Sound Source Localization by 3-D Sound Intensity Measurement using a 6-channel microphone system Part 2: Application in room acoustics

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1 Sound Source Localization by 3-D Sound Intensity Measurement using a 6-channel microphone system Part 2: Application in room acoustics OHTA Tatsuya a, ANO Hiroo b, OKOAMA Sakae c and TACHIBANA Hideki d a Graduate school, Chiba Institute Technology Tsudanuma , Narashino, Chiba ,Japan b,d Chiba Institute of Technology, Faculty of Information and Computer Science Tsudanuma , Narashino, Chiba ,Japan c Institute of Industrial Science, University of Tokyo Komaba 4-6-1, Meguro-ku, Tokyo ,Japan ABSTRACT In room acoustics, a lot of information would be obtained if the directions of the direct sound and early reflections could be determined. For this aim, the applicability of the 6- channel microphone technique using six cardioid-type uni-directional microphones was examined. With this measurement technique, the instantaneous sound intensity in a 3- dimensional space can be detected. In this study, the applicability of this method to room acoustic measurements was investigated by performing basic experiments to analyze the direct sound and discrete early reflections. As another application of this technique, sound source localization in a room was investigated. 1 INTRODUCTION The authors have been developing a 6-channel recording/reproduction system to simulate the sound in a real field in an acoustic laboratory by reproducing 3-dimensional auditory impression [1,2]. As the sound receiving device in this system, the 6-channel microphone system consisting of six cardioid-type uni-directional microphones combined at every 90 degrees in the horizontal and vertical planes have been designed. By the use of this microphone system, the instantaneous sound intensity can be determined [3,4]. This technique can be applied to various kinds of measurements of sound source localization. In this study, its application to acoustic measurements in rooms is examined by three kinds of experiments. 2 MEASUREMENT PRINCIPLE Figure 1 shows the latest type of the 6-channel microphone system developed by the authors. Through this microphone system, the sound pressure and the particle velocity of an incidence sound can be obtained and accordingly the sound intensity vector can be determined, in principle. Its real performance was checked by a laboratory test and it has been confirmed that the measurement principle hold true in the middle frequency range (octave bands of 250 Hz, 500 Hz and 1k Hz). a address: ohta@acoust.cs.it-chiba.ac.jp b address: yano@acoust.cs.it-chiba.ac.jp c address: sakae@iis.u-tokyo.ac.jp d address: pon-t@iis.u-tokyo.ac.jp

2 55 mm Figure 1: 6-channel Microphone System (Sanken, CU-6CH). 3 EPERIMENTAL INVESTIGATIONS In general, the sound field in a room is much complicated because of the existence of a lot of reflections following the direct sound. To investigate the effectiveness of separation of direct sound and reflections and the sound source localization by the use of the 6-channel microphone technique in rooms were examined by performing the following three kinds of basic experiments. 3.1 Separation of the Direct Sound and a Reflection A basic study was performed in a hemi-anechoic room under the experimental condition shown in Fig. 2. In this experiment, a swept-sine signal was radiated from the loudspeaker, and the 6-channel impulse responses were measured at the receiving point through the 6- channel microphone system. The measurement results for the octave band of 1k Hz are shown in Fig. 3; (a) shows the sound pressure impulse response obtained by summing up the outputs from the six microphones, and (b) shows the magnitude of the instantaneous sound intensity on x- and y- axes. As shown in these results, the direct sound and the sound reflected by the reflector were clearly separated in time domain, and then the sound intensity vectors for each part of the direct sound and the reflection were calculated by setting a time window of 5 ms which corresponds to the time for five waves of the sound of 1k Hz. The intensity vectors are shown in (c), in which it is seen that respective vectors indicate the directions of the direct sound and the reflected sound with a small angel error. Reflector(W=0.7 m) Hemi-anechoic room Reflector 5.1 m 4.5 m 4.5 m 6-channel microphone Loudspeaker 6-channel microphone m Loudspeaker 4.8 m Figure 2: Experiment of separation of the direct sound and reflection

3 Direct sound Reflected sound 5 ms 5 ms 50 ms (a) Sound pressure -axis Reflector Reflected sound (b) Instantaneous sound intensity -axis Direct sound (c) Intensity vector Figure 3: Result of separated direct sound and reflection 3.2 Analysis of Early Reflections in a Concert Hall In concert-hall acoustics, the construction of early reflections is very important. As a method to investigate the directions and magnitudes of these early reflections, the 6-channel microphone technique was applied. The measurement was performed in a middle-sized concert hall with a volume of 6,060 m 3 and 511 seats shown in Fig. 4. A dodecahedral loudspeaker was placed at the center position on the stage and the 6-channel microphone system was set at the receiving point in the audience area. Under this measurement arrangement, the 6-channel impulse responses in 2F Balcony seats (BL) 1F 2F Balcony seats (BR) Sound Source Measuring Point Dodecahedral loudspeaker 10m Figure 4: Measurement of impulse response in Concert Hall 6-channel microphone system

4 2-octave band covering the two octave bands of 500 Hz and 1k Hz were measured by the swept-sine method. The measurement results are shown in Fig. 5; (a) shows the sound pressure impulse response and (b) shows the magnitude of the instantaneous sound intensity and its components on x-, y-, and z axes, respectively. In the results shown in (a), it is almost impossible to separate the direct sound and the early reflections, whereas in the results shown in (b), the separation seems to be possible to some extent by examining the magnitude and the direction of the sound intensity in detail. In such a way, the instantaneous intensity was 100 ms (a) Sound pressure Magnitude Horizontal plane 0~50 ms 1 : Direct sound 2~: Reflections 5 11, axis Horizontal plane 50~100 ms -axis Vertical plane 0~50 ms Z 1 : Direct sound 2~: Reflections 8 9 5,6 4 3,7 1 2 (b) Instantaneous sound intensity Z-axis Z Vertical plane 50~100 ms , (c) Intensity vector Figure 5: Result of analysis of early reflections in Concert Hall

5 divided into some time intervals as shown in (b) and the magnitude and the direction of the sound intensity in each time interval was calculated. The results are shown in (c), in which it is seen that the direct sound was identified at the right angel set in the measurement arrangement and some predominant early reflections with different magnitude and direction were detected. (When observing the result by animation, the change of the intensity vector can be demonstrated.) In this kind of measurements, however, the results could subtly vary depending on the way of setting the time intervals to evaluate respective reflections. This point should be further examined. 3.3 Sound Source Localization in a Room In robotic engineering, it is desirable that the position of a sound source (e.g. a speaker) can be correctly detected. Under such an assumption, another application of the 6-channel microphone technique was investigated experimentally. The experiment was performed in a meeting room as shown in Fig. 6. As shown in Fig.7, a head and torso simulator (B&K Type 4128C) was used as the sound source to simulate a speaker and an announcement of radio news by male announcer was radiated from the loudspeaker set in the artificial mouth. Under such a condition, the instantaneous sound intensity measured through the 6-channel system was analyzed for the 2-octave band covering the two octave bands of 500 Hz and 1k Hz. The results are shown in Fig. 8, in which (a) shows the temporal variation of the sound pressure level measured at the receiving point, (b) shows that of the magnitude of the instantaneous sound intensity, and (c) shows that of the incidence angle in horizontal plane. From the result of the temporal sound intensity analysis, there might be various methods to identify the direction of the sound source. In this paper, the result obtained by a simple technique is presented. That is, the frequency distribution of the angle of the instantaneous sound intensity vector was analyzed. As a result, the direction to the artificial speaker was detected approximately as shown in (d). Head and torso simulator 8.5 m m 6-channel microphone Figure 6: Meeting room 4.8 m Figure 7: Configuration of measurement

6 5 s (a) Sound pressure level Incideence angle [deg.] direction -30 deg. 5s (b) Sound intensity level (c) Calculated incidence angle Detected angle -25 deg. Frequency (d) Frequency distribution of calculated incident angle Figure 8: Result of sound source localization 4 CONCLUSIONS In this study, the applicability of the 6-channel microphone technique in room acoustic measurements was investigated by performing two kinds of experiments: separation of the direct sound and reflections in a room, and sound source localization in a room. As a result, it has been suggested that this measurement technique can be applied to room acoustic measurements. However, since the sound field in a room is generally much complicated by reflection and interference, the analysis method of sound intensity, especially for averagingtime interval, and the interpretation of the measured results should be further investigated. 5 REFERENCES [1] S. okoyama, K. Ueno, S. Sakamoto and H. Tachibana, 6-channel recording/reproduction system for 3-dimensional auralization of sound fields, Acoust. Sci. & Tech., 23(2), (2002). [2] S. okoyama, H. ano and H. Tachibana, 6-channel recording/reproduction system for 3-dimensional auralization and it s applications to psycho-acoustical experiments, Proceedings of INTER-NOISE 06, paper in 06_411, 2006 [3] T. Ohta, S. okoyama, H. ano and H. Tachibana, Sound intensity measurement using 6-channel microphone system, Proceedings of autumn meeting Acoust. Soc. Jpn. (2007.9) pp (in Japanese) [4] ANO Hiroo, OHTA Tatsuya, OKOAMA Sakae and TACHIBANA Hideki, localization by 3-D sound intensity measurement using a 6-channel microphone system Part 1 :Principle and basic applications, Proceedings of INTER- NOISE 08, paper in08_518,