Ecological Psychoacoustics 4.pdf

their systems would compete with another system for the attentional resources ofthe listener (not to mention the sound output capabilities of the computers soundcard). Thus, the environment can influence both simple perception and more cog-nitive elements of the task, and researchers need to investigate this in the contextof auditory displays.

HOW PARAMETERS INTERACT WITHIN A SOUND

Pollack and Ficks (1954) found that displays using multiple parameters ofsound generally outperformed unidimensional displays measured elsewhere.Their research also indicated that the subdivision of display dimensions into finerlevels does not improve information transmission as much as increasing thenumber of display dimensions. Kramer (1996) has suggested multiple mappingsof data, whereby one series of data is used to change two or more parameters ofthe sound. For example, temperature might drive pitch, tempo, and brightness.How the resulting auditory display is perceived, and the meaning that would becommunicated, remains to be tested rigorously. We should point out the work ofHellier, Edworthy, and their colleagues, in which the perceived urgency of analarm was varied by adjusting the sound attributes (e.g., Edworthy, Loxley, &Dennis, 1991; see also Guillaume, Drake, Rivenez, Pellieux, & Chastres, 2002;Hellier, Edworthy, & Dennis, 1993). Adjusting more than one sound attribute(e.g., frequency and amplitude envelope) had a greater effect than achieved byadjusting just one attribute. All this leads to clear (and still unanswered) ques-tions about which sound dimensions to involve in a display and how to use themalone or in combination to evoke specific perceptual or conceptual responses.

LIMITATIONS IN TASKS AND COGNITIVE DEMANDS

The tasks participants are asked to do in traditional psychoacoustical researchare usually highly contrived and have limited external validity. There are oftenvery few cognitive requirements in the task. For example, detecting a signal soundamid a background masker is instructive for listening researchers but is of narrowutility to auditory display research. A real-world detection task would probablyinvolve several other cognitive elements, often including interpreting the meaningin the sounds and then deciding on an appropriate course of action as a result.Cognitive load and distraction are typically achieved through added sounds orsecondary tasks such as remembering a number or counting backward, whichhave little to do with the complex perception and cognition tasks involved in audi-tory display perception. Simple alerts and warnings might be less affected by taskdemand increases, but, as pointed out by Smith and Walker (2002, p. 365), eventhe basic task of determining the price of a stock at a given time of day caninvolve the following perceptual and cognitive subtasks: When given theopening [stock] price and asked to report the price at a given time (noon, forexample), the subject must listen to the entire graph, recall the pitch he or she

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perceived at approximately half the duration (noon time), compare it to the pitchperceived at the very onset of the graph (the opening of trading), estimate thechange in price represented by the difference between the noon-time pitch rela-tive to the opening pitch, and add or subtract that change in price to the openingprice of the stock. The cognitive demands of traditional psychoacoustics exper-iments do not even approach this level of complexity.

METAPHORS AND CONCEPTUAL MAPPINGS

The relationships between underlying data and their acoustical representationsneed not be arbitrary. At the metaphorical and symbolic end of the auditorydisplay spectrum, data are represented by sounds that often have little or no appar-ent relation to the sound. For example, pitch may be used to represent the volumeof product sales; this is an arbitrary and purely analogic mapping. It is possible,though, that particular sound attributes are simply better suited to display partic-ular data types (Barrass, 1997; B. N. Walker, 2000, 2002). A categorical sounddimension such as timbre (i.e., distinct musical instruments) might be well suitedto convey a categorical data distinction, such as different product lines (e.g., theLos Angeles offices hardware sales being played by a piano and the softwaresales being played by a trumpet). That same dimension of timbre (or instrument)would be less suited to convey the variations in a continuous variable, such assales volume. That is, we could have a sonification using the mappings {product:instrument, dollars: pitch}, but not {product: pitch, dollars: instrument}. Second,there may be a preferred polarity to the mapping. The intended listening popula-tion may agree (or, at least, there may be a majority) that increasing frequencyshould be used to represent increasing sales volume. The issues surrounding suchmapping choices have been discussed to some extent in the design literature,notably by Bertin (see also Barrass, 1997; 1981); however, this has not been atopic where the traditional psychoacoustics community has ventured (but seemore on this topic subsequently). Mapping is crucial, though, for auditory displaydesigners in a practical setting where decisions must be made, so it is encourag-ing that new approaches have been made (Edworthy et al., 1991; B. N. Walker,2000, 2002).

RIGID EXPERIMENTAL METHODS AND LANGUAGE

The psychoacoustics research field has provided many useful approaches andexperimentation techniques. This and the means to communicate the results ofsuch experiments have been listed here already as beneficial contributions.However, it is a double-edged sword: The methodological rigor and experimen-tal purity generally required in a psychophysical experiment may not berequired, or even possible, in some auditory display research. It may be impos-sible, for example, to isolate rigorously a single independent variable in a soni-fication experiment involving multidimensional data represented by a highly

Ecological Psychoacoustics and Auditory Displays 165Copyright 2004. Elsevier Academic Press. All rights reserved.




multivariate sound. In some cases, such as assessing the effectiveness of a par-ticular alarm, more qualitative methods, which can still be based on deep scien-tific foundations, may be appropriate. It is simply important for auditory displaydesigners and researchers to cast their net wide in the search for useful and appro-priate ways to evaluate their designs. Language presents further problems. It issometimes limiting to feel compelled to describe sounds in the somewhat sterileterms of frequency and envelope rather than with more descriptive terms such aswarmth, raspiness, or buzziness, especially if the participants in a study use thoseterms. It seems somehow less scientific to write more boingy, even if that cap-tures the phenomenological experience. This tension between science and, well,art is not unique to auditory displays. Not infrequently, visual display designersstruggle between the scientific world of terms such as visual angle, informationdensity, and contrast sensitivity functions and more vernacular terms such as busyand cluttered. Auditory display designers need to be conversant in both forms ofcommunication and need to feel comfortable using both traditional and nontra-ditional methods of testing and evaluation. It may be less traditional, but it neednot be less scientific.

WHAT HAS RECENT RESEARCH INTOHIGHER LEVEL PHENOMENA OFFERED TO

AUDITORY DISPLAY?

GIBSONIAN THINKING IN GENERAL

The first key contribution to auditory display from recent ecologicalapproaches to perception and acoustics has been the general acceptance of theneed to explore complex acoustic and perceptual phenomena and find new andappropriate means to do so. The possibilities have grown for conducting studiesthat do not hold all but one variable strictly constant, that are not necessarily performed in acoustically pure listening environments, and that involve more cognitive and even ill-defined tasks. This makes it feasible to study auditorydisplay design and usage in realistic conditions, raising the validity and utility ofthe findings.

COMPLEX, DYNAMIC (ECOLOGICAL) SOUNDSStudies have begun to look at sounds that change dynamically within a trial.

Walker and Kramer (1996b) took a fairly classic approach, employing reactiontime and accuracy measures to study a fictional crystal factory sonification.However, the sounds were very dynamic, with one of several possible soundparameters varying continuously on each trial. Further, the experiment involveda fairly complex monitoring task, where the listener had to interpret the problemwith the factory and make a corrective action. Thus, the participants were lis-





tening for pressure or temperature changes and not pitch or loudness.This type of conceptual listening, and the inclusion of dynamic sounds, is notrepresentative of traditional psychoacoustics studies but it certainly is relevantfor auditory display tasks.

Walker and Ehrenstein (2000) also examined reaction times, accuracy, andstimulusresponse (S-R) compatibility effects involving dynamic sounds thatchanged in pitch during a trial. The findings that pitch change can be considereda dimension (in addition to pitch itself), and that S-R compatibility effects canarise from it, pointed out the need for designers of auditory displays to considernot only the perceptual issues resulting from their sounds but also the responserequirements.

Neuhoff and McBeath (1996) have also examined perception of dynamicsounds, notably those that change in pitch and loudness as if they are movingtoward or away from the listener. The implications for train warning horns areclear; but the results more generally point out the over- and underestimation ofpitch and loudness that occur in these kinds of multidimensional sounds. Forproper calibration and maximum comprehension, auditory display designers needto consider these findings with dynamic sounds.

More realistic sounds are being used in otherwise traditional studies, as well.For example, Bonebright (2001) used multidimensional scaling techniques tostudy perception and categorization of environmental sounds. The results can helpauditory display designers anticipate the conceptual groupings that will happenwhen such sounds are used in the interface, as with earcons, or auditory icons.Ballas (1994) and Gaver (1994) have also investigated the use of real-worldsounds. These studies allow a better understanding of the recognition of, memoryfor, and meanings attributed to, common environmental sounds when used in anauditory display. In some cases, studying these reality-based sounds can alsoallow the designer to extract the salient features and thereby use parameter-basedsynthesis models to create auditory icons that can have more in common (acousti-cally) with the objects they represent. Thus, the study of realistic sounds in somecases allows auditory designers to use those with a better level of understandingand in some cases allows designers to create artificial sounds that are more efficient but equally compelling and useful.

ACOUSTIC ECOLOGIES FOR OVERALL DESIGN

As the use of sound in humanmachine interfaces increases, it is even moreimportant to know how individual sounds will blend in or stand out from the growing acoustic crowd. However, it is also important to take into consider-ation the whole sound environment and design a total auditory display environ-ment that supports the tasks as well as the persons in that environment. Thesedesign issues have been considered, although perhaps still less rigorously than they might, by the auditory perception community. For example, Walker and Kramer (1996a) point out that the nature of the humansystem interaction





(concert versus conversation modes) can inform the auditory display needs. They recommend that auditory (and visual) displays be designed to beadaptive to the interaction style. Some auditory display designers are now considering these issues in their designs, under the aegis of contextual comput-ing (e.g., Nagel, Kidd, OConnell, Dey, & Abowd, 2001), and are developingdisplays that adapt to the location, activities, and preferences of the intended listener.

METAPHORS AND CONCEPTUAL MAPPINGS

Traditional psychoacoustics has long been concerned with the issue of howlisteners perceptions of a sound (e.g., pitch) compare with the sounds physicalparameter (e.g., frequency). Psychophysical scaling functions have been deter-mined for all kinds of sound dimensions (e.g., Stevens, 1975). As auditory dis-plays fill out more of the conceptual and symbolic end of the spectrum, it iscrucial to know how listeners are interpreting the sounds, not just how they perceive them. Walker has been using magnitude estimation to determine psychophysical scaling functions between sound dimensions (frequency, tempo,brightness) and conceptual data dimensions, such as pressure, temperature, andnumber of dollars (B. N. Walker, 2000, 2002). It turns out that the best mappingsdepend not only on what the sound is but also what the data dimension is. Forexample, if frequency is used to represent temperature, the preferred polarity ispositive. That is, increasing frequency represents increasing temperature.However, increasing frequency best represents decreasing size (B. N. Walker,2000, 2002). In addition to the findings regarding polarity, this research showsthat the slope of the magnitude estimation graph can be different for each datadimension. For example, a given change in frequency may best indicate a certainpercentage change in temperature but a different change in pressure or numberof dollars. Both the polarity preferences and scaling factors are now being usedto create auditory displays that conform better to population expectancies (Smith& Walker, 2002; B. N. Walker, 2002).

One additional contribution from this line of research has been the finding thatdifferent listener populations can interpret auditory displays differently. Mostnotably, Walker and Lane (2001) found that visually impaired listeners andsighted listeners used different mental models to interpret the sounds they heard.For example, sighted undergraduates preferred the positive polarity mapping forfrequency: number of dollars. That is, a higher frequency meant a greater numberof dollars. For the blind listeners, increasing frequency unanimously meant fewerdollars. The study points out that different listening experience can affect howwe interpret the sounds we hear. These results have practical utility for auditorydisplay designers in that the designers can use different mappings for blind lis-teners. However, the research is probably just as important for its philosophi-cal contribution, pointing out that designers really do need to know their usersat a more cognitive level than had been addressed in traditional studies that





generally diminished the importance of learning and experience for interpreta-tion of sounds.

ALTERNATIVE EXPERIMENTAL METHODS

The general trend has been for researchers interested in auditory displays touse somewhat traditional methods (psychophysical scaling, magnitude estima-tion, multidimensional scaling) but employ more dynamic sounds, nontraditionallistening environments, or more cognitive tasks. However, there have been somenew experimental approaches, such as the ventriloquism effect for multimodaldisplays (A. Walker & Brewster, 2001), audio descriptive analysis for spatialsound displays (Zacharov & Koivuniemi, 2001), and the use of auditory displaysas part of the experimental task (e.g., a crystal factory, B. N. Walker & Kramer,1996b). Traditional experimental approaches would not have been very wellsuited to uncovering mental models, and previous applications might not haveeven cared much about that. Auditory displays require a new game plan.

WHERE ARE WE NOW AND WHERE DO WENEED TO GO VIS--VIS AUDITORY DISPLAY?

Research in auditory perception has progressed from the study of individualauditory dimensions, such as pitch, tempo, loudness, and localization, to the studyof more complex phenomena, such as auditory streaming, dynamic sound per-ception, auditory attention, and multimodal displays. The main research areas thatwill continue to drive sonification research forward include (1) understandingdynamic sound perception; (2) investigating auditory streaming; (3) cognitiveissues in the relationship between conceptual (data) and auditory (display) fea-tures; (4) determining how meaning is attributed to sounds, based on the sounds,the listener, and the task requirements; and (5) understanding the design and useof multimodal sonification.

Other pertinent research includes investigation of the normal variation in per-ceptual and cognitive abilities and strategies in the human population; differencesin cognitive representations of auditory displays for selected populations, such assighted and blind individuals or different cultural groups; and the role of learn-ing and familiarity in display efficacy. Study of these issues is necessary for theformulation of design guidelines for constructing efficient multivariate data displays using sounds.

Substantial questions remain in the area of multimodal perception. It is clearfrom existing research that our senses differ in their underlying abilities, butfurther research is necessary to identify what data features are perceptually most salient to each sense and to determine how to use this knowledge in design-ing effective displays. Multimodal interactions (e.g., between visual and auditorydisplays) are poorly understood, yet they critically affect most sonification





applications. When does redundant presentation (that is, presenting informationin more than one modality) improve the ability to extract data (i.e., cross-modalsynergy)? When does information presented in one modality interfere with the perception of information in another modality (i.e., cross-modal interference)?How can the total amount of information perceived across all modalities be maximized?

SYNERGIES BETWEEN AUDITORY DISPLAY ANDECOLOGICAL PSYCHOACOUSTICS RESEARCH

Many of the questions presented here regarding ecological psychoacoustics,and other questions as yet unformulated, may be best approached via the field ofauditory display. That is, abstract questions regarding auditory perception suchas How well can a person discriminate these three auditory variables? may bemore effectively approached by practical questions such as Can users of thisthree-variable display perform a task which involves discerning the relativevalues of each of the three data dimensions being represented? That is, the per-ceptual questions are addressed via a sonification task, thereby ensuring that theresults will be clearly relevant to auditory displays. At the same time, the per-ceptual questions will yield findings that also have broad theoretical relevance.

It also seems likely that auditory display research will present new questionsand perspectives to ecological psychoacoustics researchers. Display designs maypresent novel cognitive tasks, such as cross-modal pattern detection or discern-ing trends in highly multivariate displays. Fortunately, the displays presentingthese perceptual quandaries will often also provide the experimental tools fortheir exploration. Thus, a synergy is created wherein auditory display researchpushes the boundaries of ecological psychoacoustics, and ecological psychoa-coustics informs and validates auditory display research. Both fields are thusbroadened, refined, and brought to greater practical application.

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7Environmental

Acoustics:Psychological

Assessment of Noise

Seiichiro Namba and Sonoko Kuwano

From the viewpoint of environmental psychology, especially environmentalacoustics, noise is one of the important social problems. Noise is defined asunwanted sounds and brings us various adverse effects. When we are exposedto very intensive noise for some period, our hearing will be damaged. Perhapsneedless to say, exposure to such intensive noise should be avoided. On the otherhand, even if the noise is not so intensive, it may cause serious effects such asspeech interference, sleep disturbance, effect on mental tasks, and annoyance.The noise problems occur on the basis of our perception of the sound. It is impor-tant to find a method for controlling the sounds in order to reduce the effect ofnoise. There are two aspects in the subjective assessment of noise.

The first is to find the physical metrics that show good correspondence withsubjective impressions. Here, psychophysical measurement can be used as a basicmethod to find the law between physical values and subjective judgments. It isimportant to examine the validity of the law found in experiments in order todetermine the applicability of the results to practical situations. A robust law isdesirable.

The second is to measure the degree of disturbance effect of noise and deter-mine the permissible level of noise and/or noise criteria. A rating scale or cate-gory scale is often used in order to measure annoyance. It must be carefullyconsidered that the frame of reference has a great effect on the judgment using acategory scale. In the case of annoyance judgment, the frame of referenceincludes both stimulus context and social background.

175Ecological PsychoacousticsCopyright 2004 by Academic Press.

All rights of reproduction in any form reserved.

Copyright 2004. Elsevier Academic Press. All rights reserved.




In this chapter, the effect of the social and psychological frame of referenceas well as the physical stimulus context on the assessment of environmental noiseis discussed on the basis of the results from our laboratory experiments and cross-cultural studies.

PSYCHOPHYSICAL METHODS AND PHYSICALMETRICS FOR THE EVALUATION OF NOISE

Various metrics for evaluating the effect of noise have been proposed (Archi-tectural Institute of Japan, 1981). Because the metrics are used for the assessmentof and/or finding countermeasures to noise, they are required to show good correspondence with subjective impressions. Several kinds of psychophysicalmethods are often used to find good metrics for noise.

The authors have conducted a series of experiments using artificial and actualnoises and found that LAeq (equivalent continuous A-weighted sound pressurelevel, i.e., A-weighted mean energy level) shows a good correspondence withsubjective impressions. Examples are shown in Figs. 14. The results of 11 exper-iments are plotted together in Fig. 1 (Namba, Kuwano, & Kato, 1978). Each ofthe experiments was conducted independently with each sound source. Themethod of adjustment and magnitude estimation were used in these experiments.It was found that loudness shows good correspondence with the mean energylevel.

In the experiment shown in Fig. 2 (Namba & Kuwano, 1984), various sounds,such as aircraft noise, train noise, and road traffic noise, were included in a stim-ulus context and presented in random order. Their loudness was judged usingmagnitude estimation. Points of subjective equality (PSE) were calculated usinga power function of road traffic noise as a reference. It can be seen that LAeq is agood measure of the loudness of various sounds as the first approximation.

The Environmental Quality Standard of Noise was revised in Japan in September 1998 and issued in April 1999, adopting the mean energy level (equiv-alent continuous A-weighted sound pressure level; LAeq) as the index of noise cri-teria.

The median of level fluctuation (L50) was used in the former EnvironmentalQuality Standard in Japan. In order to examine the applicability of L50, the loud-ness of artificial level-fluctuating sounds was obtained using the method of adjust-ment (Namba, Nakamura, & Kuwano, 1972). The result is shown in Fig. 3. Itwas found that the loudness increases according to the mean energy levelalthough the value of the median (L50) was kept constant. It was confirmed thatthe mean energy level is a good measure of loudness. On the other hand, it isclear that loudness does not correspond with L50.

Similar results were found when subjective impression was judged using themethod of adjustment and category scale (Namba & Kuwano, 1991) as shown inFigs. 4 and 5. There is a good correspondence between the logarithmic scale of





Environmental Acoustics 177level-fluctuaiting sound (dB SPL); 1968, 1971,1972non-steady state sound (dB SPL); 1976impulsive sound (dB SPL);1976sound with rise time (dB SPL);1974road traffic simulating noise (adjustment, dB A); 1978road traffic simulating noise (ME, dB A); 1978road traffic simulating noise (dB A); 1978speech (dB A); 1978music (orchestra, dB SPL); 1978music (piano, dB A); 1977non-steady state sound (dB SPL); 1981

r = .979

N = 280RMS = 1.45

Mean energy level40 50 60 70 80 90

40

50

60

70

80

90

dB

dB

PSE

FIGURE 1 Relation between loudness and mean energy level. All 280 data points of our 11experiments are plotted together. A high correlation can be seen between them. (From Namba et al.,1978; Namba & Kuwano, 1984, Fig. 1, p. 136.)

aircraft noisesuper express train noisetrain noiseroad traffic noisespeechmusicimpulsive noiseartificial level-fluctuating noisesteady state noise

r = .973RMS = 4.40

Leq40 50 60 70 80 90

40

50

60

70

80

90

dB A

dB A

PSE

FIGURE 2 Relation between LAeq and loudness for nine kinds of sound sources judged in thesame context. PSEs were calculated from magnitude estimation. (From Namba & Kuwano, 1984, Fig.4, p. 141.)





XX

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Mean

Subject ABCD

Lin

Lso

X

SeriesFIGURE 3 Loudness of level-fluctuating sounds in relation to L50 and mean energy level.Although the value of L50 was kept constant, loudness increased as the mean energy level increased.(From Namba et al., 1972, Fig. 1, p. 253.)

90

80

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60

50

40

30

r = 0.971RMS = 5.04

30 40 50 60 70 80 90

PSE

(dB A)

LAeq [dB A]FIGURE 4 Relation between LAeq and loudness. PSEs were obtained using the method ofadjustment. The results were similar to those in Fig. 2. (From Namba & Kuwano, 1991.)Copyright 2004. Elsevier Academic Press. All rights reserved.




magnitude estimation and the linear scale of category scale as shown in Fig. 6.The fact that the same results were found with different methods suggests thatthe results of the measurements are valid.

The law of mean energy level is very robust and is applicable to various kindsof level-fluctuating sounds, except for impulsive sounds. In the case of impulsivesounds with duration shorter than about 1 second, the total energy of the sound,sound exposure level, LAE, is a good measure of loudness (Kuwano, Namba, &Kato, 1978; Kuwano, Namba, Miura, & Tachibana, 1987).

STUDIES OF NOISE CRITERIA

CATEGORY SCALE

When the permissible level of noise is determined, the responses are usuallyobtained using category judgments and the doseresponse relation between thepercentages of highly annoyed people and LAeq is often used (e.g., Schultz, 1978).However, it is well known that there is a context effect in the judgments usingthe category scale as shown in Fig. 7. It is not clear whether the context effect isreally a perceptual phenomenon or a kind of artifact by assignment of categories.The fact that a context effect was found even when reaction time (Namba,Yoshikawa, & Kuwano, 1968) and evoked potentials (Kuwano, Shimokochi, &

Environmental Acoustics 179

7

6

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3

2

1

r = 0.981

30 40 50 60 70 80 90 100

cate

gory

sca

le

LAeq [dB A]FIGURE 5 Relation between LAeq and loudness. Loudness was judged using a category scale.(From Namba & Kuwano, 1991.)





180 Ecological Psychoacoustics

1 2 3 4 5 6 70

0.5

1.0

1.5

category scale

M E

r = 0.980

FIGURE 6 Relation between loudness obtained from magnitude estimation (ME) and a cate-gory scale. (From Namba & Kuwano, 1991.)

Series 1 (50-75 dB)2 (60-85 dB)3 (70-95 dB)4 (45-95 dB)

45 50 55 60 65 70 75 80 85 90 95Sound pressure level in dB

1

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FIGURE 7 Context effect found in the judgment of the noisiness of train noise. (From Nambaet al., 1968, Fig. 1, p. 194.)





Yamaguchi, 1969) were used suggests that the context effect may exist reflectingthe perceptual phenomenon. On the other hand, it cannot be denied that subjectsmay possibly assign the given categories to the whole stimulus range, as therange-frequency model proposed by Parducci (1963; 1974) predicted. Whensuch an assignment is made, the judgment does not reflect the perception but maybe a kind of ordering. This must be avoided, especially when the permissible levelof noise is considered.

CATEGORY SCALE (ONE JUDGMENT BY ONE SUBJECT)If subjects do not know the stimulus context, the context effect does not appear

(Stevens & Poulton, 1956). In our experiment, in which the permissible level ofair-conditioner noise was examined (Kuwano & Namba, 1985a), subjects judgedthe noisiness of air-conditioner noise after an exposure for 90 minutes duringwhich they were devoted to mental tasks. Each subject judged the air-conditionernoise only once. Sometimes they reported that they did not notice the sound. Itwas suggested from this experiment that the permissible level of air-conditionernoise was about 45dBA. The context effect can be avoided by this procedure,but because there are individual differences in the judgment, many subjects areneeded.

CATEGORY SPLITTING PROCEDURE

Heller (1985) proposed the category splitting procedure. The sound is pre-sented twice. When the sound is presented first, subjects are asked to judge thesound in five categories: very soft, soft, medium, loud, very loud.When the sound is presented the second time, they judge it by subdividing thecategory they selected into 10 steps for finer grading. This means that the judg-ment is divided into 50 categories finally. It is reported that an absolute judgmentof loudness can be obtained by this procedure. It seems that by splitting the judg-ments into two steps it would be difficult to assign the sounds and absolute judg-ment can be obtained compared with conventional category judgment. Thismethod is used especially for hearing aid fitting, which requires absolute loud-ness judgment with high accuracy. Standardizing this method for audiologicalpurposes is now being discussed in an International Organization for Standard-ization (ISO) working group (ISO TC43/WG 7).

FREQUENCY OF THE RESPONSEMany social surveys of noise problems have been conducted in various places

in the world. They are usually conducted independently, and their results cannotbe compared with each other. If they could be compared with each other, morevaluable information would be obtained. A committee of the Acoustical Societyof Japan (Namba et al., 1996) has proposed the fundamental items that should

Environmental Acoustics 181Copyright 2004. Elsevier Academic Press. All rights reserved.




be included in the social surveys on noise problems so that the social surveys canbe compared. Category scale is often used in social surveys. After a great deal ofdiscussion in the committee about whether the category scale should be used, itwas decided not to use the category scale. This is partly because the word thatindicates the degreee.g., very, fairly, a littleis vague. Instead of the categoryscale, a filter question about whether a given sound is audible or not and a ques-tion about whether the sound is annoying or not were adopted. This type of ques-tion is fairly free from the problems involved in the category scale, and it wasfound that the percentage of annoyed people can be used as an index fordoseresponse relations as shown in Fig. 8 (Kuwano, Namba, & Florentine,1997). This figure shows a good correspondence between the frequency of theresponses and LAeq as usually found in category judgment (Fig. 9). At present, thefundamental items for social surveys are being discussed again by the Commit-tee of the Acoustical Society of Japan. The idea of having common items thatshould be used in social surveys on noise problems has also been discussed inter-nationally in the ISO (ISO/TS 15666:2003), and the International Commissionon Biological Effects of Noise (ICBEN).


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sleep distubance

; aircraft noise, ; train noise, ; road traffic noise

r = .980

%

FIGURE 8 Relation between LAeq and the percentages of the respondents who were disturbedby the noise. (From Kuwano et al., 1997, Fig. 2, p. 770.)





CONTINUOUS JUDGMENTS BY CATEGORY

When a vehicle is approaching, we feel that the sound is becoming louder andlouder. When it is leaving, we feel that the sound is becoming softer and softer.It is impossible to obtain this kind of continuous judgment by conventional psy-chological methods. Namba and Kuwano et al. (e.g., Kuwano & Namba, 1985b;Namba & Kuwano, 1980) have developed a method called the method of con-tinuous judgment by category in order to obtain instantaneous judgment continuously. Valuable information can be obtained by relating instantaneousjudgments and instantaneous physical properties of the sounds and by relatinginstantaneous judgments and the overall impression of a whole range of thesounds. In an experiment using the method of continuous judgment by category,subjects are asked to judge the instantaneous impression of the sound using sevencategories and to press a key on a computer keyboard corresponding to theirimpression at that time. Subjects need not respond if their impression does notchange. When they press a key, it is presented on a monitor and the impressionregistered on the monitor is considered to remain. When their impression changes,they are required to press the corresponding key. This method is free from assign-ment of numbers to the sounds because subjects are not compelled to respond allthe time.

Using the method of continuous judgment by category, many experimentshave been conducted (e.g., Kuwano, 1996). Some experiments were conductedoutdoors in which subjects were asked to judge the loudness of the sounds thatwere audible in the surroundings (Kaku, Kato, Kuwano, & Namba, 1997, 1999;


1

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5

r = 0.973

loud

ness

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LAeqFIGURE 9 Relation between LAeq and loudness judged on a five-category scale.





Kuwano, Kaku, Kato, & Namba, 1998). There were some varieties in the rangeof the sounds. The results for eight locations are plotted together in Fig. 10. Thelowest stimulus range was from 49 to 57dB (L7 in Fig. 10) and the highest was from 55 to 74dB (L5 in Fig. 10). Although there is a slight difference injudgment according to the stimulus range, it is much smaller than that found in Fig. 7.

In an experiment using the method of continuous judgment by category,assignment can be avoided and there is little end effect. This may be because sub-jects are not required to respond to all the stimuli. Subjects have to respond onlywhen their impression of the sound changes. Even if the stimulus changes phys-ically, subjects do not always have to respond to the change. This causes littlerange effect.

In addition to having little range effect, the method of continuous judgmentby category has other merits as follows: (1) it is easy for subjects to judge thestimulus, (2) judgments can be made in a situation similar to that of daily life,(3) subjects judge the sound in a stream and do not pay special attention to anyspecific stimulus, (4) the method can be applied for a long sound, and (5) variousanalyses can be done using the data stored in a computer.

Simple doseresponse relationships can be measured with any psychophysi-cal methods and scaling methods. However, in the decision about a permissiblelevel, it is necessary to find the absolute degree of the effect of noise, such as thatnoise is intolerable, serious, endurable, not annoying at all, and so on (Schick,1979, 1981). For this purpose the category scale is often used. An attempt should


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aneo

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dgme

nt

FIGURE 10 Relation between LAeq and loudness judged at eight locations outside. Althoughthere is a slight difference in the judgment according to the stimulus range, it is much smaller thanthe context effect shown in Fig. 7. (From Kuwano et al., 1998.)





be made to avoid the biases that accompany the category judgment when theabsolute effect of noise is examined. The method of continuous judgment by cat-egory is proposed as one of such trials.

THE EFFECT OF CULTURAL AND SOCIALFACTORS AS A FRAME OF REFERENCE

Noise is defined psychologically as unwanted sound. The subjectivemeaning of sounds has significant effects on the judgment of actual sounds. Ifthe sound is accepted as being pleasant, such as music, it is judged as being softerthan, for example, aircraft noise, which is usually accepted as being unpleasant.This may be a psychological frame of reference. Also, cultural and social factorscompose the frame of reference. This kind of the frame of reference should betaken into account in the evaluation of noise (Kuwano & Namba, 2000).

THE EFFECT OF THE SUBJECTIVE MEANING OF SOUND SOURCES

LAeq showed good correlation with loudness of various sounds as shown in Fig.2. Strictly speaking, however, there were slight but significant difference in theloudness among different sound sources when the sound level was high. Aircraftnoise was judged louder than train noise, for example, when their LAeq valueswere equal. In addition to the experiment, a questionnaire survey was conductedand the result suggests that the subjective meaning of the sounds has an impor-tant effect even on loudness judgment. This result suggests that LAeq can be usedas a basic metric that can be applied to various level-fluctuating sounds with thesame cognitive meaning. But the effects of noise are strongly affected by non-physical factors, such as the utility of noise sources and the degree of acceptanceof noise sources in society. It is necessary to take these factors into account todetermine the noise criteria. To avoid the effect of these nonphysical factors, it may be important to determine the permissible level of each sound source independently.

When synthesized sounds were judged that simulate the sound level patternsof actual sounds used in the former experiments with pink noise as the carrier,there was no difference among sound sources except for impulsive sounds(Kuwano, Namba, & Fastl, 1988). An example of the results is shown in Fig. 11.When the sound sources cannot be identified, the loudness is determined mainlyby LAeq.

THE EFFECT OF CULTURAL DIFFERENCE

In the evaluation of noise, cultural and social factors should be taken intoaccount. In order to examine these factors, cross-cultural study is effective.





It was found in our cross-cultural study (Namba, Kuwano, & Schick, 1986;Namba et al., 1991) that there is a cross-cultural difference in the impression ofsounds and attitude toward noise. The impression of various sounds was judgedusing a semantic differential in Japan, Germany, the United States, and China inour study (Kuwano et al., 1999). The sounds used were aircraft noise, train noise,road traffic noise, speech, music, and construction noise. The relation of loudnessjudgments between German and Japanese subjects is shown in Fig. 12. Germansubjects tend to consider construction noise louder than Japanese subjects, whilethey considered music and speech softer. There was little difference betweenJapan, the United States, and China in loudness judgments.

It is desirable to have an international standard for audio signals that tell peoplewhen something dangerous happens. In our study of danger signals (Kuwano,Namba, Fastl, & Schick, 1997), the impression of synthesized signals was judgedusing a semantic differential in Germany and Japan. It was suggested that frequency-modulated sounds are appropriate for danger signals because bothGerman and Japanese subjects judged them as being dangerous. On the otherhand, a cross-cultural difference was found in the impression of a sound such as a bell. An example is shown in Fig. 13. This signal was perceived as being


40 50 60 70 80 90Loudness of simulated sounds, Leq [dB(A)]

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[dB(A

)]

; super express train noise,; aircraft noise, ; train noise,; road traffic noise, ; speech, ; music, ; impulsive noise,; artificial level-fluctuating noise, ; steady state noise,

FIGURE 11 Relation between LAeq and loudness of sounds, the sound level patterns of whichwere simulations of actual sounds. When the sound sources were not identified, the loudness showsa good correlation with LAeq except for impulsive sounds. (From Kuwano et al., 1988, Fig. 4, p. 460.)






2 3 4 5 6 72

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loudness (J)

loud

nes

(G)

r = 0.876 A1M1

S1

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A4

R4T4

M4S4

T2T1

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C1R1

FIGURE 12 Relation between loudness judgments made by German and Japanese subjects.(From Kuwano et al., 1999.)

1 2 3 4 5 6 7

Stimulus 19

softshrillnot frighteningunpleasantsafesoftexcitingdarkpowerfultranquilrepulsiveflexiblefastvaguestrongrelaxedunpleasing

louddeepfrighteningpleasantdangeroushardcalmbrightweakbusyattractiverigidslowdistinctweaktensepleasing

(r = 0.197)G

JFIGURE 13 Semantic profiles for the sound of a bell judged by German and Japanese subjects.(From Kuwano et al., 1997, Fig. 4, p. 126.)Copyright 2004. Elsevier Academic Press. All rights reserved.




dangerous by Japanese subjects but as being safe by German subjects. Germansubjects associated the sound with a church bell, but Japanese subjects associ-ated it with a fire engine. The impression of sounds may be affected by cogni-tive differences.

CONCLUSION

There are two aspects in the subjective assessment of noise. The first is to findphysical metrics that show a good correspondence with subjective impressionsin order to predict the effect of noise. Psychophysical methods of measurementsuch as magnitude estimation are good methods for this purpose. It was foundthat there is a good correlation between the mean energy level (LAeq) and the loud-ness of level-fluctuating sounds. Moreover, the validity of the law of mean energylevel has been confirmed using various methods of measurement. It was alsofound that the law can be applied to various kinds of environmental noises.

The second is to measure the degree of the disturbance effect of noise anddetermine noise criteria. A category scale is often used for this purpose. However,biases are included in category judgment in which the given categories may be assigned to the whole stimulus range as in the range-frequency model of Parducci. This is a law of judgment, not of sensation. Various approaches toreduce the effect of range have been introduced. The method of continuous judg-ment is one of the examples in which subjects do not always have to respond tochanges in the sounds. Finally, cultural and social factors as well as physical prop-erties of the sounds have an effect on noise evaluation, and cross-cultural studiesare effective in examining the effects of cultural and social factors.

REFERENCES

Architectural Institute of Japan (Ed.). (1981). Methods for evaluating the effect of noise. Tokyo:Shokokusha.

Heller, O. (1985). Hrfeldaudiometrie mit dem Verfahren der Kategorienuntereiling (KU). Psycholo-gische Beitraege, 27, 478493.

Kaku, J., Kato, T., Kuwano, S., & Namba, S. (1997). A field experiment on the evaluation of envi-ronmental noise: An approach of the method of continuous judgment by category. Proceedingsof the Spring Meeting of the Acoustical Society of Japan (pp. 767768), Tokyo.

Kaku, J., Kato, T., Kuwano, S., & Namba, S. (1999). Predicting overall reaction to multiple noisesource. EAA/ASA Joint Meeting, Berlin.

Kuwano, S. (1996). Continuous judgment of temporally fluctuating sounds. In H. Fastl, S. Kuwano,and A. Schick (Eds.). (1996). Recent trends in hearing research (pp. 193214). Oldenburg: BIS.

Kuwano, S., & Namba, S. (1985a). Permissible level of air-conditioner noise. Journal of theergonomic Society of Japan, 21, 144145.

Kuwano, S., & Namba, S. (1985b). Continuous judgment of level-fluctuating sounds and the rela-tionship between overall loudness and instantaneous loudness. Psychological Research, 47,2737.





Kuwano, S., & Namba, S. (2000). Psychological evaluation of temporally varying sounds with LAeq and noise criteria in Japan. Journal of the Acoustical Society of Japan. (E), 21(6), 319322.

Kuwano, S., Kaku, J., Kato, T., & Namba, S. (1998). Psychological evaluation of sound environmentwith mixed sources. Proceedings of the International Congress on Acoustics (pp. 11311132).Seattle, WA.

Kuwano, S., Namba, S., & Fastl, H. (1988). On the judgment of loudness, noisiness, and annoyancewith actual and artificial noises. Journal of Sound and Vibration, 127, 457465.

Kuwano, S., Namba, S., Fastl, H., & Schick, A. (1997). Evaluation of the impression of dangersignalscomparison between Japanese and German subjects. In A. Schick and M. Klatte (Eds.),Contribution to psychological acoustics (pp. 115128). Oldenburg: BIS.

Kuwano, S., Namba, S., Fastl, H., Florentine, M., Schick, A., Zheng, D. R., Hoege, H., & Weber, R.(1999). A cross-cultural study of the factors of sound quality of environmental noise. Presentedat the EAA/ASA Joint Meeting, Berlin.

Kuwano, S., Namba, S., & Florentine, M. (1997). Style of questionnaires in social survey on noiseproblems. Proceedings of the Spring Meeting of the Acoustical Society of Japan (pp. 769770),Tokyo.

Kuwano, S., Namba, S., & Kato, T. (1978). Loudness of impulsive sounds. Journal of the Acousti-cal Society of Japan, 34, 316317.

Kuwano, S., Namba, S., Miura, H., & Tachibana, H. (1987). Evaluation of the loudness of impulsivesounds using sound exposure level based on the results of a round robin test in Japan. Journal ofthe Acoustical Society of Japan (E), 8, 241247.

Kuwano, S., Shimokochi, M., & Yamaguchi, Y. (1969). Context effect on loudness judgment and V-potential. Japanese Journal of Physiology, 31, 119.

Namba, S., & Kuwano, S. (1980). The relation between overall noisiness and instantaneous judgmentof noise and the effect of background noise level on noisiness. Journal of the Acoustical Societyof Japan (E), 1, 99106.

Namba, S., & Kuwano, S. (1984). Psychological study on Leq as a measure of loudness of variouskinds of noises. Journal of the Acoustical Society of Japan (E), 5, 135148.

Namba, S., & Kuwano, S. (1991). The loudness of nonsteady state soundsIs a ratio scale appli-cable? In S. J. Bolanowski, Jr. and G. A. Gescheider (Eds.), Ratio scaling of psychological mag-nitude (pp. 229245). Hillsdale, NJ: Lawrence Erlbaum Associates.

Namba, S., Igarashi, J., Kuwano, S., Kuno, K., Sasaki, M., Tachibana, H., Tamura, A., & Mishina,Y. (1996). Report of the Committee of the Social Survey on Noise Problems. Journal of theAcoustical Society of Japan (E), 17, 109113.

Namba, S., Kuwano, S., Hashimoto, T., Berglund, B., Zheng, D. R., Schick, A., Hoege, H., & Florentine, M. (1991). Verbal expression of emotional impression of sound: A cross-cultural study.Journal of the Acoustical Society of Japan (E), 12, 1929.

Namba, S., Kuwano, S., & Kato, T. (1978). On the investigation of Leq, L10 and L50 in relation to loud-ness. Journal of the Acoustical Society of America, 64, S58.

Namba, S., Kuwano, S., & Schick, A. (1986). A cross-cultural study on noise problems. Journal ofthe Acoustical Society of Japan (E), 7, 279289.

Namba, S., Nakamura, T., & Kuwano, S. (1972). The relation between the loudness and the mean ofenergy of level-fluctuating noises. Japanese Journal of Psychology, 43, 251260.

Namba, S., Yoshikawa, T., & Kuwano, S. (1968). Context effects in loudness judgment. JapaneseJournal of Psychology, 39, 191199.

Parducci, A. (1963). A range-frequency compromise in judgment. Psychological Monographs, 77(2,whole no. 565), 150.

Parducci, A. (1974). Context effects: A range-frequency analysis. In E. C. Carterette and M. P. Friedman (Eds.), Handbook of perception (vol. II), Psychophysical judgment and measurement(pp. 127142). New York: Academic Press.

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Schick, A. (1981). Akustik zwischen Physik und Psychologie. Stuttgart: Klett-Cotta.Schultz, T. J. (1978). Synthesis of social surveys on noise annoyance. Journal of the Acoustical Society

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8Ecological

DevelopmentalPsychoacoustics

Lynne A. Werner and Lori J. Leibold

Ecological theories of perception hold that perception involves detecting affordances, the fit between an organisms capabilities and the environmentalopportunities to exercise those capabilities. Further, the ecological view is thatthe environment provides patterns of optic, acoustic, and other information thatdesignate the nature of the affordances of objects and events (Gibson, 1986).Whereas psychophysics is the study of the relationship between characteristicsof stimuli and sensation, ecological psychophysics studies the relationshipbetween characteristics of stimuli as they occur in the real world and perceptionof real-world objects and events. Ecological psychoacoustics has dual goals. First,it identifies the available acoustic information specifying objects and events.Second, it seeks to determine what of the available information is used by lis-teners and the relationship between variations in that information and the objectsor events perceived.

Several of the chapters in this book lament the lack of information regardingauditory perception in real environments and try to understand the factors thathave led to the paucity of research in this area. In the case of perceptual devel-opment, however, there is a long-standing interest in infants or childrens perception of ecologically relevant objects and events because it has long beenunderstood that these perceptions are what drive many aspects of development.Anyone who has watched an infant playing will quickly come to the conclu-sion that the infant is learning about affordances of objects and events. Thus,although not all perceptual development research could be classified as ecologi-cal, the approach is not foreign to anyone in the field and different aspects of the ecological approach appear in many papers on perceptual development(e.g., Cooper, 1997; Gogate, Bahrick, & Watson, 2000; Walker-Andrews, 1997).

191Ecological PsychoacousticsCopyright 2004 by Academic Press.

All rights of reproduction in any form reserved.





Moreover, there is extensive literature explicitly addressing the ecologicalapproach to perceptual development, clearly summarized by Gibson and Pick(2000).

What is lacking in the perceptual development literature is information on perceptual development in the auditory domain. The purpose of this chapter is toattempt to apply the ecological approach to an account of auditory development.The chapter has two parts. First, we consider how the development of psychoa-coustic abilities would be expected to influence the information available toinfants and children about auditory objects or events. Second, we consider thedevelopment of auditory perception within the framework of ecological psychology, asking whether ecological theory provides a reasonable account ofage-related changes in complex auditory perception.

DEVELOPMENTAL PSYCHOACOUSTICS ANDTHE PICKUP OF ACOUSTIC INFORMATION

To identify an auditory object, a listener must analyze the complex soundreaching the ear in terms of its component frequencies, their amplitude, and tem-poral variations in the spectrum. Once this analysis is completed, the componentsof sound emanating from a given auditory object must be identified and usuallycoordinated with information received through other sensory modalities. Theseprocesses typically occur under conditions of uncertainty: the listener does notknow exactly when an event will occur and does not know a priori what eventwill occur. Each of these components of object identification develops duringinfancy and childhood. The question is how development in these realms influ-ences what infants and children hear.

DEVELOPMENT OF FREQUENCY, INTENSITY,AND TEMPORAL RESOLUTION

Frequency resolution is a fundamental characteristic of the auditory system.Individual components of sounds must be selectively analyzed before morecomplex processing takes place. Not surprisingly, frequency resolution abilitieshave been well examined during development. Although it is immature duringearly infancy, the findings of several studies suggest that frequency resolution ismature by 6 months postnatal age (Hall & Grose, 1991; Olsho, 1984; Spetner &Olsho, 1990).

Several paradigms have been used to examine frequency resolution duringdevelopment. In psychophysical tuning curve methods, the level of a narrowbandmasker needed to just mask a pure tone signal is estimated. Because the maskeraffects only the audibility of signals passing through the same auditory filter, plot-ting the level of the masker required to mask the signal, as a function of maskerfrequency, provides an estimate of the auditory filter.





Olsho (1984) reported psychophysical tuning curves for 6-month-old infantsusing signal frequencies ranging from 500 to 4000Hz. Average infant and adulttuning curve shape and width were similar at each of the signal frequencies tested.Subsequent studies by Schneider, Morrongiello, and Trehub (1990) and Spetnerand Olsho (1990) produced results consistent with those of Olsho (1984). Spetnerand Olsho (1990) also measured psychophysical turning curves for 3-month-oldinfants using signal frequencies of 500, 1000, and 4000Hz. Although maturetuning was observed at 500 and 1000Hz, immature tuning curves were measuredat 4000Hz. Taken together, these studies indicate that auditory filter width isimmature at high frequencies among 3-month-olds but is mature across frequencyby 6 months of age. Because similarly immature high-frequency resolution hasbeen observed in the auditory brainstem response, but not in cochlear responses(e.g., Abdala & Folsom, 1995; Abdala & Sininger, 1996; Folsom & Wynne,1987), it is believed that early age-related deficits in frequency resolution areneural in origin.

The auditory filter width paradigm has been used to examine the frequencyresolution abilities of older children. As with the psychophysical tuning curveparadigm, the task is to detect a pure tone signal in the presence of a masker.However, the masker contains a spectral notch with no acoustic energy falling inthe frequency region surrounding the signal frequency. The width of this notchis systematically varied, altering the amount of masker energy that is allowed topass through the auditory filter containing the signal. Signal threshold is thenmeasured at different notch widths, and the shape of the function describing notchwidth and threshold provides an estimate of auditory filter width. Using thismethod, mature auditory filter widths have been measured in 4- and 6-year-oldchildren (Hall & Grose, 1991)

Infant performance on measures of intensity processing appears to take longerto reach mature levels than on measures of frequency resolution (reviewed byWerner, 1996). Infant high-frequency detection thresholds in quiet improve byapproximately 40dB to near adult-like levels between birth and early childhood.Low-frequency detection thresholds in quiet, although initially more mature, takelonger to develop, reaching mature levels by about age 10 (Allen & Wightman,1994; Olsho, Koch, Carter, Halpin, & Spetner, 1988; Schneider, Trehub, Morrongiello, & Thorpe, 1986; Werner & Gillenwater, 1990).

Intensity discrimination and detection thresholds in noise follow a time courseof development similar to that of detection thresholds observed in quiet, althoughthey do not show the same high-frequency-first frequency gradient. Moreover,mature levels are typically reached for both measures at the end of the preschoolyears (Allen & Wightman, 1994; Bargones, Werner, & Marean, 1995; Jensen & Neff, 1993; Schneider, Trehub, Morrongiello, & Thorpe, 1989; Sinnott & Aslin, 1985).

It appears that the differences between infants and adults in intensity pro-cessing result from several different factors. In particular, the development of the middle ear has been shown to play a significant role in the apparent age

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differences in absolute sensitivity observed during infancy and childhood (Keefe,Bulen, Arehart, & Burns, 1993; Keefe, Burns, Bulen, & Campbell, 1994; Keefe& Levi, 1996). Contributions are also thought to come from neural immaturity,inattentiveness, and listening strategy (discussed later; Allen & Wightman, 1994;Bargones et al., 1995; Werner & Boike, 2001; Werner, Folsom, & Mancl, 1994).

Temporal resolution refers to the ability to follow rapid changes in the enve-lope of a sound. Most investigations of infants and young children have estimatedtemporal resolution using gap detection methods. In brief, gap detection para-digms measure the shortest silent gap in a sound that is detected by the listener.Using this paradigm, temporal resolution abilities in infants and young childrenappear to be quite poor. For example, Irwin, Ball, Kay, Stillman, and Rosser(1985) reported that childrens performance did not reach adult levels until age10 using low-frequency noise bands. However, only slight improvements in per-formance were observed between age 6 and adulthood using high-frequency noisebands. In a later study, Wightman, Allen, Dolan, Kistler, and Jamieson (1989)reported immature gap thresholds for noise bands centered at 400 and 2000Hzin preschoolers. Their results suggest that 3- and 4-year-olds have immature tem-poral resolution abilities, although performance was adult-like by age 5. No dif-ference in performance across noise-band frequencies was observed. Similarly,Werner, Marean, Halpin, Spetner, and Gillenwater (1992) reported no frequencygradient in the gap detection thresholds of 3-, 6-, and 12-month-old infants.Infants at all three ages tested performed poorly.

Performance on other measures of temporal processing, including durationdiscrimination and temporal integration, does not appear to mature until at leastage 6 (reviewed by Werner & Marean, 1995). In particular, studies of durationdiscrimination in 4- to 10-year-olds follow the age-related changes in perform-ance observed in the gap detection studies (Elfenbein, Small, & Davis, 1993;Jensen & Neff, 1993; Morrongiello & Trehub, 1987).

Several investigators have suggested that temporal resolution is, in fact, muchbetter than performance on gap detection tasks would indicate and that the poorerperformance observed during infancy and childhood results from immaturities in other areas of processing (reviewed by Werner, 1996). In particular, intensitycoding may affect infant performance on measures of temporal processing.Several studies report more mature temporal resolution estimates using a tempo-ral modulation transfer function (TMTF) method that separates the effects of temporal and intensity processing (Hall & Grose, 1994; Levi & Werner, 1996).TMTF methods estimate temporal resolution by determining the depth of ampli-tude modulation that is required for modulation detection as a function of mod-ulation frequency.

Thus, frequency resolution appears to be mature by 6 months of age and tem-poral resolution may well be mature by 6 months of age, but intensity resolutionfollows a more prolonged developmental course extending into the preschoolyears. There is a growing consensus that intensity processing immaturity amongpreschoolers results from processing inefficiency (e.g., Hartley, Wright, Hogan,





& Moore, 2000). Thus, by 6 months of age humans have a fairly precise, if noisy,neural representation of sounds.

DEVELOPMENT OF SOUND LOCALIZATION

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