7466 Early Sound Field Control in Critical Listening

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Sess ion E -1Preprint 4313Early Sound Field Control in Critical Listening AreasC hr is M orto nARO Technology, Prospect, SAP re sen ted a t6th Australian uo,oReg iona l C onvention10th - 12th September 1996 W orld C ong ress C en tre , M e lbourne

Thispreprinthasbeen reproducedfromthe author'sadvancemanuscriptwithoutediting,correctionsorconsiderationby theReview Board. TheAES takesnoresponsibilityorits contents.Additionalpreprintsmaybe obtainedby sendingrequestand remittancetotheAudioEngineeringSociety ,60 East 42ndStreet,New York10165,USAAll rightsreserved.Reproductionof thepreprint,or anyportionthereof,isnotpermittedwithoutdirectpermissionof theJournalof theAudioEngineeringSociety.

AN AUDIO ENGINEERINGSOCIETY PREPRINT


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EARLY SOUND FIELD CONTROLIN CRITICAL LISTENING AREASChrisW.Morton,Aro Technology,Adelaide,Australia

AbstractIn any critical listening area the properties of reflections occurringimmediately after the listener receives the direct sound are of vitalimportance to the perception of sound quality. This paper covers somepractical studies, largely related to the acoustical treatment of smalllistening rooms; however, many of the principles can be applied tolarger rooms also. Measurements are compared to listening tests,where substantial differences in sensitivity were found.1. In troductionA room's acoustical performance cannot be viewed as passive, and istheoretically never neutral. Whilst the acoustical requirements for eachroom will vary depending upon its use and other factors, such aspersonal taste, the acoustical environment within the room either canbe negative (audibly detrimental) or, conversely, can be positive, andadd beneficially to the listener's perception of the quality of soundbeing generated within that room. [1]To achieve the desired acoustical performance requires the correctbalance of the three physical acoustic properties available--reflection,absorption and diffusion. It must be noted that, in almost every casewhere there is an excess or an insufficiency of one or more of theseproperties, the room's performance as a whole will decrease, and tendto fall into the negative category.

Various studies of psychoacoustics have revealed the importance ofthe properties of reflections arising at a listener'sears shortly after thedirect sound. [2, 3]In larger rooms, all reflections up to around 80 mS after the arrival ofthe direct sound are generally considered early reflections. In manysmaller rooms, however, early reflections are typically considered to behigh level, first order reflections from the walls, ceiling and floor,which may occur with the first 20 mS or 30 mS.The time level frequency phase and directivity of these early reflec-tions are now considered important factors in determining a listener's


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appreciation of the sound quality, along with other factors, such as thereverb time/decay rate, etc.This paper is intended to be a practical discussion of the variousmeans available for controlling the properties of these early reflec-tions, and also of the differences found between listening tests andacoustical measurements.2. What Are the Options?Rooms used for the reproduction of music are often relatively small,frequently


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simple curved panel or polycylindrical diffuser, with a much widereffective bandwidth and a more even spatial distribution over thatbandwidth.In addition to spatial diffusion, Schroeder diffusers also give temporaldiffusion, with the reflected energy being spread over a time spandetermined by the total depth of the diffuser. Figures 3a and 3b showthe spatial energy distribution of a Schroeder diffuser at variousfrequencies and angles of incidence, compared to a specular reflectionoff a flat panel of identical size. The temporal diffusion performance isshown in Figure 4, also compared to a fiat panel, where the reflectionappears identical to the direct signal on an impulse response.A Schroeder diffuser does have some absorptive properties, evenwhen constructed of rigid, non-absorbent materials. This effect hasbeen studied elsewhere [7, 8, 9], and can be summarised as lossesoccurring between wells of different depths where air particle velocitychanges are greatest. In practice, it is found that this absorption isgreatest at the lower operating frequencies of the diffuser, and typic-ally peaks at around one sixth of the wavelength of the panel depth.The majority of sound energy is preserved, however, and simplyredirected in space and time. Because of this distribution, the reflec-tion level will often be in the order of 6-10 dB lower than the level ofa specular reflection off a comparatively sized fiat surface. Thesefactors need to be considered when applying such diffusers as roomtreatment. In a small room, when diffusion is increased there is also acorresponding reduction in reverb time as the reflected energy "sees"more of the absorbent surfaces within the room.3, Comparing Three RoomsThree small listening rooms are shown in Figures 5a to 5c, one beingtotally reflective, and the other two having absorbent and diffusivepanels respectively. No furnishings have been included, to exaggeratethe effect, with the time (impulse) and frequency responses at thelistening position represented in the graphs below each room.The fully reflective room is a poor choice for listeners, with the highlevel reflections and long decay rate masking any acoustical informa-tion in the material being reproduced, and distorting the perceivedstereo image.


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The room with absorbent panels shows the smoothest frequencyresponse due to a lack of interfering reflections; however, in practice,such a room will prove quite "dull" and "lifeless," with poor listeningenjoyment for most listeners' tastes, even though the stereo image maysh ow quite h igh p ositional accuracy.The room w ith diffuser p anels comes th e closest to being p referred bya typ ical listener. Th e energy d istribu tion p rovided by th e d iffusersincreases the temporal reflection density, and reduces spatial variationth rough ou t th e room . Th e decay rate falls betw een th at of th e p re-vious two rooms, but in practice it would be found that a certainamount of absorption would be needed to optimise the room's acous-tics. Th is absorp tion is used both to contro l specific reflections andalso give th e desired decay characteristic; th erefore it m ust be selectedand position for th is purpose.4. Positioning of Absorption, Diffusion and ReflectionAs mentioned earlier, the properties of early reflections reaching thelistener after the direct sound are very important to the overallperception of sound quality, and listener enjoyment. Early reflectionsare created by the nearest sign ificant surfaces, w h ich are generally theroom's side walls and floor, followed by the ceiling and end walls for aconventional 'shoe box' room. It is important that the polar dispersionch arac teristics o f th e sound source are know n as th is is vita l w h encalcu lating th e level and resp onse of early reflections at th e listen ingposition . Th e treatm ent of th ese h igh level first o rder reflec tions isnormally the first priority, and results in the most significant differenceat th e listening position . In a typ ical sm all listen ing room using conven-tional direct radiator loudspeakers, the side walls adjacent to and infront of the speakers often create the impression of a secondarysource w hen reflective. Th is causes strong comb filtering of th e fre-quency response, due to the phase difference between the direct andreflected signals, as w ell as d istortion or sm earing of th e stereo im agedue to th e confusion of th e listener's auditory response created by th eperceived secondary source. Research by Davis [10, 11, 12] resulted inthe implementation of the 'live end-dead end' approach, where thefront portion of a listen ing room was made largely absorben t, and th erear p art d iffusely reflec tive. Such room s can h ave very accu rateimaging performance, and are certainly much preferred as a listeningenvironment over a room with absorption on all planes, as coveredearlier. Th e m ost sign ifican t p roblem w ith th is ap p roach is th at th edecay rate will often be non-linear, and significant spatial variation will

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be found throughout the room. Whilst the undesired very early reflec-tions are eliminated, the amount of absorption applied will often havea substantial effect on the second and third order reflections, andtherefore the decay rate as a whole tends to become non-linear.Having a highly diffuse rear wall is certainly beneficial as this createsan impression of an enlargement of the space behind the listener, andhelps to increase the depth of image and reduce spatial variationaround the listening position. Davis later showed that large amounts ofabsorption in the front portion of the room were not necessary if thewall and ceiling geometries could be designed to direct very earlyreflections away from the listener, with absorption used to control thedecay rate only. This approach may be Meal for a professional controlroom design, however it is often too costly or impractical for manypeople working within existing non-acoustically designed rooms.5. Diffusing the Side WallsResearch by the author on the use of Schrocder diffusers to scatterfirst order side wall reflections as compared to absorptive treatment insmall rooms has provided some interesting results. In implementingsuch an approach, the designer needs to examine the distance andangle between the direct and reflected signals at the listening position,and also the polar performance of both the loudspeakers and thediffusers at all frequencies of interest. With this approach, the soundenergy that would normally be lost by using absorption is insteadredirected or d ispersed th roughout the room .This creates some interesting psychoacoustical effects at the listeningposition, with the most significant being a substantial widening of thestereo image, generally without perceivable colouration or unnatural-ness. Th e effect varies considerably w ith th e po lar resp onse of th eloudspeaker being used, and can prove particularly beneficial tospeakers with a narrow dispersion pattern.It is in teresting to no te th at acoustical m easurem ents on th e im pulseand frequency resp onse d ifferences betw een th e tw o m eth ods oftreatm ent often show only very subtle variations, desp ite th e differencebeing immediately apparent to untrained listeners. Often the diffusereflection w ill be 15 dB or m ore below th e level of th e d irect sound ,and viewed in this way the diffuser can be thought of as a semi-abso rben t su rface.


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Care must be taken if positioning either absorptive or diffusive sur-faces very close to a speaker as this can create sharp notches or 'suckout' in the direct response, particularly for speakers with a widedispersion. This effect is often found in control rooms where absorp-tive treatment on the top face of the mixing console can have anadverse effect on the frequency response of nearfield monitors posi-tioned above.Figure 6a, b and c shows the frequency response measured at thelistening position of a typical home listening room, using speakers witha very narrow polar dispersion. The three surfaces used (reflective,absorbent and diffuse) were placed within 150 mm of the speaker toexaggerate the effect, which still appears quite minimal due to thenarrow dispersion. A cancellation can be seen around 300 Hz with thediffusive treatment, which is the result of its close proximity to thespeaker, and the grazing angle of the incident sound across the frontof the diffusers. In many situations, the Iow frequency absorptioncharacteristic of Schroeder diffusers can prove beneficial when posi-tioned near loudspeakers which tend to be omnidirectional at thesefrequencies.

In most cases where pinpoint positional accuraqt of the stereo imageis desired, such as in a mixdown suite or control room, side wallreflection control is probably best achieved with wall geometry and/orabsorption if wide dispersion monitors are being used. In the majorityof other situations, however, the subtle widening of the sound stagecreated by low level diffuse reflection from the side walls can positive-ly add to the listening appreciation. Ando [131 found that interauralcross-correlation (IACC) was lowest for sounds from 4-_55 withrespect to the listener's head where phase differences are the highest.Applying a diffusive surface to the side walls of small listening roomsintroduces low level reflections around this angle, at which our audi-tory system is most sensitive. Such an approach has been recentlyapplied to a CD Mastering Room using wide dispersion, flush-mount-ed monitors. The engineer's initial reaction was that the image wasvery broad, and felt "a million miles wide." Adding a minimal amountof absorption was sufficient to audibly reduce the width, allowingadjustment to be made to suit the listener's taste. Almost withoutexception, the reaction from audiophiles and trained listeners has beenpositive.

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6. Applications in Larger RoomsThe beneficial effect of Schroeder diffusers on music performanceareas has been covered elsewhere [14, 15]. In an auditorium ortheatre, there is often a lack of support for performers on stage,where early reflections are minimal, and the sound tends to be consistmainly of reflections back from the main room. On a different scale,some side wall positioned diffusion will frequently prove beneficial tothe performers, and, in the case of a reinforced performance, theloudspeaker coverage can be enhanced, with reflections back to thestage area being !owenough to avoid triggering feedback.7 . Conc lu sionThe designer of any critical listening room must have a clear under-standing of not only the acoustical treatment available but also theproperties of the sound source(s), and the listener's personal taste.Whilst there are only three basic acoustical properties available, theirinteraction is often quite complex. The development of high perform-ance d iffusive treatm en t by Dr Sch roeder h as p roved most advanta-geous to designers and listeners alike. Th e ap p lication of th e m anyvariations of the three basic acoustical properties to listening areas isan ongoing field of research, and must always be related to psycho-acoustical research; many times it has been proved that the solelytheoretical approach may not necessarily be appreciated by a listener'sears.8. References[1] F E Olive & F E Toole The Detection of Reflection in 23ypicalRooms. JAES vol 37 no 7/8 (1989) pp 539-553[2] S Bech Perceptions of Reproduced Sound: Audibility of individualreflectionsin a complete sound field, AES 96th Convention, Am-

sterdam (1994 ) Prep rint 384 9[3] W M Wagenaars Localization of Sound in a Room with ReflectingWalls,JAES vol 38 no 3 (1990) pp 99-110[4] M R Schroeder Number Theoryin Science and Communications,Springer, New York (1985)[5] M R Schroeder Binaural Dissimilarity and Optimum Hearing forConcertHalls: More lateral sound diffusion, JASA vol 65 pp 958-963 (1979)

[6] M R Schroeder Progressin ArchitecturalAcoustics and ArtificialReverberation:Concerthall acoustics and number theory, JAES vol32 no 4 pp 194-203 (1984)

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[7] D Takahashi Sound Absorption of a QRD, Proc Wallace ClementSabine Centennial Symposium (1994) pp 149-152[8] D E Commins, N Auletta, B Suner Diffusion and Absorption ofQuadratic Residue Diffusers, Proc IOA vol 10 part 2 (1988) pp223-232

[9] C Morton Low Frequency Control and Acoustical Optimising ofSmall Rooms Using the SchroederDiffusion AES 5th AustralianRegional Convention, Sydney (1995) Prep rint 4 034[10] D Davis The Role of the Initial 77me Delay Gap in the AcousticDesign of ControlRooms for Recordingand Reinforcement_stems,AES 64th Convention, New York (1979) Preprint 1547[11] D Davis TheLEDE Concept, Audio (Aug 1987)

[12] C Davis & G Meeks The H istory and Developm ent of the LEDEControl Room Concept, AES 72nd Convention, Anaheim (1982)Prep rint 1954[13] Y Ando ConcertHall Acoustics, Springer, New York (1985)[14] P al'Antonio & J H Konnert The Role of ReflectionPhase GratingDiffusers in CriticalListening and PerformingEnvironments,AES78th Convention, Anaheim (1985) Preprint 2255[15] P d'Antonio Perfomlance Acoustics: The importance of diffusingsurfaces and the variable acoustics modularperformanceshell, AES91st Convention, New York (1991) Preprint 3118

AcknowledgmentsThe author wishes to thank the many colleagues and listeners whohave contributed comments and conducted their own experiments inthis area, and also Duntech Audio, who provided the use of theirR&D listening rooms.


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REFLECTIOH

ABSORPTION

DIFFUSIOH

Figure 1: Diagrams depicting the acoustical performance of reflec-tion, absorption and diffusion.

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t IiFigure 2: Cross-sectionof an N = 11 QR diffuser (left) and a PRdiffuser (right) .

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Figure 3b: 45 degrees off-axis spatial scattering performance of an Aro770 diffuser compared to a fiat panel of identical dimen-sions (dotted line).

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?...,P ,t..........."...................................:..................................1! 7963#sec I I,._-_ _. .i..... :.... :.... :...:...:...:iHelreS i . i! .I24.49 dS i SOUHO'SOURCE !!. .?............................ iz .If i II ""'"'"":iSvaep Rate: t_' '[' 'Il ' ' ': .... :.... :' I ...... : ' ' ' :_1692Hz/S _ : I/ . [4 id[_ . II ' ....H ;JOan, _. .i..,! .... :.... :.... :.IL..: .... :tTine Span: i i : : : '6JIStC. I 8144_)1'SecJ ':' I':fi........................... i] : . ._st Sl,,on: . [ '_ ..... ie so +e = : .......I uh,_.,!I" ':' 'l':'J":.... :.............."'"" ' _ /.qlt. : : : !lift Ti. ..: ..................'1_[ :..:.. :i..... i,.,Rs,,,,, . ,,u,. ,9.563 I /' Illld ..... i.so._ss L__J_J...-...!.U_ _. TE_., 'i

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Figure 4: Temporal energy spread reflected from an Aro QR diffuserwith a flat panel (specular) reflection shown for compari-son.

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iJIiJJrJllrIME' )' FREQUENCY )lFigure Sa: Small listening room with all reflective walls. Graphs

represent time and frequency response at the listeningposition.

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I i

' TE_ IIIIILlah,,,,,, gFigure Sb: Small listening room with absorbent panels. Graphsrepresent time and frequency response at the listeningposition.

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TI I J ! l [ l , l d [ f f j i t l j l r l f LJill[TIME , FREQUENCY '

Figure5c: Small listening room with diffusive panels. Graphsrepresent time and frequency response at the listeningposition.

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TransferFunctionMagnitudedBvolts/voltsi i ; iii;'_ ! ! ! ; i;:: M5,0 : : : :::: Li iiiill So.o i i i ilii S-5.0 : : : : :;:-10,0-Iff ,0-20.0-25.0-30,0-25,0-40.0-4_.0 .... _ ......auto

50.0 100,0 1000.0 10000.0log Frequency- Hz

Reflection alongsidespeaker.

Figure 6a: Response at listeningpositionwith reflectivesurfacealong-side th e sp eaker.

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Transfer FunctionHagnltude- dRvolts/voltsi ! _ ____ : : : : : ::5.0' !o.o...i-kil-i! _ __: i ii_ S

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-35,0-40.0-45,0auto

50,0 100,0 1000.0 10000.0log Frequency Hz

Absorptionalongside speaker.

Figure 6b: Response at the listeningposition with absorptive surfacealongside the speaker.

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TransferFunctionHagnitudedBvolts/voltss.o ....... L0,0 '"tii i ii::i i ! i i i*_*_ S

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Diffusion alongsidespeaker.

Figure 6c: Response at listening position with diffusive surface along-side the speaker.

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7466 Early Sound Field Control in Critical Listening