19 international congress on acoustics madrid, 2-7 september 2007 ...

19th INTERNATIONAL CONGRESS ON ACOUSTICS MADRID, 2-7 SEPTEMBER 2007

OBJECTIVE EVALUATIONS OF SOUND SCATTERING BY USING SCALE MODELS AND COMPUTER SIMULATION PACS: 43.55.Br 1

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Shin-ichi Sato ; Yong Hee Kim ; Hye Mi Lee ; Jin Yong Jeon School of Architectural Engineering, Hanyang University, Seoul 133-791, Korea 1 2 3 4 [email protected]; [email protected]; [email protected]; [email protected]

ABSTRACT The effect of hemisphere diffuser locations in a hall on acoustical parameters was investigated using a 1:50 and 1:25 scale models. The acoustical parameters of the hall with and without installed diffusers were compared. Comparisons between computer simulation determined values and values determined from scale model measurements were also made by using 1:50 scale models. The results of 1:50 scale model measurements showed that diffusers of the audience area decreased RT and EDT, and increased C80. Investigation of the computer model showed that the absorption coefficient used for the computer model should be decreased 25 and 30% of the original values measured in a reverberation chamber for the conditions with and without diffuser, respectively. The results of 1:25 scale model measurements showed that the RT indicated the highest correlation with the diffusion power, which is defined by the sum of each surface multiplied by the diffusion coefficients as similar manner to the absorption power. INTRODUCTION Sound diffusion by a wall structure is one of the main factors affecting the sound quality of concert halls. When the side walls and reflectors do not have sufficient diffusion, “acoustic glare” is perceived by listeners as a negative quality of the hall [1]. Tone coloration is also observed as perceptual effects of a single reflection [2]. This “distortion” can be explained as comb-filter effects due to interference between the direct sound and the reflection. Flutter echo also has a negative effect on the acoustic quality of a hall. Scattering surfaces prevent coloration is caused by a strong reflection and flutter echo is caused by multiple reflections. Also, the diffusion of a sound field affects the characteristics of the nature of the reverberation process. The effect of a scattering wall surface on diffusion in the laboratory can be assessed by determining the random-incidence scattering coefficient and diffusion coefficient [3, 4]. Jeon et al. [5] made systematic investigations to determine the optimum diffuser design for a concert hall by using hemisphere and box-type diffusers. However, the sound field in a concert hall including scattered reflections is different from the laboratory measurement condition. The audience does not locate close to the walls like the condition in which a scattering coefficient is measured. Therefore, there is a need to develop measurement and evaluation methods for determining the performance of the scattering characteristics of wall surfaces in concert halls in terms of the subjective perception of the diffused sound field. Two parameters are used as objective indices for indicating the degree of diffusion in sound fields. One is the sound diffusivity index SDI with visual inspection [6]. The other is IACCL3 [7] which is related to listener envelopment. However, these objective indices have not been fully verified through measurement in actual concert hall sound fields. More generally, conventional acoustical parameters have been used to evaluate the scattered sound field. Suzumura et al. [8] compared the sound fields with and without an array of circular columns in front of the side walls and the stage walls as diffusers in a 1:10 scale model of a concert hall. They found that columns in front of the side walls decrease IACC at seats close to these columns. In addition, ∆t1, which is defined by the delay time of the reflection with maximum amplitude in relation to the direct sound, is increased by the columns. Jeon et al. [9] investigated the effect of

hemisphere diffusers on the sidewalls close to the orchestra pit, on the side walls, and on the soffit of the side balcony in a 1:10 scale model of a multipurpose hall. They found that the effect of the diffusers on the delay time of the early reflection ∆t1 was observed in back rows and the effect on the IACC was observed at the frontal and side seating positions near the source. Fujii et al. [10] investigated the effect of circular column diffusers on the acoustical parameters of halls. They made measurements at all seats in two halls having similar floor plans, with and without columns. They found that an array of columns weakens the strong specular reflections from the sidewalls by scattered reflections. However, few studies have been done on quantifying the degree of diffusion in real sound fields. Therefore, there is a definite need for objective investigation of diffusion in real sound fields. It is difficult to install and remove diffusers in real sound fields; however, the computer and scale models enable us to compare such situations. Comparing scale modelling and simulation, acoustically precise scale modelling and measurement procedures allow reliable prediction of the sound field of a completed hall. In terms of diffusion and diffraction, prediction based on scale model results is more reliable than prediction based on computer simulations. For this reason, scale models have been used for acoustical designs of halls and studies on acoustics. Geometric room acoustic models such as ray tracing, beam or cone tracing, image source and hybrid model are utilized for determining acoustical parameters. Parameter settings such as absorption and scattering coefficients affect the accuracy of the prediction. Unfortunately, it is difficult to separate the effects of absorption and diffusion for an in-situ measurement of scattering. This study investigates the effect of hemisphere diffuser position inside a hall on acoustical parameters by using 1:50 and 1:25 scale models. Acoustical parameters with and without installed diffusers were compared to clarify the characteristics of the diffused sound field. Also, acoustical parameters obtained by the computer simulation were compared with those of the scale models. 1:50 SCAEL MODELS A 1:50 scale models of a simple shoebox and fan shape halls with a volume of approximately 3 12,000 m were investigated (Fig. 1). The absorption coefficients of the hemisphere diffusers were measured in a 1:10 scale reverberation chamber according to ISO 354 [3]. Structural walls were made of hard styrene board covered by paper coated twice with enamel. The seating area was covered by the velvet to simulate the absorption of the seats and audience. The diffusers used in this study were 7.5-mm diameter hemispheres-shaped diffusers made of lacquered wood. The diffusers were attached on 1) the stage walls and 2) the side walls of the audience area. Wall surface coverage by diffusers was about 50%. A spark source and a 1/8-in microphone were used as the sound source and receiver respectively.

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Figure 1. (a) Shoebox and (b) fan shape halls used for 1:50 scale model experiments.

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19 INTERNATIONAL CONGRESS ON ACOUSTICS – ICA2007MADRID

Table I shows the acoustical parameters averaged for 6 receiver positions with and without installed diffusers. In the text below, frequencies are given for a full-size hall. All parameter values are quoted as values, derived by averaging the results of 500 and 1000 Hz octave bands. Diffusers on the side walls of the audience area decreased RT and EDT both in the shoebox and fan shape halls while the diffusers on the stage walls did not always decrease EDT. Considering the fact that the diffusers of the audience are increased C80, it can be said that the diffusers absorb the late energy. The listening level changed little by diffusers except for the case installed stage diffusers in the fan shape hall.

Table I. Acoustical parameters measured in 1:50 scale models. Shoebox No diffuser Audience Stage No diffuser RT [s] EDT [s] LL [dB] C80 [dB]

2.17 1.53 -5.4 0.8

1.45 1.11 -5.6 1.8

1.94 1.61 -5.7 1.9

2.09 1.60 -6.4 -0.1

Fan Audience

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1.42 1.20 -6.4 1.5

1.80 1.40 -5.6 1.2

Then, the parameter settings of the computer model were investigated. The real hall has less diffusive characteristics than a reverberation chamber where the absorption coefficient is measured because the highly absorptive material (seating) located on the audience floor. If the absorption coefficient measured in a reverberation chamber is directly applied to the setting of the computer model, this may cause an error of the prediction of acoustical parameters. Nijs [14] suggested that a 20% reduction in the absorption coefficients from the reverberation chamber measurement is a good first guess for the input into a ray-tracing program. The computer model was created by the commercially available hybrid image/ray tracing model, ODEON (version 6.0). By using above models, the absorption coefficient was varied from −50 to +10% of the original absorption coefficient measured in the reverberation chamber. Scattering coefficients for the walls with and without diffusers were fixed at 0.2 and 0.7, respectively. The scattering coefficient of the seating area was set at 0.1 because the absorption of the seating and audience was reproduced by the velvet and the detailed models of the chair and audience were not reproduced. Figure 2 shows the error of EDT and C80 (parameter measured in the scale model minus parameter calculated in the computer model) as a function of the absorption change in percent. Absorption coefficient of -25 and -30% of the original values for the conditions with and without diffusers gave the minimum error in terms of RT, EDT, C80, and Ts. We investigated both shoebox and fan shape hall models, however, there was little difference between two shapes. Therefore the results of the shoebox hall were shown. The transition order is another important setting of ODEON program. Sound rays from a point source will detect image sources up to the transition order and above this order they will detect secondary sources. We also investigated the effect of the transition order on the acoustical parameters; however, there was little difference between the transition order 0 and 1. 1:25 SCALE MODEL 3 A 1:25 scale model of a shoebox hall with a volume of 5,000 m (600 seats) was investigated (Fig. 3a). The effect of the diffuser location (on the side walls, on the soffit of the side balcony, and on the ceiling) on acoustical parameters was investigated (Fig. 3b). The materials of each element were selected through absorption coefficient measurements in a scale model reverberation chamber according to ISO 354 [11]. Structural walls were made of hard styrene board covered by paper coated twice with enamel. The model seats were made from angled steel on timber strips upholstered with velvet. The diffusers used in this study were 15-mm diameter hemispheres-shaped diffusers made of lacquered wood. Wall surface coverage was about 50%. The scattering coefficients of the hemisphere diffusers were measured in a 1:10 scale reverberation chamber according to ISO 14791-1 [3].

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A spark source and a 1/8-in microphone were used as the sound source and receiver respectively. The sound source was recorded at 46 receiver positions evenly distributed on the first and second floors and in the balconies (Fig. 4). The sound level of each receiver position was calibrated by the signal measured at reference position (1 m from the source) at the same time because the spark source produced different amplitude in every measurement. Measurements were conducted under fully occupied condition with a model audience. (a)

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Figure 2. Effect of absorption changes on acoustical parameters of the model of the shoebox shape hall. (a) EDT error (value measured in the scale model minus value calculated by the computer simulation) for the condition without diffuser; (b) C80 error for the condition without diffuser; (c) EDT error for the condition with diffusers; and (d) C80 error for the condition with diffusers.

Figure 5 shows the acoustical parameters for different diffuser location. To quantify the effect of the diffuser amount on the acoustical parameters, the diffusion power (the sum of each surface multiplied by the diffusion coefficients) was introduced as a similar manner to the absorption power. RT decreased about 0.09 s by as the diffusion power was increased. RT indicated a high correlation with the diffusion power (r = 0.89; Fig. 5a) while EDT did not show a significant correlation (r = 0.31; Fig. 5b) with the diffusion power. Diffusers mainly affected the late reverberation energy of the impulse responses. Only diffusers on the soffit of the side balcony affected the early reflections of the impulse responses. C80 slightly increased according to the diffusion power (r = 0.31; Fig. 5c). Average values of C80 did not change according to the diffusion power. The difference of the average values among 8 cases was within 1 dB. On the other hand, the range of C80 decreased in cases where diffusers were installed (W 1, Wall, BF+W all) comparing with cases without diffuser (no diffuser: 8.3 dB; W 1: 5.5 dB; W all: 5.0 dB; and BF+Wall; 5.4dB). C80 showed relatively uniform distribution in cases where ceiling diffusers were installed (CR: 5.8 dB), but the range increased when the diffusers were installed on the side walls near the proscenium and on the ceiling (CR+W 1: 7.5dB). This means that excessive installation of diffuser decreased later reflection sound largely. The average of G 4 th


did not show a significant correlation with the diffusion power (r = 0.09; Fig. 5d). Also distribution on seats of sound pressure level did not show uniformly as diffusers were installed. ? t1, which is defined as delay time gap between the arrival of the direct sound and the delay time of the reflection which has the maximum sound pressure level, did not show effect of ? t1 according to increase of the diffusion power (r = 0.00; Fig. 5e).The diffusers increased the number of sound ray and rearranged the reflection paths. Ts showed the second largest correlation with the diffusion power (r = 0.57; Fig. 5f). Ts indicated opposite tendency of C80 because these two parameters have negative correlation.

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Figure 3. (a) Shape of the hall and (b) locations of diffusers installed in the hall (view from the stage). SW: side wall; BF: Balcony front; and CR: Ceiling.

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Figure 4. Source and receiver positions of 1:25 scale model measurements. (a) 1F (25 positions); and (b) 2F (18 positions including 3 positions in the balcony).

CONCLUDING REMARKS The effect of hemisphere diffuser locations in a hall on acoustical parameters was investigated using a 1:50 and 1:25 scale models. The results of 1:50 scale model measurements showed that the diffusers of the audience area decreased RT and EDT, and increased C80. The stage diffusers decreased RT and increased C80. Comparison between the scale model measurements and the computer simulation showed that the absorption coefficient used for the computer model should be decreased 25 and 30% of the original values measured in a reverberation chamber for the conditions with and without diffuser respectively in order to obtain the computer simulation results close to the scale model measurements. The results of 1:25 scale model measurements showed that the diffusers decrease RT and Ts. These parameters are related to the diffusion power. On the other hand, the average of C80, EDT, G and ? t1 did not show a significant change by the diffusers. 5 th


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Figure 5. Acoustical parameters measured in 1:25 scale model as a function of the diffusion power. Upper and lower bars show the maximum and the minimum values, respectively. (a) T20; (b) EDT; (c) C80; (d) G; (e) ∆t1; and (f) Ts. ACKNOWLEDGMENT This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (F01-2006-000-10187-0).

References [1] L. L. Beranek: Concert Halls and Opera Houses: Music, Acoustics, and Architecture. Springer Verlag, New York, 2004. [2] M. Barron: The subjective effects of first reflections in concert halls: The need for lateral reflections. J. Sound and Vib. 15 (1971) 475-494. [3] ISO 17497-1: Acoustics-Sound-scattering properties of Surfaces, Part 1: Measurement of the random- incidence scattering coefficient in a reverberation room. 2004. [4] AES-4id-2001: AES Information document for room acoustics and sound reinforcement systems characterization and measurement of surface scattering uniformity. J. Audio Eng. Soc. 49 (2001) 149–165. [5] J. Y. Jeon, S. C. Lee, M. Vorländer: Development of scattering surfaces for concert halls. Appl. Acoust. 65 (2004) 341-355. [6] C. H. Haan, F. R. Fricke: Surface diffusivity as a measure of the acoustic quality of concert halls. Proc. of Australia and New Zealand Architectural Science Association Conference, Sydney, 81-90, 1993. [7] T. Hidaka, L. L. Beranek, T. Okano: Interaural cross-correlation, lateral fraction, and low-and high-frequency sound levels as measures of acoustical quality in concert halls J. Acoust. Soc. Am. 98 (1995) 988-1007. [8] Y. Suzumura, M. Sakurai, Y. Ando, I. Yamamoto, T. Iizuka, M. Oowaki: An evaluation of the effects of scattered reflections in a sound field. J. Sound and Vib. 232 (2000) 303-308. [9] J. Y. Jeon, J. K. Ryu, S. Sato, Y. H. Kim: Subjective and objective evaluation of the scattered sound in a 1:10 scale model hall, Proc. Forum Acusticum, Budapest, 2005. [10] K. Fujii, T. Hotehama, K. Kato, R. Shimokura, Y. Okamoto, Y. Suzumura, Y. Ando: Spatial distribution of acoustical parameters in concert halls: comparison of different scattered reflection. J. Temporal Des. Arch. Environ. 4 (2004) 59-68; www.jtdweb.org. [11] ISO 354: Acoustics-Measurement of sound absorption in a reverberation room, 2003. [12] Y. W. Lam: A comparison of three diffuse reflection modelling methods used in room acoustics computer models. J. Acoust. Soc. Am. 100 (1996) 2181-2192. [13] Y. W. Lam: The dependence of diffusion parameters in a room acoustics prediction model on auditorium sizes and shapes. J. Acoust. Soc. Am. 100 (1996) 2193-2203. [14] L. Nijs, G. Jansens, G. Vermeir, M. van der Voorden: Absorbing surfaces in ray-tracing programs for coupled spaces. Appl. Acoust. 63 (2002) 611-626.

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