Hearing Musical Streams Author(s): Stephen McAdams and Albert Bregman Reviewed work(s): Source: Computer Music Journal, Vol. 3, No. 4 (Dec., 1979), pp. 26-43+60 Published by: The MIT Press Stable URL: http://www.jstor.org/stable/4617866 . Accessed: 26/04/2012 06:00 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. The MIT Press is collaborating with JSTOR to digitize, preserve and extend access to Computer Music Journal. http://www.jstor.org Hearing Musical Streams Stephen McAdams AlbertBregman Hearing and Speech Sciences Departmentof Psychology Stanford University School of Medicine McGillUniversity Stanford, California 94305 Montreal,Quebec, Canada Introduction Theperceptual effects of a soundare dependent upon haveintrigued us withcompositional possibilities. This article the musicalcontext in whichthat sound is imbedded.That is, thuspresents, in a tutorialfashion, a reviewof research(in- a givensound's perceived pitch, timbre, and loudness are in- cludingour own) whichhas directimplications for musi- fluencedby the soundsthat precedeit, coincidewith it, cians,especially for composersworking with computermusic. andeven follow it in time.Thus, this context influences the In all of ourresearch and in most otherresearch cited, com- waya listenerwill associatethe soundwith various melodic, puters(predominantly PDP-11 minicomputers) were used to rhythmic,dynamic, harmonic, and timbralstructures within synthesizethe soundspresented. They were also usedfor the musicalsequence. It thus behoovesthe composerand presentationof soundstimuli, collection of responsesfrom the interpreterto understandthe variousperceptual organizing listeners,and analysisof the data.For thoroughsummaries principlesthat affect the derivationof musicalcontext from andtheoretical treatments of this areaof research,see [2, 5, sequencesof acousticevents. We include the interpreterhere 9.] . becauseseveral musical dimensions, such as timbre,attack and decay transients,and tempo, are often not specified WhatIs An AuditoryStream? exactlyby the composerand are controlled by the performer. In this articlewe shalldiscuss principles that describe Auditorystream formation theory is concernedwith how variousmusical dimensions affect the perceivedcontinui- how the auditorysystem determines whether a sequenceof ty of music.Leon van Noorden has statedthat "insequences acousticevents results from one, or morethan one, "source." wherethe tones follow one anotherin quick succession, A physical"source" may be consideredas somesequence of effectsare observedwhich indicate that the tones arenot acousticevents emanating from one location.A "stream" processedindividually by the perceptionsystem. On the one is a psychologicalorganization that mentally represents such handwe find varioustypes of mutualinteraction between a sequenceand displays a certaininternal consistency, or con- successivetones, such as forwardand backwardmasking, tinuity, that allowsthat sequenceto be interpretedas a loudnessinteractions and duration interactions. On the other "whole."By way of example,two possibleperceptual or- hand,a kind of connectionis foundbetween the successive ganizationsof a repeatingsix-tone sequence are illustrated in perceivedtones." [28, p.1 ]. As for simultaneoussonic events, Figure1. Timeis representedon the horizontalaxis and Bregman[5, 9] has suggestedthat differentsounds are ex- frequencyis representedon the verticalaxis. The dotted lines tractedaccording to variousperceptual and cognitiveorga- connectingthe tonesin the figureindicate the streampercepts. nizationalmechanisms from the superimposedacoustic vibra- In the first configuration,six tones areheard one afterthe tions. Whilesome researchers would like to attributethese otherin a continuouslyrepeating cycle (TapedIllustration phenomenato mechanismsin the peripheralor earlycentral la)l; it is easy to follow the entiremelodic pattern. In the nervoussystem (which are surely involved to some extent, secondpercept, though, one mighthear two separatethree- see [24, 33]), prominentmembers of thismore psychophys- tone patternswhich appearto have little relationshipto icallyoriented "school" are beginning to thinkthe organiza- eachother (Taped Illustration 1b). It is difficultin this case tion is too complexto be so easilyexplained. However, rather to followthe originalsix-tone pattern. Note thatin the first thandelve into theoreticalexplanations of thesephenomena, exampleone streamis heard,and in the second,two areheard. it will sufficefor the presentpurpose to describethem in andto somesalient that general brieflyquantify parameters (1) A tapecontaining sound examples is availablefrom Mr.McAdams. Descriptions of the tapedillustrations ? 1979 by StephenMcAdams and Albert Bregman arefound in Appendix1. Page 26 Computer Music Journal, Box E, Menlo Park, CA 94025 Volume 3 Number 4 D D D B"B /B B' % F F F - I I . " II I I I Cr - O ST % % (a) (b) Time -, Time- One Stream Two Streams Figure 1. A repeating6-tone sequence composed of interspersedhigh and low tones can result in different percepts. In Figure la, with high and low tones alternatingat a tempo of 5 tones/sec., one perceptualstream is heard. In Figure 1b, at a tempo of 10 tones/sec., the high tones perceptuallysegregate from the low tones to form two streams (cf. Taped Illustration1). D 4 F t t D o 0 L. C F C U-U C"=I u A ,/ A / Time -+ Time -+ J II JU :I I:JJHigh Stream Ji I:JJJ:Low Stream HighStream ll Low Stream Figure2. Due to the competition among streamorganizations, tone F may be perceivedas belongingto either the higherstream or the lower stream but not to both. The organizationof the streams changes, among other things, the perceived rhythmic structure,as indicatedunder each diagram(cf. Taped Illustration2). Stephen McAdamsand AlbertBregman: Hearing Musical Streams Page27 In the rest of this section we will discuss some of the can group with either the higher or lower stream (Taped propertiesexhibited by a stream. For example, it is possible Illustration2). Notice that this regroupingresults in a rhyth- to focus one's attention on a stream and follow it through mic transformation as shown under each configuration. time [6, 28]. In the first taped example one can follow the Some non-musicalexamples of this phenomenonare the face- six-tone pattern without any trouble. Thus a stream must vase illusion and the reversibleNecker cubes; Escher and exhibit a certaincoherence over time. However,in the second Vasarelyhave produced art works based on such principles example, it is difficult to follow the six-tone pattern, but it is of perceptualorganization. easy to follow either three-tone pattern. Notice that one can Finally, this bringsup the relationshipbetween "source" pay attention to either the higheror lower stream,switching and "stream."A streamis perceivedas emanatingfrom a single between them at will, but that it is not possible to attend to source. So, in the first example, the pattern was fairly con- both simultaneously (Taped Illustration ib). Indeed, each tinuous and was easily recognizedas coming from one source; distance between the three-tone patternin Figure lb constitutes a separatestream but in Figure lb, the large frequency and maintainsits own temporalcoherence. While a listeneris two three-note groups introduced a sort of discontinuity paying attention to one coherent stream, other acoustic in- that caused the perceptual system to interpret the sequence formation is perceptually relegated to the background. If as resultingfrom two sources. Since at any given moment the one group of sounds is distinct enough, the foreground-back- composite pressurevariations stimulating the ear result from ground relationmay be almost involuntaryand it may require severalsources, the auditory system needs a battery of heuris- a greatdeal of attentional effort to focus on streamsinitially tics to parse, or segregate,the information into separate relegatedto the background. streams.It thus needs to build a descriptionof the acoustic The information-processingnature of the streamsegre- environmentfrom separatedescriptions of the variousstreams which the gation process is suggestedby the observationthat the segre- and the relationshipsbetween them [2]. Factors of and gation of a sequence into smallerstreams takes time to occur perceptualsystem uses to build descriptions streams, rate of occurrence of [4, 13]. In Figure lb, notice that one can hear a six-tone pat- subsequently sources, are frequency, tern for the first few cycles before it segregatesinto two events (or tempo), intensity, timbre, and attack/decay tran- the of the these will be discussedin some separatestreams (Taped Illustrationlb). It thus appearsthat sients. In rest paper are the perceptualsystem assumes things are coming from one detail. Of course it is obvious that sounds assignedpercep- the sources are at source until it acquires enough information to suggest an tually to different sources when physical this alternateinterpretation. different spatial positions. In case, intensity, spectral, all utilized to the sound into When temporal coherence is lost in