Modern auralization techniques allow to make better assessment of particular interior type for different destination and purposes. The quality and reality of acoustic recording and reproduction systems increase so the results of this kind of research are much more reliable. The article shows the psychoacoustic comparison of different reverberant interiors. The auralization was provided using 1st order ambisonics spatial impulse responses convoluted with anechoic choral music. Listening tests were conducted within the 16-channel sound system. The subjects were tested using the pair comparison method and the results were analyzed with the multidimensional scaling approach

Malecki, Pawel

DepositOnce (Techn. Univ. Berlin)

Proc. of the EAA Joint Symposium on Auralization and Ambisonics, Berlin, Germany, 3-5 April 2014     AURALIZATION OF SEVERAL CHURCHES AND LISTENING COMPARISON USING MULTIDIMENSIONAL SCALING APPROACH. TEMPLATES FOR DAFX04, NAPLES, ITALY Pawel Malecki   1Department of Mechanics and Vibroacoustics, AGH – University of Science and Technology Krakow, Poland pawel.malecki@agh.edu.pl    ABSTRACT Modern auralization techniques allow to make better assessment of particular interior type for different destination and purposes. The quality and reality of acoustic recording and reproduction systems increase so the results of this kind of research are much more reliable. The article shows the psychoacoustic comparison of different reverberant interiors. The auralization was provided using 1st order ambisonics spatial impulse responses convoluted with  anechoic choral music. Listening tests were conducted within  the 16-channel sound system. The subjects were tested using the  pair comparison method and the results were analyzed with the multidimensional scaling approach. 1. INTRODUCTION Although there is an increase of popularity and diversity of  modern auralization techniques, they are not very often employed in a room acoustics validation and evaluation. Early works on that subject were conducted using  few channels sound systems with many simplifications [1], [2]. Modern ambisonics tech-niques [3], [4], sound-field synthesis [5] and other complex systems [6], [7] are well known and commonly exploited but hardly to previously mentioned goals. Actual work focuses on that subject with respect to orthodox churches, so on interiors, where the acoustics features are extremely important and con-nected with a sense of characteristic space. The chosen orthodox churches from the southern and eastern parts of Poland were examined in the context of the orthodox choral music. The Or-thodox church (referred as a building) is inseparably connected with spirituality and  philosophy of the eastern Christianity. Its architecture is based on orthodox liturgy and implicates a natural cultural landscape of the east part of Europe. The acoustic meas-urements in orthodox churches were marginalized by now and only a few buildings were examined.   The first order ambisonics is employed in a measurement pro-cess and in listening experiments. The article shows the research methodology and the main statistically developed results.   2. MEASUREMENTS OF SPATIAL IMPULSE RESPONSES IN THE CHOSEN ORTHODOX CHURCHES IN POLAND  Measurements of SIRs (Spatial Impulse Responses) were con-ducted in eleven chosen orthodox churches in Poland from June to October 2012.  The measurements and the overview of basic features were pro-vided for the churches with  different architecture style, size and  interior design. In order to measure SIRs, EASERA Pro software was employed as well as authors algorithms implemented with the Matlab computing language. The ISO3382 standard [8] was followed during the measurements as possible. Due to the specif-ic goals of the research, some simplifications were made. As it was mentioned before, first order ambisonics microphone - Soundfield ST350 was used, and instead of an omni-directional source, an active loudspeaker JBL EON 515 was employed. Because of the main destination of SIRs (listening experiments), the number of measurement locations was also limited. The main placement of the measurement instruments was common for all the churches. The sound source was placed in front of the  ico-nostasis and the microphone behind a tetrapod. This is a typical place where a priest, a deacon sings or the choral music concerts take place. Behind the tetrapod (a small table about six meters in front of the iconostasis) there are the best first seats for the audi-ence. On the basis of the B-format pressure component, the room acoustic parameters were also calculated and some of them are listed in table 1. Except for  the  previously explained setup, the measuring points were located at characteristic places for ortho-dox churches: • in a central point of the church (at least at two different po-sitions), • under the main dome, • under the gallery,  • at the back of the church,  • at a side naves (at least three different positions), • at other positions if the church shape suggested this. Except for the small local wooden churches at Zdynia and Konieczna, all of the churches were made of bricks. The most Latin style characterizes the medium size Hole Trinity Church in Sanok. The churches in Wlodawa and Hrubieszow are the exam-ples of the Byzantine-Russian style. The church in Tomaszow Lubelski is also a medium size as well as a St. Peter and Paul in Siemietycze, both in the Neo-Russian style. The churches in Bialystok, Hajnowka and the Resurrection Church in Siemiatycze are the largest ones.. The church in Hajnowka is well known from the International Festival The Hajnowka’s Orthodox Church Music Days were best choirs from all over the world perform. The church of Divine Wisdom in Bialystok is interesting due to the fact that it is a miniaturized copy (1:3) of Hagia Sophia in Istanbul.   151Proc. of the EAA Joint Symposium on Auralization and Ambisonics, Berlin, Germany, 3-5 April 2014   Table 1: Acoustic parameters of church SIRs used for listening tests.  No. Church of the…\ Parameter RT [s] C80 [dB] C50 [dB] ts [ms] LEF STI G Vol. [m3] 1 Holy Trinity in Sanok 1,83 4,2 2,3 65 0,47 0,58 7,3 1650 2 Sacred Virgin Mary in Włodawa 2,09 2,6 0,8 93 0,57 0,58 6,6 2400 3 Assumption in Hrubieszow 1,82 4,6 2,5 59 0,21 0,61 3,3 1700 4 St. Nicholas  in Tomaszow Lubelski 2,87 3,4 1,7 105 0,25 0,58 3,8 3050 5 St. Peter and Paul in Siemiatycze 2,2 0,7 -1,2 109 0,76 0,51 7,7 1500 6 Holy Spirit in Bialystok 6,53 0,2 -0,4 235 0,22 0,49 6,6 9500 7 Holy Trinity in Hajniwka 4,59 3,9 3,0 106 0,1 0,56 3,7 6050 8 Resurrection in Siemiatycze 4,74 3,9 3,0 106 0,14 0,56 5,8 6400 9 Divine Wisdom in Bialystok (Hagia) 6,14 0,9 -0,7 198 0,27 0,46 5,7 5950 10 Protection of the Mother of God in Zdynia 1,07 6,9 4,1 41 0,51 0,68 4,7 750 11 St. Basil in Konieczna 1,17 7,7 5,4 37 0,46 0,68 5,2 900 3. LISTENING SETUP For the listening test, the 16-channel setup was prepared using the RME converters and Genelec 6010 monitors. The selected loudspeakers are quite small but their sensitive is 93 dB SPL with flat frequency response from 74 Hz to 18 kHz (±2,5 dB). The loudspeakers are spherically placed around the listener, whose configuration is shown in Fig. 1. This setup was installed at the anechoic room of AGH-UST using microphone stands with sphere diameter of 3,2 meters.  Figure 1: The idea scheme of loudspeakers placed on sphere configuration The system was positioned with a laser angular meters and then calibrated and phase checked.   Figure 2: Listening stand overview photo   Loudspeakers of the system are situated on a sphere around a listener in three planes – above, under and on the height of listen-er’s head. Exact angular coordination in respect to the listener are presented in the table 2. Table 2: Loudspeakers angular coordination in respect to the listener position Loudspeaker no. φ [°] θ [°] 1 0 0 2 45 0 3 90 0 4 135 0 5 180 0 6 -135 0 7 -90 0 8 -45 0 9 68 -45 10 158 -45 11 -113 -45 12 -23 -45 13 23 45 14 113 45 15 -158 45 16 -68 45 4. STIMULI PREPARATION The human voice is often regarded as one of the most beautiful musical instruments. When the human voice is multiplied (like in choir), as the result of this mixture there is a sound of a very rich and interesting timber. Choral music is inseparable from Ortho-dox church and its tradition so as a samples in listening test, choral music exerts were used.  4.1. Raw sounds recording In order to obtain raw audio material (without any reflections or other type of response), choral free field recording were made. Recording of chamber orthodox choir (8 singers) was performed at anechoic room of AGH-UST. The whole choir was recorded using multitrack setup using one type of microphones (Rode NT5 with cardioid capsule) for each voice. Soprano, alto, tenor and bass tracks were separately recorded and prepared for next re-152Proc. of the EAA Joint Symposium on Auralization and Ambisonics, Berlin, Germany, 3-5 April 2014   search stages. Recorded music samples were in moderate tempo and not complex harmony, with neutral, and universal lyrics (halleluiah). 4.2. Raw audio samples convolution with SIRs Finally, obtained SIRs were convoluted with a raw choir music samples. In order to place each voice in the right position of sound plan, each convolution was performed for every voice separately. As a result there were 176 convolutions (11 churches x 4 ambisonics components x 4 choir voices). After that, the B-format tracks were converted to 16-channel setup, acording to [3] and the coordinates shown on table 2. Choir voices placement were performed using VBAP (Vector Based Amplitude Panning) technique [9]. Voices (phantom images) were placed typically for regular SATB choir according-ly to data in tables 2 and 3. Angular positions of particular voices in table 2 are the trigonometric result of typical distances be-tween singers and 6.5 m distance from listener position. Table 3 shows calculated gain factors for specific loudspeakers of listen-ing setup, based on VBAP. Table 3: Gain factors of specific loudspeakers of setup Voice φ [°] Loudspeaker no. Gain factor Soprano -20 1 0,78 8 0,63 Alto -7 1 0,98 8 0,19 Tenor 7 1 0,98 2 0,19 Bas 20 1 0,78 2 0,63 5. LISTENING TEST The main purpose of the performed psychoacoustic procedure is to ranking measured churches in sense of acoustic esthetics. It is difficult to precise this kind of criteria. So, in order to find out relations between the listener preferences and the physical fea-tures of the rooms, the MDS (Multidimensional Scaling) method was employed [10]. Obtaining data about objects similarity and preferences for MDS analysis was performer using paired com-parison. For 11 churches, there are 55 unique possible pairs to compare. Each test round was designed using 2IFC procedure (2 stimuli, intervaled forced-choice). After two choral music ex-cerpts of length about 15 s each, located in different churches, the listener’s task was to choice which of them sounds better and how much do they differ. To clarify and unify listeners task, the exact question was stated before each survey: • How similar are the following sound excerpts representing different interiors, where 0 denotes to totally different and 100 refers as identical? {similarity test} • In which of presented auralized interiors would you prefer to listen to this kind of music? In other words: Which of the presented sounds you like more? {preference test} Listeners answered to these questions after each round of two sounds using tablet touch interface. Fig. 3 shows the application form window used during the tests.  Figure 3: Form window used during the tests Actual tests were followed by a few training rounds to introduce test subjects with exemplary stimuli and to practice whole test procedure. Test subjects were placed in the center of loudspeaker sphere individually to achieve similar sweet spot condition for  every listener. During the tests, only the subject and the operator remained in the anechoic chamber. Whole test last for about 40 minutes so a short break in the middle of it was obligatory. Every test round was introduced by a lector, and the listeners had the opportunity to repeat every round if needed. Listening group counted of 20 people from 20 to 35 years old within 6 female. Listeners had different listening or acoustics experience and different confession. None of the listeners had impaired auditory system.  6. STATISTICAL SIGNIFICANCE OF RESULTS Actual work discusses results of a preference test only. First step of statistical analysis is the correlation between personal listeners results. Average value of Pearson’s coefficient equals 0,38 for n = 20. Zero hypothesis which assumes no correlation between answers  of listeners was not rejected in 40 cases on 180 possible pairs, at 0,95 significance level. The conclusion is that alternative hypothesis assuming significant correlation of the particular results cannot be accepted. For more intuitive and visual correla-tion analysis, metric MDS [10] is used to visualize correlation matrix (fig. 4). It could be noticed that there are two clusters of subjects relatively close to each other. This suggests to perform analysis with respect to noticed division – separately for both groups, marked in fig. 4 as group A and B.   Fig 4: Graphical interpretation of correlation matrix of results of particular subjects   Group A Group B 153Proc. of the EAA Joint Symposium on Auralization and Ambisonics, Berlin, Germany, 3-5 April 2014   7. RESULTS Collected data was processed in two ways. Fig. 5 shows prefer-ences test results using Thurstone-Mosteller’s least square ap-proach [11]. Presented results are based on the probability analy-sis of listeners preferences in paired comparison test. Mentioned probability is calculated on the base of quantity when object i, was marked as preferred accordingly to other object j. This kind of scaling is performed for both statistically significant groups of subjects.   Fig 5: Listeners preferences calculated using Thurstone-Mosteller’s least square approach of data obtained in paired compari-son test  After comparison of diagrams in fig. 5 it could be noticed that the main difference between considered groups is for objects 6-11. Group A prefers objects 10 and 11 much more than objects 6, 8 and 9. Opposite tendencies can be noticed for group B. Both groups prefers objects 2 and 4 the most.  Second approach to calculation is based on non-metric MDS scaling. Difference between objects is calculated on the basis of probability of preferences according to transformation in [10]. Graphical interpretation of preferences for both statistical groups are shown in fig. 6. Applying non-metric MDS scaling within 2 dimensions resulted with stress factors of 0,11 for both groups. For random data and the same analysis parameters stress factor would equals 0,21. So according to [10], actual fitting is rated as good.   Fig 6.  Graphical interpretation of preferences for both statistical groups calculated using non-metric MDS  For both diagrams in fig 6 there are two geometrically arranged aggregations of objects and some separated ones. For group A this separated objects are very big and reverberant and for group B very small and acoustically dead compared to others. On the basis of fig 5 and 6 it can be concluded that noticed statistical groups differs mostly in sense of negative preference. Analysis of individual subjects does not suggest any special features of any groups (age, acoustic-music experience, sex etc.) Smaller group Scaled preferences Group A Scaled preferences Group B   Church no. Church no. 154Proc. of the EAA Joint Symposium on Auralization and Ambisonics, Berlin, Germany, 3-5 April 2014   B contains three orthodox listeners versus one orthodox in a group A. This clue seems to be not strong enough to make some general conclusion about it. 8. CONCLUSIONS Measurements of 11 SIRs in several orthodox churches were conducted as well as listening test using auralized samples of choral music. Tests were conducted in order to find listeners preferences about highly reverberant rooms in aspect of choral music. Auralization was performed using ambisonics and VBAP technologies. First order ambisonics was employed at every stage of research. Conducted acoustic measurements in orthodox churches in central Europe on that scale are one of the first.  Listening experiments were performed using psychoacoustic methodology and modern statistical MDS approach. Correlation analysis of results revealed two different groups of listeners. They had different preferences especially about extreme rever-berant conditions. They had completely opposite opinion about very long and short reverberation time in context of choral ortho-dox music. Based on conducted research very interested conclu-sion can be stated that listeners were divided not with aspect to what do they like, but rather in what do they not like. All listeners rated  medium size churches with big central dome as most pre-ferred. Future research on current subject is planned with em-ployment of higher order ambisonics and different approach in on place SIR measurement in order to achieve even more realis-tic auralization. 9. REFERENCES [1] Y. Ando, “Subjective preference in relation to objective parameters of music sound fields with a single echo”, J. Acoust. Soc. Am, vol. 62, pp. 1436–1441, 1977. [2] M. Barron, “The subjective effects of first reflections in concert halls – The need for lateral reflections”, J. Sound Vibr, vol. 15, pp. 475-494, 1971. [3] A. Farina, R. Glasgal, E. Armelloni and A. Torger, “Ambi-ophonic Principles for the Recording and Reproduction of Surround Sound for Music”, Proceedings of the 19th AES Conference on Surround Sound, Techniques, Technology and Perception, Schloss Elmau, Germany, 2001. [4] C. A. Dimoulas, “Sound source localization and B-format enhancement using soundfield microphone sets”, Proceed-ings of the 122th AES Convention, Vienna, Austria, 2007.  [5] S. Spors, H. Wierstorf, A. Raake, F. Melchior, M. Frank and  F. Zotter, "Spatial Sound With Loudspeakers and Its Percep-tion: A Review of the Current State", Proceedings of the IEEE, vol.101, no.9, pp. 1920 - 1938, 2013. [6] V. Pulkki, “Spatial Sound Reproduction with Directional Audio Coding”, J. Audio Eng. Soc., Vol. 55, pp. 503-516, 2007. [7] W. Woszczyk, D. Ko and B. Leonard, „Convolution-based virtual concert hall acoustics using aural segmentation and selection of multichannel impulse responses”, The Proceed-ings of INTER-NOISE 2009, Ottawa, Canada, 2009. [8] ISO 3382, Acoustics – Measurement of the Reverberation Time of Rooms with Reference to Other Acoustical Parame-ters. Geneva, ISO, 1997. [9] U. Zolzer et al., DAFX: Digital Audio Effects, John Wiley & Sons Ltd, 2011. [10] I. Borg and P. J. F. Groenen, Modern Multidimensional Scaling. Springer, 2005. [11] K. Tsukida and M. R. Gupta, How to Analyze Paired Com-parison Data. University of Washington, 2011. 155

Auralization of Several Churches and Listening Comparison Using Multidimensional Scaling Approach

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Auralization of Several Churches and Listening Comparison Using Multidimensional Scaling Approach

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