Synthetic soundfields for the rating of spatial perceptions

Synthetic sound®elds for the rating of spatialperceptions

JoÈ rg Becker *, Markus SappInstitut fuÈr Elektrische Nachrichtentechnik, RWTH-Aachen, D-52056 Aachen, Germany

Received 10 August 1999; received in revised form 27 October 1999; accepted 27 June 2000

Abstract

This paper focusses on new listening tests and their results for the quantities apparent

source width (ASW) and listener envelopment (LEV). In these tests, binaurally recordedsound ®elds are compared to synthetic sound ®elds. These synthetic sound ®elds can be ®ttedin real-time to the sound ®eld under test by the test person. Thus the sound ®elds under testcan be measured absolutely for ASW and in relation to the synthetic sound ®eld in case of

LEV. Investigations on the suitability of the synthetic sound ®elds and test signals for therating of ASW and LEV have been made. The results of these investigations lead to limits forASW and LEV as single number quantities for the characterization of sound ®elds in rooms.

For listening tests concerning spatial hearing, binaural recording techniques are state of theart even though di�erent recording and playback techniques led to better or worse localiza-tion abilities of the test persons. Therefore recordings from three di�erent arti®cial heads are

compared. Playback with headphones sometimes evoke in-the-head localization and front-to-back inversions which are said to appear less frequent for sound ®eld reproduction via cross-talk-cancellation. In order to examine these in¯uences the listening tests were made with both,equalized headphones and cross-talk-cancellation (CTC) techniques. # 2000 Elsevier Science

Ltd. All rights reserved.

1. Introduction

In room acoustics, the characteristics of an enclosure are usually determined in theform of single number quantities. In order to judge the acoustic quality of the roomthese quantities can be compared with results from the literature [1]. Several singlenumber quantities for an objective measurement which are highly correlated withthese perceptual properties have been derived by di�erent authors [2±6] (e.g. IACC,early lateral energy fraction, center of gravity time, etc.). In [7], the term spatial

Applied Acoustics 62 (2001) 217±228

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* Corresponding author. Tel.: +49-241-80-7676; fax: +49-88-88196.

E-mail address: [email protected] (J. Becker).

impression is devided into the three subcomponents spaciousness [8,9], size impres-sion and reverberation. The subcomponent spaciousness is divided into two moresections, ASW and LEV.Since spaciousness is without doubt an important term for characterizing the

acoustic quality of a concert hall or a room, it is of great interest which sound ®eldevokes what amount of spaciousness and what are the physical reasons for it. Thesole search for physically measurable quantities which highly correlate with spa-ciousness bears the danger of not noticing all parameters which are responsible forthis spatial perception.If the psychoacoustic mechanism which evokes spaciousness is known, a synthetic

or real sound ®eld could be rated objectively with the help of a binaural modelwhich rates hearing sensation with regard to spaciousness aurally adequate. Then itis possible to ®nd the required properties of such a sound ®eld more easily. If theseproperties are well de®ned, their physical origins can be investigated.A de®nition of ASWandLEV can be found in [7,10], while Griesinger [11] gives a good

description of the di�erence between ASW and LEV.While ASW is closely connected tothe sound source and describes its apparent width, LEV describes the impression of``feeling inside the music''[5]. Measuring ASW has the advantage of its concreteimage of the perceptual impression connected with the concrete physical meaning. Otherterms, for instance LEV, describe the perceptual impression as good as ASW but theyhave a lack in missing a directly measurable physical correspondence. For LEV onlysome highly correlating single number quantities exist [12]. In recent literature, ASW isrelated to the early re¯ections while the reverberant sound ®eld is responsible for LEV.ASW can be measured directly as the solid angle occupied by the performing

entity. For the measurement the left and right edges of the apparent sound sourcehave to be determined. For the rating of LEV it is useful to ®nd synthetic binauralsound ®elds which can be produced and scaled in real-time to gradually evoke thedesired hearing sensation. These synthetic sound ®elds can be compared with measuredsound ®elds of existing rooms. If the achieved measurement accuracy is high enoughit is possible to examine the in¯uence of di�erent arti®cial heads and di�erent play-back systems like headphones and cross-talk-cancellation (CTC) on the rating ofASW and LEV. The in¯uence of the source signal on ASW and LEV can be examinedto optimize test signals for appropriate listening tests.Furthermore with the help of this data it is possible to develop binaural models of

hearing [13±15] for the rating of spaciousness. These models can be used to searchfor correlations between psychoacoustic perception of spaciousness and the acousticsof the enclosure.

2. Description of the experiments

2.1. Recording system

For an aurally accurate sound ®eld recording arti®cial heads were used. In orderto research the in¯uence of the characteristics of each arti®cial head the examinations

218 J. Becker, M. Sapp / Applied Acoustics 62 (2001) 217±228

were made for three di�erent dummy heads (head acoustics HMSOÂ I and HMSOÂ II,prototype digital arti®cial head, Institute for Technical Acoustic RWTH-Aachen).The room impulse responses which had to be rated were measured in the seminar

room of our institute (20 m length�12 m width�5 m height). They were determinedon three positions for the three di�erent arti®cial heads with a normal loudspeakerwith a higher directivity in comparison to a human speaker (Tannoy System 800).The axis of the coaxial speaker system was positioned about 1.7 m above the ¯oorand 3 m in front of the front wall. The arti®cial heads were placed in normal seatingpositions (height of the axis through the ears 1.2 m) in 2, 6 and 10 m distance facingthe sound source (rows 2, 5, 8). In order to have a symmetric angle of sound inci-dence the sound source and the arti®cial head were placed on the middle axis of theroom. The reverberation time of the room is about 1.6 s for the unoccupied state.The impulse responses have been measured for this state.

2.2. Playback systems

The signals were played back in an anechoic chamber using cross-talk-cancellation(CTC) as well as equalized headphones. To avoid disturbing spikes in the equalizationtransfer function the measured transfer functions are smoothed with a gliding onethird octave band. This leads to a better compatibility between the head relatedtransfer functions of the arti®cial heads and the di�erent test persons. The cross-talk-cancellation used here includes ®ve iteration steps. Further information on CTC canbe found in [16].Prior to the listening test the test persons had to make an identi®cation test in

order to verify the function of the CTC and the equalization of the headphones. Thispre-test was made for each head in each playback situation. As shown in Fig. 1 theidenti®cation results for white noise for the CTC playback situation are slightlybetter than for headphones. Because of the acoustical di�erence of recording person(arti®cial head) and listening person (test person) the identi®cation diagrams showfront to back inversions especially for the headphone playback systems.

2.3. Source signal

Three di�erent source signals were chosen. The ®rst is a piece of music from theoverture to ``Ruslan and Lyudmila'' by Glinka taken from the Denon CD ``Anechoicorchestral music recording''. The second signal is an anechoic recorded male speakertaken from the Westra digital audiometer disk no. 2 speaking three sentences fromthe ``Marburger Satztest''. The third signal is pulsed white noise with 2 s durationinterrupted by 2 s silence. The 10% percentile value of the SPL of all three signalswas set to 70 dB during the listening test. This leads subjectively to an approximatelyequal loudness for all signals. Fig. 2 gives an impression of the temporal variationsof the SPL of the three signals. While the noise shows nearly no variations except forthe pulsing the SPL of the orchestral recording varies noticeably. This e�ect is evenstronger for the speech signal. In order to examine the in¯uence of the SPL on ASWthe noise signal was also presented at a level of 60 and 80 dB.

J. Becker, M. Sapp / Applied Acoustics 62 (2001) 217±228 219

2.4. The subjects

The listening tests have been performed with 10 persons aged between 23 and 45.Some of the test persons had experience in hearing arti®cial head recordings viaCTC and also some untrained test persons were chosen. Before the beginning of thetest all persons were instructed about the di�erence in hearing sensation betweenASW and LEV. In an explanation they were introduced in the di�erent spatial

Fig. 1. Identi®cation listening test results for the horizontal plane for three arti®cial heads. Top: playback

by cross-talk-cancellation; bottom: playback by headphones.

Fig. 2. SPL of the source signals. Solid line orchestral music recording; dotted line speech recording;

dashed line noise.


impressions they have while they listen to sound sources in enclosures. They were toldthat there are two main spatial perceptions, one directly connected with the soundsource called ASW which seems to broaden the sound source and another one calledLEV which leads to the feeling to be enveloped by the sound. For the ASW the testpersons had to determine the left and right edge of the sound source they are hearing.For LEV the test persons had to judge the amount of sound coming from the wholesphere which could not be directly associated with the sound source and which causesto feel inside the sound ®eld and not looking at a sound through a window.

3. ASW listening test

3.1. Procedure

Fig. 3 shows di�erent possible experimental setups for measuring ASW by usingthe concept of pointers. The task for the test persons is to outline the borders of thesound source under test by using the pointers. The left picture shows the measure-ment concept of [2] in combination with a stereophonic playback. In opposite to theexperimental setup with the visual pointer the aim of the work presented here was tomeasure the ASW without combining two human senses. Because of the strongin¯uence between visual sensation and hearing sensation [17] an experimental setupwith only acoustic components was chosen. The concept of the moving speakershown in the center picture has the disadvantage of in¯uencing the sound ®eld undertest by the moving speaker. This leads to unsolvable problems for the playback viacross-talk-cancellation.For this paper, the third listening test setup from Fig. 3 was built up. One

advantage of adding a virtual acoustic pointer to the source signal is that an addi-tional controlling feature of the playback situation is given. If the CTC of the sourcesignal fails, e.g. as a result of head rotations or shifts, the image of the virtual pointerwill be destroyed, too. This would be recognized by the test person very easily. Dis-tortions of the sound ®eld reproduction would e�ect the source signal in the sameway as the pointer and so they compensate to a certain extent. Another advantage inopposition to the concept of the moving speaker is the direct transferability of the

Fig. 3. Three di�erent methods for measuring ASW using a pointer.


system to a headphone playback system, which is the easiest playback system forbinaural recorded sound ®elds. The FIR ®lters for the calculation of the CTC onlyhave to be replaced by the equalization FIR ®lters for the headphones and thecomplete playback with pointer and signal source can be switched from CTC toheadphones.The virtual pointer concept introduces the problem of designing an acoustic

pointer which works similarly to a visual pointer. Everybody has an image of how apointer has to look in the visual world. A long object like an arrow with a sharpmarked tip in order to locate exactly the things we want to show. Corresponding tothe sharp tip of the visual pointer, the localization blur of the acoustic pointer signalhas to be very small in order to indicate directions as exactly as possible; for that,clicks or even speech would be the best [18]. Clicks have the problem of gettingsuppressed by the source signal and speech has the problem of in¯uencing the sourceunder test. So an amplitude modulated broad band noise was chosen as the pointersignal. The period of the rectangular modulation was 1 s with a duty cycle of 500 mswhere the rising and the falling edges were smoothed. The signal was audible duringthe whole test. The left or right pointer could be chosen alternatively by the testpersons.The signal is convolved with measured head related impulse responses for the

speci®c arti®cial head measured in steps of 2� in the horizontal plane. The elevationwas neglected because in normal listening situations the audience looks at the soundsource which means that the sound is direct in front of them. Moving the pointer,the corresponding head related impulse responses are crossfaded in real-time on aDSP-system [19]. The width of the pointer was determined to be less than 4� for thefront direction.

3.2. Results for ASW

The diagrams in Fig. 4 show the mean and standard deviation over all test personsof the left and right edges of the sound source. The measured ASW is the di�erencebetween the values of the two corresponding lines. Although the original soundsource was positioned right in front of the arti®cial heads, both left and right borderof the source under test had to be measured due to the fact that di�erent strongre¯ections from one side can broaden the sound source more to one direction thanto the other.The results for the three di�erent playback levels are shown in Fig. 4a. First pub-

lished by Keet [2] there is a known dependence between ASW and SPL (1.6�/dB forstereophonic recording and playback techniques). The result of our listening testshows a weak dependence of about 2�/dB for all binaural recordings an both play-back techniques. This e�ect has its origins in the nonlinear signal processing of thehuman sense of hearing.In [20], Barron points out that in his opinion this dependence on the SPL is not

essential for the rating of the room due to the fact that SPL is also a property of thesound source but, since there is a dependence of ASW on SPL, if a characterizationof the room should be done by the perceptual quantity ASW and not by highly


correlating physical parameters like early lateral fraction energy it is essential tode®ne a test signal at a certain sound pressure level for the sound source or to con-sider the gain of the room. A synthetic signal-like noise should be used to providethe test persons for judging their personal image of a sound source and not the originalsound source. Due to the dependence of ASW of the spectral composition of the testsignal [9] the noise should be adapted to the purpose of the rooms to rate, e.g.speech or music. To improve the listener's ability to localize the sound source atemporal structured signal like a pulsed noise should be chosen.Fig. 4b shows the results for di�erent playback signals. In Fig. 4b, the in¯uence of

the kind of source signal music, speech and noise are layed out. It can be seen fromFig. 4b that there is a dependence between measured ASW and the signal of thesource under test. The music signal seems to broaden the sound source. The tem-poral and spectral composition of the signal could be the reason but the music signalhas medium temporal variations and a medium bandwidth compared to the otherpresented signals (see Fig. 2) so that these properties cannot explain the high valuesmeasured for ASW for this signal. In our opinion the test persons judge a virtualorchestra playing in front of them and not a signal coming from a relatively closelylocated speaker.Fig. 4c and d show the results for the di�erent recording and playback situations

for the noise signal. No signi®cant in¯uences of the di�erent dummy heads can befound. All diagrams show no signi®cant di�erence between headphone and CTCplay back.

Fig. 4. Results for ASW listening tests. (a) di�erent SPL; (b) di�erent source signals; (c,d) di�erent arti-

®cial heads. Solid line: head a; dashed line: head b; dash-dotted line: head c. Black: cross-talk-cancella-

tion; gray: headphones.


4. LEV listening test

4.1. Procedure

Due to the fact that LEV cannot be rated directly by the test persons, a syntheticsound ®eld has to be generated which the test persons can compare to the sound®eld under test. By changing one parameter of this synthetic sound ®eld, the testpersons should be able to gradually change their perception from a discrete monoticsound source to a fully enveloping sound ®eld. Since Kuttru� [21] shows anapproximate relation between lateral energy fraction and IACC the ®rst attempt wasto decorrelate the two ear signals in steps. Therefore, two ®lters for left and right earsignal with the following transfer functions [22] were designed:

Hl f� � � cos c:log 2�f� ��

Hr f� � � cos c:log 2�f� �� '� �

The correlation between the left and the right ear signal can be controlled byvarying the phase angle ' were chosen in away that the IACC for a monotic signalcan be varied in linear steps from IACC=1.0 ' � 0o� � to IACC=2.0 ' � 90o� �. Theconstant c was set to 100.Even though the ideal set up would apply the ®lters only to the reverberation part

of the signal, to simplify matters in the setup used here the whole signal is processedby the ®lters. Pre-tests have shown that only the apparent source width is in¯uencedby these ®lters, not the listener envelopment. For that reason, a®lter concept had tobe chosen. In [23], Schroeder describes an all-pass ®lter which evokes envelopingsound to the listener. The all-pass structure leads to a comb®lter function in thegroup delay time. If the same factor g is used for the two channels with oppositesigns the notches in the group delay of the left channel lay exactly on the spikes ofthe right channel. This leads to strong interaural time ¯uctuations which are thereason for the perception of envelopment [24]. Fig. 5 shows the complete realized®lter structure. The gray box contains the all-pass structure suggested by Schroeder.In combination with a synthetic reverberation algorithm in front of this structure itproduces much envelopment. In order to control the amount of envelopment theoutput of the all-pass structure is scaled by a factor h and added to a direct part. To

Fig. 5. Filter structure for generating LEV.


keep the energy of the resulting signal constant the direct path is scaled by a factor1ÿh. By changing the value of h, the binaural recorded anechoic signal can be variedfrom a small extended sound source with no envelopment to a fully di�use ®eld asshown in Fig. 6.Fig. 6 shows the perceived envelopment depending on the all-pass scale factor h.

During this test, the subjects had to compare a synthetic sound ®eld with a randomlychosen all-pass scale factor h with a binaural anechoic recording and a binauralrecording made in the reverberation chamber of the institute. The anechoic recordingwas de®ned as reference for 0% envelopment and the recording from the reverberationchamber (reverberation time 6 s) was de®ned as 100% envelopment.The reverberation time of the synthetic sound ®eld was chosen to 1.5s for the

reference sound ®eld for this and further tests. In further listening test its envelopmentis compared to synthetic sound ®elds with di�ering reverberation times. During thelistening test the test persons have to compare the measured sound ®eld of the roomunder test with the synthetic sound ®eld in an A±B test. While the playback of thesound ®eld under test was always the same, the synthetic sound ®eld could beadapted to the measured sound ®eld via the factor h with regard to the LEV. TheSPL of both sound ®elds was set to a value of 70 dB.

4.2. Results for LEV

Fig. 7 shows the mean and standard deviation over all test persons of the scalefactor h as a measure for LEV. In the ®rst test (Fig. 7a) two synthetic sound ®eldswere compared in order to examine the in¯uence of di�erent simulated reverberationtimes. From this test resulted that a longer reverberation time of the simulatedmonotic reverberation leads to more envelopment. Due to the fact that the scalingfactor h which reaches from 0 to 1 was kept very low in the hole test there is no needof a longer reverberation time even if sound ®elds with longer reverberation timeshad to be judged. The ``headroom'' of envelopment of the presented listening testsetup is also large enough for rating rooms with much longer reverberation time andmore envelopment as shown in Fig. 6. Fig. 7b shows the in¯uence of the source

Fig. 6. Listener envelopment depending on all-pass scale factor h. Black: cross-talk-cancellation; gray:

headphones.


signals on the quantity LEV. Because of the kind of listening test the di�erencesbetween the sources signals could only base on di�erent interactions between signaland room, so the di�erences in LEV between the source signals are comparable small.Figs. 7c and d show the in¯uence of the di�erent arti®cial heads and the playback

situations. The headphone playback leads to more envelopment due to the fact thatin-the-head localization and front-to-back inversions (Fig. 1) appear more often whenusing headphones than with CTC. The di�erent heads also lead to di�erent listenerenvelopment. This problem is caused by the fact that the heads can only be equalizedfor one direction, normally front direction but not for the whole sphere. Therefore thesignals of the measured room impulse responses have slightly di�erent spectralcomponents which leads to the di�erent LEV especially for the low frequencies [25,26].The e�ect of the di�erent characteristics of the three arti®cial heads is in¯uencing ASWnot in the same amount as LEV. This is based on the fact that the dominant energypart of the sound for the ASW listening test comes from the front direction.

5. Conclusions

In this paper new listening test set ups for determining ASW and LEV are pre-sented. The in¯uences of di�erent recording and playback systems for these listening

Fig. 7. Results for LEV listening tests. (a) di�erent synthetic sound ®elds; (b) di�erent source signals; (c,d)

di�erent arti®cial heads. Solid line: head a; dashed line: head b; dash-dotted line: head c. Black: cross-talk-

cancellation; gray: headphones.


tests have been examined. Research on the e�ect of di�erent SPL on ASW and dif-ferent source signals on ASW and LEV has been made.Two new approaches were realized for listening tests with real-time modi®cation

of synthetic sound ®elds. These new approaches allow to broaden a sound sourcein®nitely variable and to evoke variable listener envelopment. With the help of thesesound ®elds it is possible to examine ASW and LEV very exactly. Basic research onbinaural models for the rating of spatial perception could be done with these test set ups.The concept of the acoustical pointer has been introduced in order to avoid auditory±

visual interactions when determining the apparent width of the sound source.The in¯uences of di�erent arti®cial heads have been examined. Because the

recordings from the dummy heads can only be equalized for one direction, usuallythe front direction or for a di�use ®eld, there are di�erent spectral componentswhich lead to di�erent LEV. Due to the fact that the dominant energy part of thesound for the ASW listening test comes from the front direction the e�ect of thedi�erent characteristics of the three arti®cial heads is small compared to the e�ect onLEV. The in¯uence of di�erent arti®cial heads should be realized when makingmeasurements for LEV, but ®nally this leads back to the problem of designing anoptimal arti®cial head.Two binaural playback systems have been compared. If equalization is made

carefully the results using headphones are comparable to CTC although the locali-zation abilities of the subjects di�er depending on the playback technique. In ouropinion cross-talk-cancellation is the best with regard to a better virtual image of thesound source and less potential for in-the-head localization. Due to the more com-fortable hearing situation and the advantages mentioned above the test personspreferred CTC.The listening tests have shown that di�erent source signals have an in¯uence on

the rating of ASW. The e�ect of di�erent sound pressure levels with a dependence ofabout 2�/dB shows results comparable to Keet (1.6�/dB). The in¯uence of di�erentsource signals is related to psychological as well as measurable reasons. Due to thatfact, either the listening test have to be done with many di�erent source signals orone should apply a consistent source signal at a certain source level to make di�erenttests comparable.

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Synthetic soundfields for the rating of spatial perceptions