects occur in settings where language is not used. Early studies showed that activities involving language were useful vehicles for understanding the effects of noise on strategies, but it was unclear whether tasks which only involved relatively simple manual responses were also sensitive. Serial reaction is an example of a task involving little verbal processing. In this task, subjects are required to make a speeded, manual response to the onset of one of a number of lights. Each response brings on the next light, usually after a short delay, in an unpredictable sequence. Subjects are required to undertake the task in sessions lasting up to forty minutes. Serial reaction is known to be sensitive to noise at or above 95 dB, but hitherto this effect had only been observed on the frequency of unusually long responses, known as gaps (e.g., Wil- kinson 1963) or errors (Hartley 1973). Recent work has confirmed that continuous noise has a detrimen- tal effect at a lower level of 90 dB(C) (Jones 1984) and that intermittent noise produces effects on mean response time at levels as low as 80 dB(C) (Lahtela et al. 1986).

Such an effect could be produced by a response of the mechanical and involuntary type, but studies which have manipulated the demands of the task have shed light on the strategic nature of the response. In its usual form each of the lights in the serial reaction task appears with equal probability, but in a version used by Smith (1985) some lights were more proba- ble than others. With this version of the task, moder- ate intensity noise (at 85 dB(C)) increased the time it took subjects to respond to signals which appeared less frequently and decreased the time to respond to the more frequent signals. Noise therefore appears to alter the distribution of attention, away from low- probabil i ty sources and towards high-probabili ty sources, echoing the classical finding of Hockey (1970). Smith was also able to show that noise ap- pears to have made the subjects inflexible in their adoption of this attentional strategy: they continued to use it even when, unannounced, the signal proba- bilities were restored to normal. The strategic effect is not a fleeting one therefore, and the period for which it persists may be relatively short (Cohen 1980; Jones 1984) or long (Smith and Broadbent 1982).

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The observation that the effects of noise extend beyond the period of exposure is not a novel one, but the suggestion that these effects are strategic, rather than associated with the general state of arousal, is novel. In the last two decades much has been done to discover the factors responsible for aftereffects of noise, but little is understood about the recovery from them. In early work on aftereffects (e.g., Glass and Singer 1972) it was difficult to distinguish whether the relation of exposure to postexposure performance was one of "hangover" or "rebound" (Jones and Broad- bent 1979). This was largely because performance was not usually measured during exposure. These recent studies show that performance continues to deteriorate in the period following exposure, which tend to confirm that a hangover interpretation is the more appropriate.

In settings where the cognitive psychologist's un- derstanding of strategies is less well understood, there seems to be a corresponding lack of certainty about the way in which noise influences performance. This is particularly true of vigilance tasks, which have traditionally been associated with noise research and are still the occasional focus of research interest. In a very thorough review of the noise and vigilance literature, Koelega and Brinkman (1986) were de- spondent about the chances of progress. They opened their account with the assertion that, "the literature on the effects of auditory noise on visual perfor- mance presents an outstanding example of confu- sion". Possibly, if vigilance tasks were more clearly understood in terms of the strategies which are de- ployed during performance, a more coherent picture would emerge.

Unfortunately, the bulk of the research on noise and vigilance has been devoted to testing theories other than those of the strategic type. But for those vigilance tasks incorporating a significant cognitive component (usually using verbal stimuli and employ- ing some memory component), some headway has been made in understanding the effects of noise. For example, Miles et al. (1984) found that the effect of noise depended on both signal probability and prior training. They argued that the effect of prior training and noise were minimal if the signals were frequent: subjects were more entrained to their present task context. Only if the probability of a signal was low, with the attendant uncertainty about the likely ap- pearance of signals, would loud noise (at 95 dB(C)) have an effect.

Strategic effects in verbal tasks

In contrast to serial reaction, which has no verbal component and where the range of strategies may be very limited, most complex cognitive tasks involve language, and this means they can be executed in a variety of ways. Hence, in noise, subjects may have more opportunity to exercise control over the deploy- ment of strategies.

One of the more important findings in this area is that when a variety of strategies is available, noise seems to bias the subject toward strategies of the verbal type, especially ones which involve rote re- hearsal of a list of items. For example, in a study by Hartley et al. (1986) subjects had to verify whether a sentence and a picture were compatible. To make their judgement, some subjects preferred to use a spatial strategy, converting words into a visual men- tal image, while others preferred to use a verbal strategy, transforming the picture into a description based on words. Noise did not increase the tendency to use such strategies, but rather hindered those who preferred to use a spatial strategy, presumably by promoting a mixed (verbal/spatial) strategy. In con- trast, the performance of those already employing the verbal strategy was facilitated by noise, again sug- gesting that verbal strategies are adopted in noise, but this time to advantage.

Similar results appear if, rather than studying sponta- neously adopted strategies, the experimenter instructs the subject on which strategy to use. In noise, effi- ciency is improved if subjects are instructed to employ a verbal strategy. If, on the other hand, the subject is instructed to use a spatial strategy, performance dete- riorates in noise (Hartley et al. 1987). This will be true as long as the particular verbal strategy is the optimal one. For example, the verbal strategy of repeating the material over and over to oneself is good for preserving order information.

In noise, subjects appear to adopt strategies involv- ing rote rehearsal, unless instructions induce an alter- native strategy, in which case noise reinforces the use of the alternative strategy. Moreover, the induced strat- egy will be adopted even if in objective terms it is not the best one (Wilding et al. 1982). This explains a range of findings: the improvement in ordered recall in noise (e.g., Daee and Wilding 1977; Hamilton et al. 1977; Millar 1979; Wilding and Mohindra 1980); a reduc- tion in the tendency for words of similar meaning in a list to be recalled together, the so-called category clustering effect (Smith et al. 1981; Wilding et al. 1982).

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Effects at encoding and retrieval

In most of the recent evidence the emphasis has been on impairments due to noise at the encoding stage of memory, that is, when the items are being learned. It has been suggested that the effect of noise is restricted to encoding and rehearsal-maintenance strategies rather than to retrieval from memory (Bell et al. 1984). However, as Smith et al. (1981) note, noise may also influence performance in those cases where there is some flexibility over the retrieval strategy. This has been demonstrated in a task involv- ing running memory. In this type of task, a stream of digits is shown and when the stream is stopped un- predictably the subject is required to recall as many items as possible from the end of the list. The effect of noise crucially depends on how many items the subject is required to recall: if it is relatively few, noise exaggerates the tendency to recall the early items first. But if the subject is asked to recall as many as eight items from the list, noise increases the subject's predilection to recall the last items first (Smith 1983). In both cases the prevailing dominant tendency is reinforced.

Effects on the speed of reading

Even when serial order of the material to be re- membered does not have to be preserved, noise at relatively low levels of intensity may disrupt perfor- mance. For instance, the Stroop test can be shown to be sensitive to noise as low as 85 dB(C). Interpreta- tion of early work on this test had been hampered by methodological differences in the level of noise and the duration of noise exposure. In addition, the early work only used part of the Stroop test, a fault which recent work has been careful to remedy. The conven- tional Stroop test comprises three forms: one in which patches of colour are named (C), another in which monochrome colour words are read (W), and an in- terference condition (CW) in which colour words are printed in incompatible hues. This last condition is the one usually referred to as "the Stroop interference condition". With the CW card, the subject has to resist the natural and compelling tendency to read the word and instead name the ink in which it is printed. The C and W cards act as measures of reading speed for the two types of material. In most early studies on the effects of noise on performance, only the interference condition was included. Without the C and W conditions as a check, this left open the pos- sibility that the measured effect was due to changes in the speed of reading rather than to change in interference. However, Smith and Broadbent (1985) conducted a series of experiments using the control

material and found rather complex effects. In contin- uous noise, efficiency was impaired only in the read- ing of words and this only happened after the subject had been in noise for some time. One possibility is that this is another manifestation of the tendency for verbalisation to become more emphatic in noise, a phenomenon which may underpin the tendency to- ward rote rehearsal in list learning, but in this case it acts to slow down the rate of work.

In sum, these results suggest that both the benefi- cial and deleterious effects of noise can be reconciled by understanding the degree to which verbal process- ing is important to the proper execution of the task. We may summarise the position with the study of strategies as follows: 1) when learning lists, noise seems to induce a verbal strategy, usually one involv- ing rote rehearsal; 2) where a verbal strategy is in use, noise will lead to its more vigourous adoption; 3) strategies may persist some time after the noise has been turned off; and 4) non-verbal tasks also show strategy effects but thus far only the study of serial reaction has yielded systematic results.

EFFECTS OF AROUSAL

That the general state of alertness or reactivity of the person is elevated by loud noise is long established. Primarily, the reason for supposing that arousal is important is that the effects of noise mod- ified by other stressors whose action is primarily on the bodily state of the person. Two main ways have been used to classify stressors into similar types: in the first, the individual action of stressors is judged to see whether they show qualitative similarities; the second approach monitors the joint action of stressors for evidence of a more than additive or a subtractive effect. Both approaches are reviewed here.

Time of day as an arouser

As an example of the first approach, Breen-Lewis and Wilding (1984), in separate experiments, exam- ined the effect of noise at 85 dB(C) and time of day. It is by now well established that the person's state of arousal changes over the waking day. Changes in psychological reactivity are thought to be under- pinned by biochemical rhythms of the body. Evi- dence of the same pattern of response to both noise and time of day would be suggestive but not conclu- sive evidence that they have a similar type of action. In the study in question, results pointed to the fact that the two stressors were independent: morning performance encouraged subjects to engage in a more active processing strategy regardless of test expecta- tions, while noise appeared to adopt the prevailing

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strategy more vehemently. In a complementary fash- ion, Smith (1985) showed that the effect of noise on five different performance tests at three different times of day (0900, 1200, and 1800) showed no evi- dence of an interaction. Moreover, Smith and Miles (1986a) failed to find an interaction between moder- ate intensity noise and the variability in alertness during a night shift. However, this same group of researchers found that noise did interact with the dip in performance which usually follows a meal (Smith and Miles 1986b). One way to reconcile these find- ings is to suppose that post-meal effects are qualita- tively different to other time of day effects, but the evidence for this is at present equivocal.

Arousal and prolonged work

Some convergence has occurred in the work on strategies on the one hand and arousal on another. For instance, Holding et al. (1983) examined the effect of task length and noise on behaviour in a complex task involving choice between various routes to a task goal. The alternatives are weighted in terms of the risk of failure and likelihood of success, and also for the amount of effort required for their successful completion. Holding et al. showed that noise was similar in its action to prolonged work, causing a shift from high-effort, low-risk choices in quiet to low-effort, high-risk choices in noise (at 95 dB). These studies are particularly important in illustrat- ing the general point that our understanding of noise effects needs to embrace effects on the general state of the person and also on the selection of perfor- mance strategies.

Individual differences in arousal

Not only are preferences for the payoff factors in the task important in shaping the effect of noise, the individual's preference for the level of noise is obvi- ously important. Recent studies have reinforced the view that individual differences in arousal are re- sponsible for establishing the person's tolerance for the overall level of broad-band noise. This in turn may shape the nature of performance effects. The personality dimension of introversion has long been identified with the person's state of arousal: Eysenck (1967) suggested that extroverts tend to seek out, and are able to tolerate, higher levels of stimulation than introverts. Using a technique which allowed subjects to adjust the level of noise to an optimum, Geen (1984) showed that extroverts selected higher inten- sities. Physiological parameters also showed the introverts to be more aroused than the extroverts. Subjects were then exposed to noise either at, above,

or below, their optimum while undertaking a paired associate learning task. Introverts only showed dec- rements in performance when the level was above their optimum. Performance of extroverts on the other hand declined if performance was either above or below their optimum. Hence any divergence from the optimum may, but will not always, bring about poor performance. The important point is that the opti- mum is different for the two groups, but unfortu- nately it is difficult to specify in advance,

When more complex material is being learned, the main personality variable of importance appears to be neuroticism rather than extraversion. Neuroticism is often thought to be reflected in self-oriented thoughts, worry, and anxiety, each of which act as a distractor from learning and recall (Mandler and Sarason 1952). Noise tends to impair the perfor- mance of neurotics and improve the performance of stable individuals. This is particularly true of com- plex tasks such as retrieval from semantic memory (yon Wright and Vauras 1980) or the learning of a passage of prose (Nurmi and yon Wright 1983).

The analysis of performance in studies on individ- ual differences has not yet incorporated the approach of studying strategies. Perhaps the most fruitful way forward is to adopt an approach which combines strategy and arousal approaches. For example, such a view might suggest that extraversion could dictate the preferred level of stimulation, neuroticism on the other hand may dictate the process of strategy selec- tion, either by dictating the payoff associated with different outcomes or by changing the rate at which progress of a particular strategy is reconsidered.

The levels of processing approach

It has long been argued that the joint action of stressors has important implications for their classi- fication into various types. Knowing which stresses modify the action of others is essential in construct- ing a model of stress interaction. Part of the difficulty in assembling such a model is that the experiments are themselves rather time-consuming and expen- sive, even if only two values of each stressor are included in a study. But the main difficulty is that the most sophisticated theory in this area, due to Broad- bent (1971) has not been put to direct test. This model incorporated a mechanism whereby perfor- mance was governed by two mechanisms: a "lower" mechanism whose purpose was to formulate and execute well established responses and an "upper" mechanism to regulate and monitor the lower system, to detect and remedy any departures from the opti- mum.

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Within the model, performance can be couched in terms of the change in efficiency of the lower level and the degree to which the upper mechanism con- trols the lower. Several possible scenarios can be envisaged. For example, the upper level can govern the actions of an inefficient lower level which means that efficiency can be sustained, but only if the bur- den of processing is not too great, or if the person has been doing the task for only a relatively short time. Alternatively, the upper level may deteriorate prema- turely, leaving the action of the lower level to be unregulated. For researchers, a major difficulty arises in operationalising these notions in terms of task performance. Thus far, this has been done on a post hoc basis, but one approach from our laboratory shows some promise. It uses the Levels of Control Task, which has been designed to investigate Broadbent's theory (Matthews 1989).

The advances in our understanding of the effects of arousal are less well advanced than the other two areas considered by the review, nevertheless three advances may be summarised: 1) variations in arousal due to the time of day at which the person is tested seem to interact with noise in an inconsistent fashion; 2) arousal effects of noise and prolonged work appear to be qualitatively similar; 3) individual differences in arousal (as measured on the extraversion dimen- sion) in part dictate the preferred level of noise which, in turn, predicts the degree of disruption in perfor- mance by the noise.

IRRELEVANT SPEECH

A range of studies has shown that if speech is heard when a subject reads or remembers verbal material, performance declines markedly. This was first re- ported by Colic and Welsh (1976), but most of the theoretical headway in this area of research has been made by the pioneering work of Salame and Baddeley (e.g., 1987). Here we will use the term "irrelevant speech" in preference to "unattended speech" used by Salame and Baddeley since the latter term appears to prejudge the fate of the speech in the attcntional system. Two main and apparently different areas of action can be identified for irrelevant speech: on memory and on reading.

Effects on memory

Well-established findings. Three key findings can be stated with a good degree of confidence:

1) The effect of irrelevant speech is independent of its intensity. This is true in the range 55 dB(A) to 95 dB(A) (Colle 1980; Jones et al. 1990 ; Salame and

Baddeley 1987). This result has profound repercus- sions upon abatement of speech noise: attenuation of the signal, hitherto found satisfactory for reducing the effects of broad-band noise, will be insufficient, and only total abolition will remove any possibility of disruption.

2) The meaning of speech is unimportant (at least in disrupting short-term memory performance). This point may be demonstrated in a variety of ways. Speech in a language unfamiliar to the listener may be used; effects of irrelevant speech have been shown with German (Colic and Welsh 1976); Arabic (Salame and Baddeley 1982); and Welsh (Jones et al. 1990). Alternatively, speech may be rendered meaningless by employing either nonsense material (Salame and Baddeley 1982) or by playing speech backwards, a procedure which preserves the spectral properties of speech but introduces some novel prosodic features. Results by Jones et al. (1990) show this quite clearly. They compared the effect of English (forward and reversed), Welsh (forward only), and white noise of roughly equivalent level. Of particular note was that all the speech conditions produce a deterioration of the same pattern and extent. This suggests that specific phonological features of the speech arc not important, and that apparently quite dissimilar speech features can produce disruption, a point to which we shall return in due course.

3) The effect of irrelevant speech is one on mem- ory rather than upon attention. Salame and Baddeley (1982) suggested that irrelevant speech interfered with memory at the stage after the words had been converted to some kind of phonological code in mem- ory. Broadbent (1984) was rather sceptical of this view, arguing instead that the competition between the two streams of information was before they en- tered memory. However, the weight of evidence over the last five years has built up in favour of the mem- ory interpretation. Evidence for it takes four forms. First, irrelevant speech does not disrupt tasks in which the memory component is extremely small. For ex- ample, tasks in which subjects were required to clas- sify visually-presented pairs of consonants on the basis of either case or rhyme showed no effect caused by irrelevant speech (Salame and Baddeley 1986). Second, irrelevant speech has a roughly equivalent effect if it is presented either while the visual mate- rial is being shown or when it is being rehearsed in memory. If the effect was one of the perception of material, it should be confined to presentation phase only.

A third line of evidence is based on studies with the Stroop test. Hearing spoken colour words while

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experiencing Stroop interference makes little impact on efficiency (Miles et al. 1989; Miles and Jones 1989). In fairness, some doubt has been expressed as to whether this represents an adequate test of the effects of irrelevant speech on perception. Cowan (1989) has argued that Stroop interference is predom- inantly an output phenomenon and hence less suscep- tible to the effects of irrelevant speech. A fourth point is that the attentional explanation fails to take into account details of the effects of phonological simi- larity, that the disruption by irrelevant speech is re- lated to the phonological similarity between the words which were heard and those which were remembered (Salame and Baddeley 1982). Both points of view predict that phonological similarity is important, but the attentional theory relies on items in the two streams being present at or about the same time. But by virtue of the procedure used in Salame and Baddeley's study, the chances of phonologically similar words (such as "one" and "tun) being present at about the same time as the phonological representation of a visually-pre- sented integers were rather small. Only if we suppose that the phonologically similar words are stored for an appreciable time can we reconcile the findings with a phonological similarity effect. Hence the weight of evidence is in favour of the irrelevant speech effect in memory.

Uncertainties about the locus of the effect. Residual uncertainties surround the nature of the interference in memory and those features of speech which qual- ify it for entry into the memory system. Further work is required to clarify the nature of disruption in mem- ory. Although there is some indication that phonolog- ical similarity is important, a number of puzzling features about its action remain. The first is that evidence from several studies shows that sounds of very different phonology produce effects of a very similar kind. As the data of Jones et al. (1990) show, reversed speech, English, and Welsh all show roughly similar effects on memory, despite the fact that they seem to have quite different phonology from the visually presented material.

One possibility is that in Salame and Baddeley's experiment the contrast of phonological similarity confounded some speech feature. For instance, the dissimilar words were bi-syllabic and similar words were mono-syllabic. Perhaps the important factor is the degree of similarity of the speech envelopes rather than the phonology per se (which comprises temporal and spectral properties of the speech signal). Another possibility is that only the short-term aspects of pho- nological similarity are important. This means that only if the auditory stream consists of words uncon-

nected by either meaning or prosody the effect of phonological similarity will become important. The results of Jones et al. (1990) are based only on irrel- evant speech comprising connected prose spoken with natural intonation and hence their results may only show the effect of manipulating long-term features of speech.

Some details are only just becoming clear about which portion of memory is being disrupted by irrel- evant speech. Roughly speaking, the short-term ver- bal memory system can be conceived as having two main parts. The first is a phonological store in which items are held in phonological form for a short time. This store helps to buffer items into a second articu- latory store. Here items are in articulatory form (that is, in a code which corresponds to the movements of the vocal apparatus required to make a speech sound). It can be demonstrated that the effectiveness of artic- ulatory store is impaired by requiring the subject to say "the" repeatedly (Baddeley et al. 1984). Even when this articulatory suppression technique is used, the phonological store is still available, but only to items which are presented auditorily. They have di- rect access to the phonological store, in contrast to visually presented items which are converted to their phonological form via the working memory system. The main point of interest for noise researchers is which store is disrupted by irrelevant speech?

One set of experiments suggests that the irrelevant speech effect occurs by interfering with phonological storage. Hanley and C. Broadbent (1987) used audi- tory presentation of the list coupled with judicious use of articulatory suppression. They found that even when the capacity of the articulatory loop had been taken up by suppression, irrelevant speech still pro- duced disruption. This implies that the phonological store is responsible for the effect. However, this does not rule out the possibility that some additional disruption of the articulatory loop could take place when presentation is visual. Neither do these results entirely rule out Broadbent's (1984) view that the effect is an attentional one. The task used by Hanley and C. Broadbent (1987) is quite unlike those used conventionally: the subject had to cope with two auditory streams of information, rather than one vi- sual and one auditory. Articulatory suppression adds a third source of auditory interference. It is entirely possible that the locus of interference between two auditory streams is, at a very early stage of process- ing, more properly associated with perception than memory and the results may not generalise readily to the usual irrelevant speech paradigm.

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Characteristics of the auditory filter. Some filter mechanism is responsible for governing which sounds qualify for entry into the working memory system. By manipulating the content of irrelevant speech, future work should clarify the nature of this filter. However, some progress has already been made in this direction. For example, Salame and Baddeley (1987) have shown that the effect of instrumental music is much less marked than music with a vocal content. We may add another qualification to this: a tune when hummed is less disrupting than its sung or spoken counterpart. These results suggest that some feature of speech which is connected with words rather than simply with vocal sounds is responsible for the disruption in memory, such as features that delimit words or specific unvoiced features such as plosives or fricatives which are absent from hummed sound.

These results also suggest that speech itself is not a sufficient condition for disruption. One technique used in our laboratories to show this is to sample a pitch pulse from a human utterance and manipulate it is various ways. We found that, for example, if the pitch pulse is either continuous, periodically inter- rupted, or amplitude-modulated with low-pass noise, there is no effect on memory. However irrelevant speech in the form of a small subset of simple speech sounds based on letters of the alphabet was sufficient to produce the usual disruption (Miles et al. 1988).

Generality of the effect. The susceptibility of work- ing memory to the effects of irrelevant speech sug- gests that a whole range of tasks with which working memory has an intimate connection, such as reason- ing, mental arithmetic and problem solving, will also be vulnerable to disruption (Salame and Baddeley 1982). Evidence is beginning to emerge that semantic processing (in this case checking whether sentences made common sense) and syntactic reasoning (check- ing whether assertions such as "A follows B - BA" are correct) are immune to disruption by continuous meaningless noise, but performance on both types of task declines if the subject is exposed to conglomer- ate noise containing irrelevant speech (Smith 1985). Verbal reasoning has also been shown to be suscep- tible to disruption by irrelevant speech. Dornic et al. (1982) found irrelevant speech at 65 dB to be signif- icantly more disruptive than an 85 dB white noise source on this type of task. Nevertheless, the mental task of counting is immune to the effect of irrelevant speech, a somewhat surprising outcome given the heavy involvement of working memory in this activ- ity (Logic and Baddeley 1987).

Effects on reading

Recent experimentalfindings. The susceptibility of complex mental tasks to disruption by irrelevant speech suggests that reading, with its reliance on memory and other verbal processing functions, may also be at risk. It is already established that intermit- tent office noise and bursts of speech impair reading (Weinstein 1974, 1976) but at least some of this disruption may be due to arousing effects of sound bursts. Evidence of a direct effect upon cognitive processes have emerged from a series of experiments who (Jones et al. 1990) who examined the effects of irrelevant speech on proofreading. They distinguished different types of proofreading error, namely, contex- tual errors which could be detected only by reference to the prevailing meaning of the text, and non-con- textual errors, such as typographical errors, whose detection does not depend on the analysis of mean- ing, and may only be detected by the analysis of the physical features of the words. The results of the experiments showed some similarities with action of irrelevant speech on short-term memory but also they exhibit some marked differences.

Like the results of memory studies, the effect is independent of intensity [at least in the range 50 to 70 dB(A)]. Some of the other results were not straight- forward, however. The effect of irrelevant speech was sensitive to meaning, with meaningful speech being more disruptive than meaningless speech. Moreover, the loss of efficiency was confined to non-contextual errors. These findings are appar- ently paradoxical. Meaning gives rise to the disrup- tion, but its effect is manifested in the detection of errors which do not have meaning as their basis. One way of reconciling these facts is to suppose that the analysis of graphemic features is of relatively low priority and that the intrusion of meaningful speech makes the task of analysing meaning more difficult. Hence, the lower priority analysis of graphemic fea- tures is neglected. In turn, this suggests that when the text is less meaningful, or if the reader is prevented from extracting meaning from the text, the effect of irrelevant speech will be diminished. Evidence for this view was obtained by Jones et al. (1990) in a study which compared two displays, one showing one line of text and the other showing five lines of text. It was argued that meaning was more readily ex- tracted from the display with five lines. Meaningful irrelevant speech produced a deterioration in the de- tection of contextual errors in the five line version only, a result which confirms the idea that the effect seems to depend on the level of meaning which can be extracted from the text.

Human performance in noise 455

A parallel discovery was made by Martin et al. (1988) using a slightly different technique. They as- sessed text comprehension, using a task interpo- lated between learning and recall to minimise the effects of short-term memory. First, they estab- lished that irrelevant speech, but not music, inter- fered with comprehension of prose passages. Second, they established that the detrimental effect of irrele- vant speech was due to its meaning rather than to its phonological properties. By showing that both non- words and speech in Russian were less effective than either narrative speech or word lists, the results again suggest the importance of meaning and the small part played by similarities in the phonology between the visual and auditory streams.

Practical implications. The work on irrelevant speech has extremely important practical implications. Wherever speech is likely to accompany reading, there is likely to be loss of efficiency. The number of such settings is very great and, importantly, with the burgeoning of what has been referred to as "the in- formation age," their incidence is likely to increase. In many cases the effect on efficiency will be marked, and steps should be taken toward abatement. There will be special groups for which this will be particu- larly important. For instance, the effect of irrelevant speech on the learning and development of reading in children is a topic which the research community should regard with high priority. That younger chil- dren are more distracted by auditory stimuli such as music is by now established (Higgins and Turnure 1984). However, systematic studies of the effects of irrelevant speech are few and, because of difficulties of procedure or interpretation, do not provide any firm conclusions (Baker and Mandell 1965; Collins- Eiland et al. 1986; Zimmer and Brachulis-Raymond 1978). From what is known of the action of irrelevant speech in adults, it is not too daring to speculate that its effects on children will be nothing short of dra- matic.

In settings where efficiency is paramount, such as flight decks, command and control systems, in pro- cess control applications, and so forth, judicial man- agement of low-priority auditory input may be a remedy, especially if the sound is relayed by elec- tronic means.

Perhaps the most alarming outcome of the research is that, in the abatement of irrelevant speech, mere attenuation by the order of 10 dB may not be suffi- cient. The insensitivity of reading and short-term memory effects to intensity point to the fact that attenuation of several 0.1 dB will have to be achieved. But the importance of the results are not just con-

fined to the effects of noise on task performance. They are instructive also for the further understand- ing of annoyance, specifically, those mechanisms re- sponsible for allowing the privileged entry of speech to working memory, also govern access to our consciousness and, in turn, the intrusive quality of speech. Hence, those factors which are responsible for interference with memory may also govern the degree of annoyance generated by background speech. Studies of the annoyance generated by a range of office noises and other physical factors place the intrusiveness of conversation in a position of preem- inence (Brookes 1972; Nemececk and Grandjean 1973).

The major challenge for researchers in this area is to establish which features of speech give it its intru- sive quality. This is undoubtedly a major undertaking and it is problematic to know where to begin the search. However, several clues to suggest the factors that govern entry could emerge from considering the evolutionary importance of certain speech features (Lieberman 1984, for a general discussion). In addi- tion, information processing analyses of speech per- ception may yield valuable evidence (Broadbent 1975, for an analysis).

Great progress has been made in understanding the effects of irrelevant speech, and, although several issues still need to be resolved, we may summarise the main findings as follows: 1) the effects of inten- sity of speech is unimportant, both for proof reading and memory. This means that roughly the same de- gree of interference will occur whether the speech is played loud or soft; 2) the effects on reading are qualitatively different from those on memory; the effect on reading is primarily a function of the mean- ing of the speech; 3) in a memory task, speech pro- duces its effect not when the material enters the information processing system, but rather when the material is stored in a phonological form; 4) speech is a necessary but not sufficient condition for disrupt- ing memory; 5) vocal music has a greater disruptive effect than instrumental music, but this is only true if the vocal component is not hummed. Only if the words are properly articulated will the irrelevant speech have an effect.

EMERGING ISSUES

The stage is set for some important advances in the study of human performance in noise. A number of approaches are converging, in particular those in- volving strategic explanations and ones involving arousal. The work on irrelevant speech seems to be in quite a separate category, largely devoid of arousal and strategy effects. Much needs to be done in under-

456 D. Jones

standing irrelevant speech, particularly in field stud- ies validating those effects found in the laboratory. In summary, the main areas of concern for the future are as follows:

The study of strategies in non-verbal tasks

Little is understood of how less complex tasks, particularly those that do not employ verbal materi- als, are disrupted by noise. Vigilance tasks are an example of this class. But much needs to be done by way of furthering our understanding of strategies within vigilance tasks before any systematic work can be done with noise.

Arousal and strategy selection

The ways in which arousal determines the choice and persistence of a strategy need to be more clearly understood. Available evidence indicates that the over- all context of instruction from the experimenter, pre- vious experience of the task and the person's state determine the level of performance. Some progress has already been made in this regard by showing that the presence of noise can alter the costs associated with certain courses of action. Extensions of this work focussing on the choice of action over a greater range of tasks now needs to be undertaken.

Individual differences in strategy selection

The choice of action in a particular setting is partly determined by the person's predilection for certain states and actions. Again, it would seem advanta- geous to adopt the strategic approach to understand- ing performance. The work already undertaken on the role of individual differences in establishing prefer- ences for noise level should be extended to include a study of noise types. This work needs to be under- taken in conjunction with studies of strategy choice in complex information processing tasks.

Studies of the interaction between stresses

There needs to be a very much greater database from which to assemble a general picture of interac- tions between noise and other stresses. In logistic terms, the programme of work in this area is indeed formidable, but it should go hand in hand with theo- retical developments, along the lines of the levels- of-control approach (Broadbent 1971).

The nature of the auditory filter

Among the more important of the emerging themes is the discovery of those features of speech which give it privileged access to higher levels of mental activity. Some work on irrelevant speech has already

made headway in this respect, but further detail is required, particularly on the effects of the phonolog- ical similarity between visual and auditory streams of information. Extensions of this work are important to understanding both the effects on processing ac- tivities, such as memory or reading, and why some sounds are more intrusive or annoying than others.

Irrelevant speech and reading

Results from laboratory studies have suggested that reading is susceptible to disruption by irrelevant speech, and the description of the mechanisms re- sponsible for such an effect is now becoming clear. Two topics need further consideration. First, a pro- gramme of studies should seek to establish whether children's reading is slowed by the presence of speech, and more especially, whether the development of their reading skills is delayed by exposure to irrele- vant speech in open-plan classrooms. Second, the degree to which irrelevant speech interferes with performance of tasks in offices should be assessed. In particular, field studies should seek to establish confirmation of trends found in the laboratory in an attempt to judge the seriousness of the disruption in personal and economic terms.

The analysis of performance effects of noise con- tinues to be a popular focus of research interest. In theoretical terms, the central concerns of the area have changed very markedly in the last ten or fifteen years. The practical concerns have also changed, they are now directed toward meeting the challenge, not j u st of broadband noise in simple repetitive tasks, but of understanding the intrusive quality of verbal noise on a range of intellectually demanding tasks. These sounds and tasks have special relevance to present day work environments.

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