Dau, Torsten

Epp, Bastian

Käsbach, Johannes

Marschall, Marton

Online Research Database In Technology

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Downloaded from orbit.dtu.dk on: Dec 20, 2017The relation between perceived apparent source width and interaural cross-correlationin sound reproduction spaces with low reverberationKäsbach, Johannes; Marschall, Marton; Epp, Bastian; Dau, TorstenPublished in:Proceedings of DAGA 2013Publication date:2013Link back to DTU OrbitCitation (APA):Käsbach, J., Marschall, M., Epp, B., & Dau, T. (2013). The relation between perceived apparent source widthand interaural cross-correlation in sound reproduction spaces with low reverberation. In Proceedings of DAGA2013The relation between perceived apparent source width and interauralcross-correlation in sound reproduction spaces with low reverberationJohannes Ka¨sbach, Marton Marschall, Bastian Epp, Torsten DauCentre for Applied Hearing Research, Technical University of Denmark (DTU), Email: johk@elektro.dtu.dkIntroductionApparent source width (ASW) is a perceptual attributethat describes the perceived width of a sound image. Theinteraural cross-correlation (IACC) is commonly used inroom acoustics as an objective measure for ASW. Earlyreflections in a room cause a decorrelation of the two earsignals, i.e., a reduction of IACC, which leads to a largerASW. In a dichotic listening condition using headphones,the ASW changes when varying the correlation betweenthe two channels. Blauert and Lindemann [1] showedfor bandlimited noise signals that the ASW increaseswith decreasing inter-channel correlation (IC). For agiven bandwidth, the ASW was found to increase withdecreasing center frequency of the noise while keepingthe IC constant. In such conditions, the sound imageis internalised, i.e. perceived inside the head. SinceASW relates to the spatial extent of a sound source,this measure becomes more meaningful for externalisedsources, such as produced by two loudspeakers. Aphantom source is then perceived in the middle of thetwo loudspeakers ([2]). In contrast to the headphone(HP) presentation, the loudspeaker (LS) presentationproduces an interaural cross-talk (CT) referring to thecross paths between left LS and right ear and viceversa. The present study investigates the influence of theinteraural cross-talk on the perception of ASW in roomswith low reverberation that are typically used for soundreproduction systems and virtual sound environments.MethodThree experiments were performed: 1) LS presentation ina listening room; 2) HP presentation of the same listeningroom using head and torso simulator (HATS) recordingsand 3) HP presentation of an anechoic listening conditionusing a HRTF database.ApparatusThe experiments were conducted in the Immersive Pres-ence Lab at CIRMMT, McGill University, with a re-verberation time of T60 = 0.2s. A typical stereo-setupwith an opening angle of α = 30 ◦ was installed usingGenelec 8030A loudspeakers. The listener was seated ata distance of d = 2.3m (Experiment 1). Room impulseresponses (IR) were measured for each loudspeaker in thesame room with a B&K head and torso simulator (HATS)of Type 4100 placed at the listener’s position. The signalswere convolved with the IRs and presented to the listenervia headphones. The resulting stimuli were then eitherpresented including or excluding the interaural cross- talk(Experiment 2). For the anechoic listening condition, theCIPIC HRTF database [3] was used (Experiment 3).Stimuli and experimental procedureThe stimuli were bandlimited noises with center fre-quencies of fc = 0.25kHz or 1 kHz and a bandwidthof ∆f = 1.5 octaves. The signals were generatedfrom a 2-dimensional multivariate normal distributionwhere each dimension corresponded to one loudspeakerchannel. The bandpass filter was a digital 4th-orderButterworth filter with 24 dB/octave roll-off. The signalswere amplitude modulated with a modulation depth of60% and a modulation frequency of 8 Hz. All stimuli werepresented with constant spectral density of 35dB/Hz (atIC = 0) and had a duration of 4s.The experiments were separately tested using aMulti Stimulus test with hidden reference and anchor(MUSHRA) [4], excluding anchor and reference. InExperiment 1, the ASW was measured for three inter-channel correlation (IC) values of 0, 0.6 and 1 at the twocenter frequencies of 0.25 and 1 kHz. In Experiments 2and 3, the two conditions with and without interauralcross-talk were considered for the same noise signals andICs. Subjects were asked to rate the stimuli relative toeach other in terms of ASW on a scale from 0 (narrow)to 100 % (wide). In addition, the task was to identifythe narrowest and the widest stimulus with 0 and 100,respectively. The stimuli were presented in randomorder and repeated six times. Thirteen normal-hearingsubjects participated in Experiments 1 and 2, and sixof these participated in Experiment 3. The results wereanalysed with a 3-way ANOVA with the null hypothesisthat all stimuli produced the same ASW. The threefactors were stimuli, subjects and repetitions. The posthoc analysis were based on Least Significant Difference(LSD) bars. Differences were considered significant forp < 0.05.IACC-based predictions of ASWAccording to [5], the IACC describes the correlationbetween the left-ear signal, pl(t), and the right-ear signal,pr(t), normalised with their rms values. The resultingIACC coefficient corresponds to the maximum of thecross- correlation function ρlr(τ), calculated with a delaytime interval of |τ | ≤ 1ms and using a time window oft2 − t1. It takes values between zero and one:ρlr(τ) =∫ t2t1pl(t)pr(t+ τ)dt√∫ t2t1p2l (t)dt∫ t2t1p2r(t)dt(1)IACC = max|ρlr(τ)| (2)The IACC was calculated for the same stimuli as pre-sented to the listeners, i.e. the binaural room impulseresponses, measured with the HATS, were convolved withthe noise signals. The integration time window t2 − t1was chosen to be the duration of the stimuli.ResultsThe results of the three experiments are shown in Figures1 to 3. The measured ASW scores (open symbols)are indicated as median values and 25th and 75thpercentiles for the three considered IC values (0, 0.6and 1). The corresponding interaural cross-correlationcoefficients were calculated and are represented as 1-IACC. They are indicated as filled symbols connectedby solid lines in the figures for comparison with themeasured ASW scores.The results from Experiment 1 are shown in Figure 1.The measured ASW scores were similar for the two noisebands centered at 0.25 kHz (open circles) and 1 kHz(open squares). At both frequencies, ASW decreasedwith increasing IC. At both frequencies, a comparabledynamic range (of 0.8 − 0.9) was obtained, representingthe difference between the largest and smallest values.In contrast to the data, the calculated IACC values weredifferent for the two bands. Larger IACC values wereproduced at 1 kHz than at 0.25 kHz. Furthermore, areduced dynamic range of only 0.2 was obtained for thelow-frequency band, compared to the high- frequencyband with a dynamic range of 0.4. The statisticalanalysis revealed no significant differences for the twofrequency bands for IC = 0 and 0.6, but the differencefor IC = 1 was significant.Figure 2 shows the measured data from Experiment 2obtained in the conditions with cross-talk (open squares)and without cross-talk (open circles) using the HRTFsrecorded in the CIRMMT lab. The results for 0.25 kHzare shown in the top panel and the results for 1 kHz arerepresented in the lower panel. The data show that, inthe case of cross-talk, lower ASW scores were obtainedthan in the absence of cross-talk, for all specified ICvalues. The statistical analysis revealed that there was asignificant difference between the two conditions at bothfrequencies. The median of the ASW scores was foundto be shifted downwards by the same amount of about0.4 in both conditions. Thus, the dynamic range for theASW values as a function of IC was similar with andwithout cross-talk which was the case for both frequencybands. The only exception was the high-frequency bandFigure 1: ASW scores and calculated 1-IACC values asa function of the inter-channel correlation in Experiment1. Two frequency bands centered at fc = 0.25 and 1 kHzwere considered in the CIRMMT lab, presented via twoloudspeakers. The subjective results are represented by theirmedian and 25th and 75th percentiles.for IC = 1 where a similar ASW score was observedwith and without cross talk. In contrast, the IACCpredictions, indicated by the filled symbols connected bysolid lines, resulted in a shallower curve in the conditionwith cross-talk (filled squares) than in the conditionwithout cross-talk (filled circles). At 1 kHz, this resultedin an intersection of the two IACC curves at IC = 0.4.Figure 3 shows the results from Experiment 3 for the twoconditions with and without cross-talk in the anechoiclistening environment. The results were similar to thoseobtained in Experiment 2 and the statistical analysisrevealed that there was a significant difference betweenthe two conditions at both frequencies. Lower ASWscores were obtained when the cross-talk was present.The difference between the two conditions was, however,slightly smaller than in Experiment 2. The IACCpredictions for the 1-kHz band provided similar values forICs between 0.2 and 0.6 and decreasing values for outsidethis IC range. This is in contrast to the measured ASWvalues that showed a monotonic decrease with increasingIC.DiscussionThe results demonstrated that the IACC predicted areduced dynamic range of values in the cross-talk condi-tions whereas the measured ASW scores showed a similardynamic range in the conditions with and without cross-talk and a downward shift in the cross-talk condition.This discrepancy from the data can be explained by con-sidering the effect of cross-talk on the cross-correlationfunction ρlr(τ) from Eqn. (1) as illustrated in Figure 4(left panel). ρlr(τ) is shown for a broadband signal forfive inter-channel correlation values. Besides the peak atzero lag which varies with IC, two additional constantpeaks occur at time lags corresponding to the angularoffset of the loudspeakers from the median plane. Dueto this offset, the signal from the left LS will reachthe left ear before the right ear, and vice versa for theFigure 2: ASW scores and 1-IACC predictions as a functionof IC in Experiment 2. The two conditions with and withoutcross-talk were presented via headphones in the CIRMMTlab using HATS recordings, for two frequency bands. Themeasured data are represented by their medians and 25thand 75th percentiles.right LS. For low correlation values between the two LSsignals (from IC = 0 to IC = 0.4), the peaks caused bythe delay are larger than the actual specified IC value(peak at zero lag). According to Eqn. (1), the IACCcoefficient corresponds to the maximum of ρlr(τ), whichis equal to the value of the side peaks unless the peak atzero lag becomes larger. A monotonically decreasing 1-IACC with increasing IC is thus only resulting in theabsence of cross-talk. When bandlimiting the signal,the peakwidths increase and interfere with each other.The middle panel of Figure 4 shows ρlr(τ) for the 1-kHzfrequency band considered in Experiment 3. The patternshows a minimum at IC = 0 for a delay time of 0ms and aroughly constant maximum value (defined by the two sidepeaks) for ICs between 0.2 and 0.6, consistent with thecorresponding 1-IACC function shown in Figure 3 (filledsquares in the lower panel). For the low-frequency bandat 0.25 kHz (right panel of Figure 4) from Experiment2, the width of the peaks increase such that they are notdistinguishable from each other and result in one largepeak. This explains the reduced range 1-IACC valuesshown in Figure 2 (filled squares in the upper panel).The outlined differences between data and predictionsbased on the IACC suggest that other cues might beFigure 3: ASW scores and 1-IACC results as a function of ICin Experiment 3. The two conditions with and without cross-talk were presented via headphones in an anechoic listeningenvironment using the CIPIC database [3], for two frequencybands. The measured data are represented by their mediansand 25th and 75th percentiles.available to a listener. More advanced models, suchas the one proposed in [6], suggest that the stimulusenergy below 500 Hz and the variation of interauraltime differences above 500 Hz provide important cuesfor the perception of ASW. The analysis of these cueswas outside the scope of the present study. However, thelisteners mentioned a difference in timbre between stimuliwith different ASW. A stimulus with a ”brighter” timbrewas often associated with a narrower sound source, suchas in conditions with high IC values, particularly inthe headphone-based Experiments 2 and 3. Further-more, image splitting was reported which refers to theperception of two or multiple sources ([1]), in contrastto one fused sound image. While in the LS-basedexperiment (Experiment 1) only one listener reportedimage splitting, six listeners in Experiment 2 and twolisteners in Experiment 3 reported this effect. In caseof image splitting, subjects based their judgement onthe overall ASW as they reported after the experimentalprocedure. Most subjects mentioned that the task ofevaluating ASW was easiest for the LS presentation inExperiment 1. Nevertheless, it should be noted thatthe measured data showed remarkably stable resultsobtained with the chosen method.Figure 4: The normalised cross-correlation function ρlr(τ) inthe conditions with cross-talk, for five values of IC and threestimulus conditions. Left: for a broadband signal; middle:for the noise band centered at 1 kHz from Experiment 3;and right: for the noise band centered at 0.25 kHz fromExperiment 2.The maximal delay time interval of |τ | ≤ 1msis indicated by the vertical lines.ConclusionsA stereo set-up was used to study the perception of ASWfor partially correlated bandlimited noise signals betweenthe two loudspeakers and to test the performance ofthe IACC that is commonly used to predict ASW.The following results were obtained: 1) In a listeningenvironment with low reverberation, ASW was foundto be frequency independent for ICs of 0 and 0.6, incontrast to the case of dichotic HP presentations asinvestigated in [1], whereas a slight frequency effect wasobserved for IC = 1; 2) With and without cross-talk,the ASW decreased monotonically with increasing IC.The cross-talk generally caused a smaller ASW than theconditions without cross-talk, but the dynamic range ofthe ASW ratings was preserved. This was the case bothfor the listening room with a low reverberation time andthe anechoic listening condition and was found to beindependent of frequency. In the absence of cross-talk,the predictions based on the IACC were consistent withthe measured data. However, in the presence of cross-talk, the IACC did not describe the data correctly. Thechange of ASW with IC was strongly underestimatedat 0.25 kHz. At 1 kHz, the IACC based predictiondid not account for the monotonic decrease of ASWwith increasing IC. The discrepancies are caused by theinterferences in the cross-correlation function introducedby the cross-talk delays. More advanced predictors areneeded to correctly account for the measured data.References[1] Blauert, J. and Lindemann, W.: ”Spatial Mappingof intracranical auditory events for various degrees ofinteraural coherence”.J.Acoust.Soc.Am. 79 (3), 1986[2] Plenge, G.: ”U¨ber das Problem der Im-Kopf-Lokalisation”.Acustica, Vol.26, Heft 5, 1972[3] Reference to the CIPIC HRTF databaseURL: http://interface.cipic.ucdavis.edu/sound/hrtf.html[4] Recommendation ITU-R BS.1534-1. ”Method for thesubjective assessment of intermediate quality level ofcoding systems”.The ITU Radiocommunication Assembly, 2001-2003.[5] de Vries, Diemer, Hulsebos, Edo M. and Baan, Jan.”Spatial fluctuations in measures for spaciousness”.J. Acoust. Soc. Am. 110 (2), Aug. 2001[6] van Dorp Schuitmann J.. ”Auditory modelling forassessing room acoustics”. PhD Thesis, Laboratoryof Acoustical Imaging and Sound Control, Facultyof Applied Sciences, Delft University of Technology,Delft, The Netherlands, 2011.

The relation between perceived apparent source width and interaural cross-correlation in sound reproduction spaces with low reverberation

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The relation between perceived apparent source width and interaural cross-correlation in sound reproduction spaces with low reverberation

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