A DIAGNOSTIC SYSTEM MEASURING ORTHOGONAL FACTORS OF SOUND FIELDS IN A SCALE MODEL OF AUDITORIUM

M. SAKURAI
Graduate School of Science and Technology, Kobe University, 1 Rokkodai, Nada, Kobe 657-8501, Japan, and Yoshimasa Electronic Inc., 1-58-10 Yoyogi, Shibuya, Tokyo 151-0053, Japan

S. AIZAWA
Yoshimasa Electronic Inc., 1-58-10 Yoyogi, Shibuya, Tokyo 151-0053, Japan

Y. SUZUMURA AND Y. ANDO
Graduate School of Science and Technology, Kobe University, 1 Rokkodai, Nada, Kobe 657-8501, Japan




SUMMARY

Based on the model of auditory-brain system which consists of the autocorrelation mechanism, the interaural crosscorrelation mechanism between the both auditory pathways, and the specialization of human cerebral hemispheres [Ando, Architectural Acoustics, Blending Sound Sources, Sound Fields, and Listeners, AIP Press/Springer-Verlag, New York, 1998], a new diagnostic system was developed. After obtaining the binaural impulse response, four orthogonal factors including the SPL, the initial time delay gap between the direct sound and the first reflection, the subsequent reverberation time and the IACC can be analyzed for the calculation of the scale values of both global and individual subjective preferences. In addition, two more factors extracted from the interaural crosscorrelation function, τIACC and WIACC, can be figured out. Also, the sound energy, Φ(0), the effective duration, τe, and fine structures of autocorrelation function of sound signals including the magnitude of first maximum, φ1, and its delay time, τ1, can be analyzed. As an example of the measurement, effects of reflectors' array above the stage in a 1/10 scale model of auditorium at each seat are discussed here.



1. INTRODUCTION

In order to measure orthogonal factors, SPL, Dt1, Tsub, IACC, τIACC, and WIACC [1-6], and also the running ACF of sound field at each seat in a scale model as well as in a real auditorium, a diagnostic system is developed. Based on the model of auditory-brain system which consists of the autocorrelation mechanism, the interaural crosscorrelation mechanism between the both auditory pathways, and the specialization of human cerebral hemispheres as shown in FIGURE 1 [1], a diagnostic system was designed. 

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FIGURE 1. A model of auditory-brain system.

The system works on PC for Windows with an AD & DA converters, there is non need for special additional devices. After obtaining the binaural impulse responses, four orthogonal factors including the SPL, the initial time delay gap between the direct sound and the first reflection, the subsequent reverberation time and the IACC are analyzed. These factors are used for the calculation of both the scale values of global and individual subjective preferences. In addition to the four factors, two more factors, τIACC and WIACC as defined in FIGURE 2, extracted from the interaural crosscorrelation function can be figured out for evaluating the image shift of sound source and the apparent source width [6], respectively. 

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FIGURE 2. Definitions of the IACC, τIACC and WIACC in the interaural crosscorrelation function.

Also, the averaged sound energy, F(0), the effective duration, τe, defined by the delay at which the envelope of normalized ACF becomes 0.1 (FIGURE 3), and fine structures of autocorrelation function of sound signals including the magnitude of first maximum, f1, and its delay time, τ1, of source signals are analyzed. 

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FIGURE 3. A practical example of determining effective duration of ACF defined by the ten-percentile delay, with the straight line-fitting envelope of ACF from 0 to -5 dB.

In order to examine effects of reflectors' array above the stage in a 1/10 scale model of auditorium, the IACC measurements are demonstrated here.



2. OUTLINE OF A DIAGNOSTIC SYSTEM

Because the complex requirements made the system difficult to evaluate, an advanced diagnostic system and a high-power computer was used. The measuring system was utilized to obtain the binaural impulse response at each listening position. The sound was created using an omni-directional loudspeaker fed with a maximum-length signal produced by a diagnostic system in a notebook PC. The period of maximum-length signal (MLS) was between 1024 and 524288 samples, and the sampling rate can be changed between 8 kHz and 48 kHz. The acoustic signal amplified from the two microphones placed at the entrances of ears of a 1/10 scale model of dummy head (a sphere with the diameter of 25 mm) was sampled after passing through a low pass filter (FIGURE 4). The binaural-impulse-response measurement may be performed by a summation of the output data from the linea system, without any multiplication operation [7.8]. The measurement was done automatically within only a few seconds by pushing a single button. It took another few seconds for the analysis of the orthogonal acoustic factors and the scale value of the subjective preference. And at the same time this program can took the result to compute the acoustic parameters and prepare the reports.

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FIGURE 4. A block diagram of the measurement system.



3. MEASUREMENT OF ORTHOGONAL ACOUSTIC FACTORS

3.1. PROCEDURE

The diagnostic system developed may examine effects of scattered reflections of complex boundary conditions of the room. The reflectors above the stage are designed mainly for the performer obtaining the preferred reflections according to the program sources. We measured the IACC of sound field at each seat to find effects of reflectors' array above the stage [10]. The effective direction of reflections to listeners for the 2000 Hz range is centered on +- 18 degrees from the median plane, which might be realized by the reflectors above the stage [1]. Therefore, the IACC of the 2000 Hz-frequency band is selected here to be examined.
In order to obtain reliable results, measurements were repeated several times until obtaining the same results of the binaural impulse responses. The sound is produced using an omni-directional loudspeaker fed with the MLS produced by the diagnostic system in a notebook PC. FIGURE 5 shows the window on PC of actual diagnostic system with the data obtained from the impulse responses. In the measurement, a special attention should be made to maintain a suitable value of the signal to noise ratio adjusting the power level of the loudspeaker.

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FIGURE 5. An example of display window of the diagnostic system, with binaural impulse responses.



3.2. RESULTS

As mentioned above, in order to examine effects of reflectors' array on the IACC for the 2000 Hz range at 15 seating positions shown in FIGURE 6, measurements were performed with and without reflectors above the stage. As indicated in this figure, the location of the sound source is marked by a star, and triangular reflectors` arrays [10] are installed above the stage. FIGURE 7 shows the measured results of the IACC without reflectors, and FIGURE 6 shows those with reflectors` array. The IACC values of the 2 kHz-frequency band for a real room were measured at the 20 kHz frequency band in the 1/10 scale model.

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FIGURE 6. Measured IACC for the 2000 Hz-frequency band with reflectors above the stage.

　

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FIGURE 7. Measured IACC for the 2000 Hz-frequency band without reflectors above the stage.

As shown in these figures, the reflectors decrease results of the IACC at the 9 measuring points, so that acoustic quality is much improved. Especially, the decrement of IACC values was remarkable in the frontal area close to the stage in audience floor except for the center, due to the reflections from above the stage to the listeners.



4. REMARKS

It is shown that measurements in the 1/10 scale model for acoustic parameters by the diagnostic system may prove effects of reflectors' array and other scattering elements which may not available by calculation at the design stage. In order to examine the sound fields after the construction of the auditorium, the diagnostic system measuring orthogonal factors may be applied. Also, keeping the subjectively optimal conditions, this system may be applied for the automatic control of sound fields by the use of electro-acoustic systems.



REFERENCES

1. Y. Ando 1998 Architectural Acoustics - Blending Sound Sources, Sound Fields, and Listeners, AIP Press/Springer-Verlag, New York.

2. H. Sakai, Y. Ando et al. 1997 Inter-individual differences in subjective preference judgments of sound fields in Music and Concert Hall Acoustics, Y. Ando and D. Noson (Eds.), Academic Press, London, chapter 13.

3. Y. Ando et al. 1997 Global subjective evaluations for design of sound fields and individual subjective preference for seat selection in Music and Concert Hall Acoustics, Y. Ando and D. Noson (Eds.), Academic Press, London, chapter 4.

4. M. Sakurai et al. 1997 A sound simulation system for seat selection in Music and Concert Hall Acoustics, Y. Ando and D. Noson (Eds.), Academic Press, London, chapter 6.

5. S. Sato, Y. Mori and Y. Ando 1997 The subjective evaluation of source locations on the stage by listeners in Music and Concert Hall Acoustics, Y. Ando and D. Noson (Eds.), Academic Press, London, chapter 12.

6. S. Sato and Y. Ando 1997 The apparent source width (ASW) for music source in related to the IACC and the width of the interaural crosscorrelation function (WIACC), 134th meeting of ASA, San Diego.

7. Alrutz. H 1981 Ein Neuer Algorithmus zur Auswertung von Messungen mit Pseudo-Rausch Signalen, Fortschritte der Akustik DAGA 81, 525-528, Berlin.

8. Y. Ando 1985 Concert Hall Acoustics, Springer-Verlag, Heidelberg, appendix E.

9. Y. Suzumura, Y. Ando, M. Sakurai, M. Oowaki, T. Iizuka, and I. Yamamoto, 1999 An Evaluation of Scattered Reflections in Sound Fields (Unpublished)

10. T. Nakajima, Y. Ando, and K. Fujita 1992 Lateral low-frequency components of reflected sound from a canopy complex comprising triangular plates in concert halls, J. Acoust. Soc. Am., 92, 1443-1451.