Effects of Stage Volume on Concert Hall Acoustics as a Design Element

Toronto, Canada International Symposium on Room Acoustics 2013 June 9-11 ISRA 2013 Effects of Stage Volume on Concert Hall Acoustics as a Design El...
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Toronto, Canada

International Symposium on Room Acoustics 2013 June 9-11

ISRA 2013

Effects of Stage Volume on Concert Hall Acoustics as a Design Element Hocheul Park ([email protected]) Hyung Suk Jang ([email protected]) Yong Hee Kim ([email protected]) Jin Yong Jeon ([email protected]) Department of Architectural Engineering Hanyang University Seoul, 133-791, South Korea

ABSTRACT This paper reports on the investigation of the three-dimensional size of a concert hall stage, hereinafter referred to as the “stage volume,” as a dimensional aspect for stage design in concert halls. From a survey of hall measurement data, smaller stages tend to provide higher sound. The effect of this stage design element was investigated further by computer simulation. A rectangular hall was modeled with various stage volumes. The results showed that a smaller stage volume generally enhanced sound strength in the audience area. The effects of stage volume were thus applied to various hall types, and an optimum value of stage volume was investigated for each hall. 1

INTRODUCTION

Stage enclosures including platforms in concert halls have been acoustically evaluated mainly in terms of stage support parameters. Early support (STearly) is a typical stage measure because it had been validated with a series of field measurements and auditory tests with musicians.1,2 Recently, the distribution of STearly in a concert hall stage has been investigated by scale-model testing, and the relationship between stage size and stage support parameters was reported.3,4 However, there is still little knowledge about the effect of stage elements on auditorium (audience) acoustics. As the most important design element in concert hall acoustics, the stage enclosure is regarded as the main surface for early reflections. Stage design elements include stage width, depth, height, area, volume, and seating behind platform. Accordingly, there are many kinds of stage shapes based on the dimensions (height, width, and depth). Stage volume can also be regarded as a dimensional aspect for stage design, which includes the platform area. However, stage absorption is also important in determining the characteristics of early sounds. Finishing materials of the stage enclosure, audience seating behind the platform, and onstage performers are the major sound absorbers. Therefore, stage absorption together with stage size is important for identifying acoustical influences and determining the stage shape in a concert hall design. In the present paper, the effects of stage volume on concert hall acoustics were investigated in a rectangular concert hall using computer simulations. 1

2 2.1

METHODOLOGY STAGE DESIGN ELEMENTS

Stage volume ratio (VS/V) is defined as the stage volume (VS) divided by the total volume of the hall (V). The stage volume (VS) includes the upper volumes of both the platform area (So) and the seating area behind the platform (Sc). Figure 1 shows the distribution of the stage volume ratios of existing concert halls (26 rectangular-shaped halls). The hall volumes ranged from 10,500 to 25,000 m3, and the capacities ranged from 1,200 to 2,900 seats. The average stage volume ratio was 24.1% (ranging from 11 to 49%).

Figure 1: Stage volume ratio in rectangular-shape concert halls.

2.2

HALL DESCRIPTION

The simulation model was based on the Boston Symphony Hall to investigate the effects of controlled stage dimensional parameters on the audience acoustics. The model hall had 2,625 seats with 18,750 m3 of the hall volume. The stage of the model hall was simplified. The stage area (So) was 130 m2, and the stage volume (VS) was 1,651 m3. 2.3

COMPUTER SIMULATION

The acoustical simulation program ODEON with a hybrid ray tracing method was employed to estimate the acoustical parameters from the model hall. The computer model had fitted acoustical parameters based on the measurements of the real hall, such as STearly for stage acoustics, and reverberation time at the midpoint of an occupied hall (RTmid,occ.), early decay time at the midpoint of an unoccupied hall (EDTmid,unocc.), C80 in the 3rd balcony of an unoccupied hall (C803B,unocc.), and sound strength G at the midpoint of an unoccupied hall (Gmid,unocc.) for auditorium acoustics. In the simulation, the sound source was located 3 m from the stage front on the center line. As shown in Figure 2, the receivers were distributed uniformly at 27 positions throughout the audience areas.

2

3 3.1

EFFECT OF STAGE VOLUME RATIO (VS/V) Simulation settings

Stage dimensions of a rectangular hall model were modified in 13 steps to evaluate the effect of stage volume on the auditorium acoustics. As shown in Figure 3 and Table 1, height, width, and depth of the stage were simultaneously extended according to the increased stage volume, which ranged from 1,650 to 5,982 m3. Based on the distribution of the stage volume ratios of the existing halls, the stage volume ratio of the simulation model was controlled within the range of 9 to 26%.

(a) Floor plan

(b) Section

Figure 2: Floor plan and section of the computer model with receiver positions.

(a) Floor plan of stage

(b) Section of stage

Figure 3: Variations of stage volume with source and receiver positions in computer simulations. Table 1: Descriptions of the stage dimensions according to variation in stage volume Case No.

So [m ]

2

VS [m ]

3

Va [m ]

3

V [m ]

3

VS/V [%]

1

130

1,650

17,100

18,750

8.8

2

145

1,900

17,100

19,000

10.0

3

160

2,175

17,100

19,275

11.3

4

175

2,450

17,100

19,550

12.5

5

190

2,735

17,100

19,835

13.8

6

208

3,078

17,100

20,178

15.3

7

225

3,423

17,100

20,523

16.7

8

241

3,790

17,100

20,890

18.1

9

257

4,065

17,100

21,165

19.2

10

285

4,470

17,100

21,570

20.7

11

299

5,039

17,100

22,139

22.8

12

324

5,550

17,100

22,650

24.5

13

345

5,982

17,100

23,082

25.9

3

3.2

Results

Figure 4 (a) shows the simulation results of STearly according to variations in VS/V. By increasing Vs/V by 17% (averaged from 9 to 26%), STealry in the stage acoustics evaluation was significantly reduced by 6 dB (from −12.4 to −18.4 dB). STearly showed a very strong linear relationship with the stage volume which was consistent with previous studies.2,4 Since the early reflection at 20–100 ms mainly influences STearly calculation, the smaller stage volume reflects more early energy back to the stage. As discussed by Beranek5, the stage volume ratio should be designed to be 15% or less to produce a desirable range of STearly (−14.4 dB to −12 dB). Acoustical parameters in the audience area were investigated in terms of loudness, reverberance, and clarity. For loudness evaluation, G and G80 (early sound strength up to 80 ms) were calculated. As shown in Figure 4 (b), both G and G80 were decreased—G80 was more sensitive—by an increase in VS/V. It seemed that the stage volume mainly affected the early energy level in the audience area. As shown in Figure 4 (c), both RT and EDT increased with an increase in Vs/V. It was confirmed that a larger volume yields more reverberation. However, when Vs/V was around 20%, there were breakpoints which came from the coupling effects between the stage and auditorium. C80 for the clarity factor showed similar results to RT and EDT as shown in Figure 4 (d). C80 dropped by 1.27 dB with an increase in Vs/V above 17%. When Vs/V was 16% or less, a larger stage volume decreased music clarity. However, stage volume had little influence on clarity when Vs/V was over 16%.

(a) STearly

(b) Sound strength (G)

(c) Reverberation time (RT), early decay time (EDT)

(d) Clarity (C80)

Figure 4: STearly, G, G80, RT, EDT and C80 values in the audience area by variation of stage volume ratio. 4

4

EFFECT OF STAGE VOLUME RATIO (VS/V) IN VARIOUS HALL SHAPES

The effect of stage volume ratio on auditorium acoustics was evaluated for five concert halls including Boston Symphony Hall using the computer simulation. Table 2 shows the capacity (N), stage volume (VS), total volume (V), the original volume ratio (VS/V) with the ranges of VS/V used in the simulations shown in parentheses, and the sound strength (G) for the original configuration of each hall. Figure 5 illustrates the stage parts that were modified to vary the stage volume in the simulation models. Vs/V was varied from 9 to 31% in these simulations. Figure 6 shows the results of the G values in the audience area due to changing Vs/V. It was verified that an increase in the stage volume causes the sound strength (G) to decrease by a factor of 0.06. It was confirmed that stage volume affects sound strength in the audience area. Similarly to the Boston Symphony Hall results, it was verified that an increase in the stage volume causes the sound strength decrease in the other concert halls as well. Table 2: Changes of stage volume ratio in the five concert halls Hall name

N

VS[m3]

V[m3]

Boston Symphony Hall, Boston

2,625

1,650

18,750

Royal Festival Hall, London

2,901

4,209

21,950

2,337

3,975

29,737

1,750

3,500

18,500

2,523

5,877

23,330

Gasteig Philharmonie, Munich Incheon Art Center, Incheon Seoul Art Center, Seoul

(a) Royal Festival Hall, London

VS/V[%] 9 (9–26) 19 (19–24) 13 (11–13) 19 (16–22) 25 (17–31)

(b) Incheon Art Center Hall, Incheon

(c) Gasteig Philharmonie, Munich

(d) Seoul Art Center, Seoul

Figure 5: Stage volume variations in four concert halls 5

G[dB] 4.3 2.4 3.2 5.8 3.7

Figure 6: G as a function of △Vs/V in various halls. 5

DISCUSSIONS AND CONCLUSIONS

In the present study, the effects of stage dimensions on auditorium acoustics were investigated using computer simulations. Stage volume ratios for dimensional aspects were manipulated as stage design elements. The existing halls showed a diverse range in their stage volume ratios from 11 to 49%. In the computer simulation models of the rectangular halls, it was found that a large stage volume tends to reduce stage support, audience sound level, and clarity, but it increased reverberation in the audience area. Particularly, early energy level and early sound decay are more affected by stage volume. The coupling effect between the stage and auditorium spaces was observed in the variation of RT, EDT, and C80 values. Evaluations by performers are important for designing a stage enclosure. However, the effect of stage design elements on the audience area should be considered. From the results in this study, the stage enclosure should be minimized for better stage and audience acoustics. This is natural, but many design cases ignore the importance of the stage enclosure. In the schematic design phase, the hall volume is normally determined according to a given reverberation time. In the same manner, the stage volume should be determined according to the target sound strength. It must be considered that the relative ratio of stage and auditorium spaces is appropriate and the stage volume is minimized to meet the target values of both RT and G. References 1 A.C. Gade, “Investigations of musicians' room acoustic conditions in concert halls. I. Method and laboratory experiments” Acustica 65, 193-203 (1989). 2 A.C. Gade, “Investigations of musicians' room acoustic conditions in concert halls. II. Field experiments and synthesis of results” Acustica 69, 249-262 (1989). 3 J.Y. Jeon and M. Barron, “Evaluation of stage acoustics in Seoul Arts Center Concert Hall by measuring stage support,” Journal of Acoustical Society America, 117(1), 232-239 (2005). 4 W. Chiang and Y. Shu, “Acoustical design of stages with large plane surfaces in rectangular recital halls,” Applied Acoustics 64(9), 863-884 (2003). 5 L. L. Beranek, “Concert Halls and Opera Houses,” Springer-Verlag, New York (2004). 6