The influence of architectural design on the acoustics of concert halls

Applied Acoustics 31 (19903 207-214

The Influence of Architectural Design on the Acoustics of Concert Halls A n d e r s C. G a d e The Acoustics Laboratory Building 352, Technical University of Denmark, DK-2800 Lyngby, Denmark

A BS TRA CT Room acoustical and architectural data from 32 concert halls have been subjected to statistical analyses in order to look for relationships between the position averaged acoustical data and various design variables. It isfound that volume and the classical room acoustic parameter~reverberation time are the main factors governing the behaviour of the newer and more sophisticated measures of level, reverberance and clarity. However. geometrical factors can also have a major it~uence on clarity and--in particular--on spaciousness and on the conditions for musicians on the orchestra platform. The results point towards the possibility of forming empirical, linear regression fornzulae. which can predict the values of the newer parameters in halls with given R T and geometry.

In the field of subjective room acoustics, intense research activity in 1960s and 1970s has resulted in a high degree of concensus regarding (1) what aspects are important in listeners' perception of r o o m acoustic quality, and (2) how these aspects can be measured objectively by means of room acoustic parameters. Thus, a set of new objective parameters has been found, which are subjectively more relevant than the classical reverberation time (RT) but which are much more difficult to predict, unless expensive scale or computer models are applied. The reason for this is that these newer parameters are more sensitive to changes in the early reflection sequence, and thus are highly dependent on the geometrical shaping o f the room. The c o m m o n l y used Note: Since May 1989 further analyses have lead to a more detailed prediction formulae than

those presented in this paper. These are described in a new paper submitted to JASA. 207 Applied Acoustics 0003-682X/90/$03"50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain

Anders C. Gade

208

practice of looking at delay times of single reflections is not sufficient for guiding architects on the choice of room shape; we need to know h o w - - a n d how much--the design may, or should, be changed before significant changes in the objective parameters appear. However, at present there are still very few data available which can provide the acoustician with this knowledge, although this is essential for participation in the early discussions of choice ofshape for a new hall. In this decade, however, a number of researchers, e.g. Refs 1-4, have carried out systematic measurement surveys in existing--often internationally well known--halls, with the purpose of learning about the behaviour of these newer parameters in different hall designs. In the Danish survey of 21 halls, 3 we tried to establish rules for how much the objective parameters change as a function of geometry by subjecting the acoustic and geometrical data to analysis of regression. In order to be able to (1) extend the range of validity of the relationships to larger halls than those most common in Denmark, and (2) have enough data to make statistical analyses on selected groups of halls, we have lately extended our measurements to a number of halls in Europe, including Concertgebouw (CG) in Amsterdam, Gasteig Philharmonie in Munich (GM), Musikvereinsaal in Vienna (MW), Grosses Festspielhauss in Salzburg (FS), Usher Hall in Edinburgh (ED) plus the Royal Festival Hall (FH) and Barbican Concert Hall (BA) in London. This paper deals with the first results of analyzing this larger set of data, based on measurements in 32 halls in total. 3.t

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Influence of architectural design on acoustics

An idea of the range of the room acoustic spectrum covered by this material can be obtained from Fig. 1, showing corresponding values of R T and volume. The data points corresponding to the halls mentioned above have been marked by the two letter code in parentheses.

2 PARAMETERS MEASURED The objective parameters measured have been listed in Table 1, along with the subjective qualities which each one intends to describe. Besides the wellestablished parameters related to listener conditions, the table also contain a number of our own suggestions for measures describing the conditions for musicians on the platform. The exact definitions of the parameters and further details about the measurement techniques can be found in the literature (Refs 5-10) referred to in the table. In all cases the halls were empty during the measurements. In the following, only frequency and position averaged values will be discussed, since the design aspects dealt with are related to the hall as a whole.

3 M U T U A L C O R R E L A T I O N B E T W E E N T H E OBJECTIVE PARAMETERS Before entering the discussion of ties between the m a n y parameters and the design, it is helpful to take an overview of how the objective parameters are interrelated, since a high mutual correlation between two parameters means TABLE l

Objective Parameters and Subjective Qualities Room acoustic parameters

Early decay time Centre time Clarity Level/strength Latent energy fraction EDT on platform Clarity at I m Early ensemble level Support-100 ms Support--200 ms

Symbol EDT TS

C L LEF EDTP CS EEL ST1 ST2

Associated subjective aspect

Reverberance/clarity Reverberance/clarity Clarity Relativelevel Spatialimpression Reverberance for musicians Reverberance for musicians Easeof ensemble Ease of ensemble Support

Reference

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that they will be related to the design in the same way. Figure 2 provides this overview by showing the weights of the first two factors of each parameter in a three-dimensional, rotated, factor space. The three factors explain 90% of the total variance in the data. It is clearly seen that the reverberance/clarity measures: RT, EDT, EDTP (equals the EDT measured on the orchestra platform), TS and C are all highly mutually correlated and comprise the first dimension, whereas the level measures, L, plus all platform parameters apart from EDTP, are also highly interrelated and form the second dimension. The only parameter which does not come out with a strong correlation with any of these two dimensions is seen to be LEF. This is because LEF is the main contributor to the third dimension (on which its weight is 0"86). Therefore, when averaged over frequency and position, the ten parameters measured only describe three different aspects of the acoustic conditions of concert halls. In other words, the number of parameters could be reduced to three (as long as one is not interested in more detailed analyses, including position differences). 4

R E L A T I O N WITH P R E D I C T E D VALUES

Applying classical diffuse field theory, a formula for expected values for each of the parameters (except LEF)can be calculated as simple functions of RT

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i.e. EDT~ RT. The lowest correlation was found for C: r=0:74, with regression line given by: C = - 0 " 4 + 0 - 9 C¢~p where C~xp= lOlog(exp(l'lO4/RT)-l)dB

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This means that in the range covered by our data (see Fig. 4), the measured values will normally be between 2 and 3 dB lower than expected from diffuse field theory alone. Figure 4 show a limited spread around the regression line,

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Influence of architectural design on acoustics

213

indicating that L can be predicted with a rather high degree of accuracy from (3) without further consideration of geometric properties of the hall.

5 CORRELATION WITH GEOMETRIC PROPERTIES Concerning C, which was not that well predicted by the regression in Fig. 3 alone, the deviation from the expected value, C - C ~ v, was found to be positively correlated with hall width as well as the angle between the side walls, i.e. in wide or fan shaped halls, C will tend to be higher than in narrow or rectangular halls. The most significant geometrical relationship was found for LEF, which, based on the data from all 32 halls, has a fairly high correlation with hall width, r = 0"73. However, if the analysis is restricted to 16 rectagular halls, r increases to - 0 . 8 2 , and the regression model becomes: L E F = 0-47 - 0-0085 width

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This line is the one shown in Fig. 5 along with the data points for all 32 halls.

6 CONCLUDING

REMARKS

To a large extent, classical diffuse field theory and R Texplains the behaviour of the new r o o m acoustics parameters. Still, it is found that the predictions can be refined by using empirical statistical relationships, which also include the influence of geometrical variables. With further refinements, it is believed that such empirical predictions can be of great value in the early stages of auditorium design.

REFERENCES 1. Barron, M. & Lee, L.-J., Energy relations in concert auditoriums, I. JASA, 84 (1988) 618-28. 2. Bradley, J. S., Experience with new auditorium acoustic measurements. JASA, 73 (1983) 2051-8. 3. Gade, A. C. & Rindei, J. H., Akustik i Danske Koncertsale. Publication No. 22, The acoustics Laboratory, Technical University of Denmark, 1984 (in Danish). 4. Tachibana, H., Yamasaki, Y., Morimoto, M., Hirasawa, Y., Maekawa, Z. & POsselt, C., Acoustic survey of auditoriums in Europe and Japan. J. Acoust. Soc. Jpn, 10 (1989) 73-85. 5. Jordan, V. L., Room acoustics and architectural acoustics development in recent years. Appl. Acoustics, 2 (1969) 59-81.

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Anders C. Gade

6. Kiirer, R., Zur gewinnung von einzahlkriterien bei impulsmessung in der raumakustik. Acustica, 21 (1969) 370-2. 7. Reichardt, W., Alim, D. A. & Schmidt, W., Definition und messgrundlagen eines objektiven masses zur ermittlung der grenze zwischen brauchbarer und unbrauchbarer durchsichtigkeit bei musikdarbeitungen. Acustica, 32 (1975) 126-37. 8. Lehmann, P. & Wilkens, H., Zusammenhang subjektiver beurteilungen yon konzerts/ilen mit raumakustischen kriterien. Acustica, 45 (1980) 256-68. 9. Barron, M. & Marshall, A. H., Spatial impression due to early lateral reflections in concert halls: The derivation of a physical measure. J. Sound Vib., 77 (1981) 211-32. 10. Gade, A. C., Investigations on musicians" room acoustic conditions in concert halls. II: Field experiments and synthesis of results. Acustica, 69 (1989) 249-62.