Acoust. Sci. & Tech. 23, 2 (2002) TECHNICAL REPORT

6-channel recording/reproduction system for 3-dimensional auralization of sound fields

Sakae Yokoyama1;*, Kanako Ueno2;{, Shinichi Sakamoto2;{ and Hideki Tachibana2;} 1Graduate School, University of Tokyo, Komaba 4–6–1, Meguro-ku, Tokyo, 153–0041 Japan 2Institute of Industrial Science, University of Tokyo, Komaba 4–6–1, Meguro-ku, Tokyo, 153–0041 Japan ( Received 13 August 2001, Accepted for publication 11 October 2001 )

Abstract: In order to simulate three-dimensional sound fields in laboratory experiments, a 6-channel recording/reproduction system has been contrived. To record the sound in a real sound field, six uni- directional combined at every 90 degrees are used. As the reproduction system, six are set in an anechoic room and the recorded signals in each direction are reproduced. The advantages of this system are that the principle is quite simple and the listening area is not strictly limited. In this paper, the principle of the system and the reproduction accuracy are reported.

Keywords: 3-dimensional sound field simulation, 6-channel recording/reproduction system, Aur- alization

PACSnumber: 43.50.Rq, 43.55.Gx, 43.55.Lb, 43.55.Mc, 43.66.Qp, 43.66.Yw

are recorded through a system consisting of six 1. INTRODUCTION uni-directional microphones onto a tape When performing subjective judgment tests on room recorder and then reproduced through six loudspeakers in acoustics and environmental noise problems, it is desirable an anechoic room [4–6]. This system (called 6-ch. system, to reproduce the actual sound fields under consideration hereafter) is very simple in principle and has the advantage with 3-dimensional information. For this purpose, such that the constraint on the listening position is relatively auralization techniques as the trans-aural system using a easy. In this paper, the principle and reproduction accuracy head and torso simulator (so called dummy head micro- of the system are presented. phone system) and a 2-channel reproduction system have been designed [1–3]. For the trans-aural system, however, 2. OUTLINE OF THE RECORDING/ head-related transfer-functions for both ears have to be REPRODUCTION SYSTEM measured for each listener, and complicated signal Figure 1 shows the outline of the 6-ch. system processing is needed to cancel the cross-talk components schematically. The microphone system consists of six between the 2-channel system and both ears. uni-directional microphones combined at every 90 degrees In addition, the listener’s head has to be fixed when in the horizontal and vertical planes. Figure 2 shows an applying this method. The 2-channel signals recorded example of the system which consists of six uni-directional through a dummy head system can be reproduced through microphones (, C-48) with cardioid directional , but in this case the accuracy of sound source characteristic (see Fig. 3). In principle, the microphones localization is not sufficient and it is not easy to control the should be combined as closely as possible. In this case, the sound pressure in the headphones on the listener’s head. distance between the opposite microphones is 135 mm. As Accordingly, the authors have invented a new record- seen in Fig. 2, an omni-directional microphone of a sound ing/reproduction system, in which environmental sounds level meter is located at the center point of the microphone set for the measurement of the absolute sound pressure level in the sound field under measurement. *e-mail: [email protected] {e-mail: [email protected] The 6-channel signals recorded onto a digital audio {e-mail: [email protected] (SONY, PC208A) are reproduced through six }e-mail: [email protected]

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Fig. 2 6-ch. microphone set composed of 6 uni-direc- tional microphones (SONY C-48).

vertical planes in the same way as for the microphone set. In this 6-ch. system, any special signal processing is unnecessary in principle, but six spectrum-equalizers (SONY, SRP-E210) are used to correct the frequency characteristics of the total system to be within Æ1 dB between octave bands from 63 to 4 kHz. 3. REPRODUCTION ACCURACY 3.1. Reproducibility of the Direction of Arriving Sounds In order to examine the accuracy of the sound field reproduction by the 6-ch. system, the following physical Fig. 1 Outline of the 6-ch. recording/reproduction system. and subjective studies were performed. For these studies, impulse responses at every 15 degrees around the receiving system in the horizontal plane were measured in the loudspeakers (TANNOY, T12) arranged on a spherical anechoic room using the time stretched pulse (TSP) surface of 2 m radius in an anechoic room. The loudspeak- technique [7,8]. In this measurement, a loudspeaker sound ers are arranged at every 90 degrees in the horizontal and source was located at a point 7 m apart from the receiving

Fig. 3 Directivity characteristic of the uni-directional microphone (SONY C-48) measured in octave bands using a pink noise.

98 S. YOKOYAMA et al.: 6-CHANNEL RECORDING/REPRODUCTION SYSTEM system. measured in the same way as in the former case. The (1) Sound intensity vector in the horizontal plane results are shown in Fig. 5, in which it is seen that the The 6-channel impulse responses for each direction of reproducibility of the intensity vector has much improved the sound source measured through the 6-ch. microphone in 1 kHz and 2 kHz bands, whereas some errors still remain system were convolved with a pink noise. These synthe- in 4 kHz band. This result indicates that the uni-directional sized signals were reproduced from the 6-channel loud- microphones should be combined as closely as possible. speaker system and the sound intensity in each octave band (2) Subjective judgment of the direction of incident sound in the horizontal plane was measured through an intensity Next, the reproducibility of the 6-ch. system was probe (B&K, 4181) at the center point of the reproduced examined by subjective experiment. For this experiment, sound field. In this measurement, the sound intensities in x- the 6-channel impulse responses measured in every 30 and y-directions were measured separately and the intensity degrees were convolved with an intermittent pink noise (1 s vector was obtained from these results. on-time and 0.5 s off-time, three bursts) and they were Figure 4 shows the measurement results for the sound reproduced by the 6-channel loudspeaker system. The intensity vector. In these results, it is seen that each vector subject sitting at the listening point was asked to judge the points in the true direction in the cases of octave bands direction of the test sound. In this experiment, two kinds of from 125 to 1,000 Hz, and the magnitude of the vector is arrangement of the recording and reproduction systems almost uniform in every direction. In the 2 kHz and 4 kHz were examined. They are the ‘‘plus-figure arrangement’’ in bands, however, some discrepancies are seen. This error which the four microphones and loudspeakers in the can be attributed to the fact that the microphones of the horizontal plane are set as shown in Fig. 6(a) and the ‘‘X- receiving system are arranged with finite distance between figure arrangement’’ in which they are set as shown in Fig. them as mentioned above. 6(b). As test subjects, seven graduate students with normal To examine this finite difference error, the 6-channel hearing ability participated in this experiment. When impulse responses were again measured using a uni- judging the direction of the sound, the subjects were directional microphone by rotating it in 90 degree steps at allowed to turn their heads. the measurement point. After the same convolution The average of the judgment results by all the subjects processing as mentioned above, the intensity vector was is shown in Fig. 7, in which the diameter of each circle

Fig. 4 Reproducibility of sound intensity vector (The 6-ch. microphone set was used).

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Fig. 5 Reproducibility of sound intensity vector (One uni-directional microphone was rotated in each direction).

Fig. 8 Results of the judgment test on the direction of incident sound (X-figure arrangement).

Fig. 6 Arrangements of the recording and reproduction system. arrangement for practical arrangement in the laboratory. Therefore, it was decided to adopt the X-figure arrange- ment as the standard experimental configuration and then the direction judgment test was again performed. In this experiment, the judgment test for the direction in the median plane was added to that in the horizontal plane. As test subjects, ten graduate students participated in this experiment. The experimental results are shown in Fig. 8, in which error judgments are scarcely noticeable. As a result, it can be said that the direction of the incident sound can be correctly judged using the 6-ch. recording/ reproduction system.

3.2. Reproducibility of the Sound Fields in Halls Fig. 7 Results of the judgment test on the direction of As an application of the 6-ch. system to actual sound incident sound. fields, 6-channel impulse response measurement was performed in the four concert halls shown in Table 1.In indicates the relative number of the response. As seen in this measurement, a dodecahedral omni-directional loud- these results, it can be said that the direction of the incident speaker system was set at the center point of the stage and a sound was correctly judged by the subjects in both TSP signal was generated. Based on the experimental study recording/reproduction arrangements. In a comparison mentioned above, the X-figure arrangement was also between the two arrangements, the X-figure arrangement adopted in this case. seems to be more convenient than the plus-figure The impulse response signals measured in the original

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Table 1 Outline of the concert halls under measurement. field. In other cases, almost the same resemblance was Variation Volume (m3) Seats Reverberation obtained. time (s) As typical room acoustical indices, the values of A 21,000 2,006 2.5 ‘‘definition’’ D50, ‘‘clarity’’ C80, ‘‘center time’’ TS and B 14,800 1,521 1.6 reverberation time in the two-octave band from 354 Hz to C 5,800 552 1.9 1.4 kHz were calculated from the impulse responses D 19,600 1,800 2.5 measured in the original sound field and the simulated E sound field. Figure 10 shows the correspondence of the values between the original sound fields and the simulated sound field. In every result, the plots lie almost on the 45 sound fields were reproduced in the anechoic room and the degree line; this means that the constructions of impulse impulse responses at the center point of the simulated response in the original sound fields are well reproduced in sound field were measured through an omni-directional the simulated sound field. microphone and a dummy head system [Head Acoustics, (2) Study of the listening area by subjective test HMM-II]. As the actual listening condition in the simulated sound (1) Echo-diagram and room acoustical indices field, it is desirable that the listener be able to move his/her Figure 9 shows two examples of the comparison of head to some extent. In order to examine the extent, the echo-diagram (envelope of the impulse response) between following subjective experiment was performed using the the results measured in the original sound fields and those binaural recording/reproduction technique. measured in the simulated sound field in the laboratory. At first, the binaural impulse responses were measured The envelope wave forms were obtained by passing the in the simulated sound field using the dummy head impulse response signals through a numerical RMS microphone system by changing its position from the detector with a 1 ms time constant. In these figures, it can center point to the point 50 cm from the center in 10 cm be visually judged that the echo-diagrams in the original steps. For each hall, six pairs between the result for the sound fields are well reproduced in the simulated sound center point and that for each of other points (including the center point) were made and pair comparison tests were performed. In this experiment, two kinds of test sounds

Fig. 10 Correspondences of typical room acoustical indices between the results measured in the original Fig. 9 Reproducibility of echo-diagram in concert halls. sound field and those in the simulated sound field.

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Table 2 Five step categories for the pair comparison test.

These two sounds are 1) not different at all. 2) slightly different. 3) moderately different. 4) very different. 5) extremely different.

were used: one is the raw impulse response and the other is the sound synthesized by convolving the impulse response with violin music (Gavotte by J. S. Bach) of 6 s duration. These test sounds were presented to the subject through a headphone set and the subject was asked to give his/her impression in the five step categories shown in Table 2. Five graduate students participated in this experiment as the test subjects. In the experimental results, high repeatability was found in every result by each subject and mutual correlation between each subject was high; therefore, the arithmetic average of the category number answered by all the subjects was calculated. The results are shown in Fig. 11, in which it is seen that the subjective difference increases with the increase of the distance from the center point. Assuming that categories 1) and 2) are within the permissible range, 20 cm for the impulse response sound and 30 cm for the music are the permissible distance from the center point. This result indicates that the listener may Fig. 11 Results of the pair comparison test on the move his/her head within these areas in the simulated difference of hearing impression. sound field. 4. CONCLUSIONS general audio recording/reproduction system. In such a case, 6 channels are not necessarily needed and 5 channels The principle and the accuracy of sound field (4 in the horizontal plane and 1 overhead) or 4 channels (all reproduction of the newly developed 6-channel recording/ in the horizontal plane) would also be effective. reproduction system have been presented. Although very When making recordings in actual fields, the receiving simple in principle, this system can accurately reproduce system should be as small as possible from a practical the original sound fields with 3-dimensional information viewpoint and the authors are now working on the and therefore it can be applied to various kinds of psycho- miniaturization of the microphone system. acoustic experiments. The authors have been applying this simulation technique to such psycho-acoustic experiments ACKNOWLEDGEMENTS as the study of acoustic environments in public spaces [9], The authors would like to thank Hikari Mukai, noisiness of road traffic noise [10] and loudness and Katsuhiro Yasuda and Masato Ikeda for their assistance annoyance assessment of HVAC noises [11,12]. As a in examining the performance of the system. similar recording/reproduction system to that introduced This research was partially supported by Grant-in-Aid here, the authors have designed a method to detect 6- for Scientific Research (B07455231, B09450214) from the channel impulse responses at the player’s position on the Ministry of Education, Science and Culture, Japan. stage by rotating a uni-directional microphone and simulate the acoustical conditions on the stage. By means of this REFERENCES simulation technique, the authors have been making [1] M. R. Schroeder and B. S. Atal, ‘‘Computer simulation of investigations on stage acoustics [13]. sound transmission in rooms’’, IEEE Conv. Rec., pt. 7, 150 Although the 6-ch. system has been developed as an (1963). auralization tool for the psycho-acoustic experiments as [2] D. H. Cooper and J. L. Bauck, ‘‘Prospects for transaural recording’’, J. Audio Eng. Soc., 37, 3 (1989). mentioned above, this technique could be expanded to a [3] H. Hamada, N. Ikeshoji, Y. Ogura and T. Miura, ‘‘Relation

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