Design of room acoustics and a MCR reverberation system for Bjergsted concert hall in Stavanger

Applied Acoustics 19 (1986) 465-475

Design of Room Acoustics and a MCR Reverberation System for Bjergsted Concert Hall in Stavanger S. Strem, A. Krokstad, S. Sorsdal and S. Stensby The Laboratory of Acoustics/ELAB--SINTEF, The Norwegian Institute of Technology, Trondheim (Norway) (Received: I July. 1985)

SUMMA R Y Bjergsted concert hall in Stavanger, Norway, was opened in 1982 as a multi-purpose hail, but primarily intended for concert~'. The hall has been rebuilt frem an oM dome-shaped exhibition hall. The new shape qf the hall was designed using computeri:ed sound ray tracing, in a threedimensional mathematical model. The hall i~ fan-shat)ed and additional lateral reflections are pro~'ided .by large reflectors hanging freely below the ceiling. Because of the marginal voh,ne per seat ~,nd the hall's man), light-weight con.~tructions, we have designed, .~pec(ficL,41)'for this hall, an clectroacoustic multi-channel ret'erheration '¢)'stem. This increases the reverberation time over a broad frequency range and, in addilion, the sound lerel is also increased.

INTRODUC'.'ION The Laboratory of Acoustics/ELAB was the acoustical consultant for the rebuilding of an old exhibition hall in Stavanger, in co-operation with architect, Ola Natvig. The hall was dome-shaped, having a circular audience area of 2100m ~"and a maximum height of 14½m. The dome was a light-weight wood construction with sound-absorbing plates facing the hall. The hali was primarily intended as a concert hall for the Stavanger Symphony Orchestra but we also had to design for shows, congresses and cinematic presentations. The greatest acoustical problem 465 Applied Acoustics 0003-682X/86/$03-50 ~, Elsevier Applied Science Publishers Ltd. England, 1986. Printed in Great Britain

466

S. S'.rorn, A. Krokstad, S. Sorsdal, S. Stensby

was the design of a new room shape. Within the existing dome one could easily find a focusing effect in the audience area. A new ceiling and walls had to be designed, incorporating the architect's intention of keeping some of the old airy feeling due to the dome. The hall had to be wide and fan-shaped. Preliminary calculations showed uneven sound energy distribution. In particular, the early lateral reflections in the middle of the audience area were weak. Bearing in mind our experience with the Grieg Memorial Hall project,' we realized that the hall had to be unconventional. The building committee had decided that the new hall must seat an audience of more than 1000. This would give a marginal volume per seat. We therefore had to use sound reflecting materials in every construction, to achieve the necessary reverberation time. We also had to improve the sound insulation of the dome, because of the traffic noise from nearby roads, but the light construction of the dome only allowed for small additional loads. Owing to fire restrictions, the choice of materials was restricted (wood and plastics could not be used). These restrictions, the difficult starting conditions and the instructions given to us by the architect and the building committee made for a long design period. Many calculations of the sound energy distribution had to be made. However, after an adjustment period, when we designed and installed an electroacoustic multi-channel reverberation system, the reports from the musicians and the audience were very favourable.

COMPUTER MODELLING OF THE ROOM SHAPE Since the Bjergsted hall and the Grieg Memorial Hall t had nearly the same problems, due to design and the intended use, we were able to copy the acoustical guidelines from the former project. 2 Again, we had to plan for an even distribution of reflected sound energy on the audience area, a certain balance between early and late reflections, strong early lateral reflections and satisfactory stage acoustics. The use of diffusing elements of different dimensions was taken into consideration in our design. The investigation of many alternative room shapes in the Grieg Memorial Hall project made it easier to find the best solution for the Bjergsted project. Again, we used the idea of freely hanging reflectors below the ceiling to obtain the effect of a smaller hall (Fig. 1). The size of the plane

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reflecting front surface was 7-10m 2 and the angle of the surfaces was carefully calculated. Owing to the load restrictions of the dome construction, these reflectors had to be thin light-weight plates. The side walls were designed to give strong early reflections in the audience area. As t~e wall elements have dimensions of 15-25 m 2, we also obtained fairly good reflecting properties for the lower frequencies. Parts of the side walls have curved reflectors, tilted about five degrees. The large reflecting ceiling above the stage and the front part of the audience area is responsible for the even distribution of early reflections to the musicians and '.he audience. The curved cross section of Fig. I gives a good balance between early and late reflections (this also increases the lateral reflected energy). ENER{:Y

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The final hall design gave a satisfactory distribution of sound energy in the audience area, as Fig. 2 shows. The calculated echograms indicate a good balance between early and late energy, necessary for concert hall acoustics (Fig. 3). Figure 4 shows calculated values for 'definition' and 'centre of gravity time'. The measured impulse responses were in good agreement with the computed data (Fig. 5). MATERIALS A N D COblSTRUCTIONS A sufficiently low background noise level is an important design criterion for a concert hall (not more than N - 20 or 25 dB(A)). Because of the

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traffic noise we had to improve the sound insulation of the dome construction (Fig. 6). New brick walls were used all around the stage and the audience area. The curved side wall reflectors, the diffusing elements on the rear wall, the freely hanging reflectors and the ceiling are made of vibration damped 0.7 mm aluminium panels. The concrete floor has a vinyl layer and the 1070 seats are heavily upholster~-.d. Figure 7 indicates that the reverberation time is too short, especially in the lower frequency range. ELECTROACOUSTICS The loudspeaker system fcr speech reinforcement has one spec::ally designed unit, centrally positioned just in front of the stage opening. This unit is a directive array loudspeaker system.

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Both the musicians and the audience noticed the distinct sound reproduction and the short reverberation time ('dry' acoustics). We therefore decided to install a specially designed multi-channel reverberation system (MCR system). This is a broad band system designed to give the greatest increase in reverberation time at the low frequencies. Our bases for the design were the investigations published by Franssen, ~ Sehroeder 4 and Schroeder and Kuttruff. s Krokstad 6 has given a discussion of these theories and shown the advantage of this system compared with other relevant systems. Each channel in the system, consisting of a microphone, an amplifier and a loudspeaker, can only contribute 0.5 per cent to the increase in the reverberation owing to the stability of the system. The different channels do not need a time delay unit, they need only amplify the reflected sound. The system will increase the energy density in the hall and therefore the sound level will be higher. The distance between the microphones, between the microphone and the loudspeaker and between the loudspeakers should preferably be more than a wavelength at the lowest frequency. The microphones and the loudspeakers should also be. placed in such a way that the sound from the loudspeaker,; is delayed relative to the direct sow.ad from the orchestra at every position in the audience. Our calculations indica,.ed that a forty channel system would give a satisfactory increase in reverberation time, owing to the moderate corrections needed. A sensitivity analysis of the electroaco~lstic system showed that components with tight tolerances and good st~bility while ageing were needed. Accordingly, we used omnidirectional condenser microphones, high quality amplifiers and specially designed loudspeaker units. Each unit contained one 10in woofer and one I in dome tweeter in a closed box. Twenty of these mxits were placed on the top of the brick walls, 3-7 m above the audience. The remaining twenty units were designed as spheres of I m diameter, hanging freely 2 m below the ceiling (Fig. 8). The amplifiers have the microphone preamplifier, the correction network and the power amp~.ifier built together in one unit. Each amplifier is placed as close to the microphone and the loudspeaker as possible. The correction networks were designed as fixed parametric equalizers. The frequency response compensations were based on two kinds of data--first, ~he measured (uncorrected) reverberation times in the hall and the corresponding increase we wished to achieve with the MCR

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shown in Fig. 7. The deviations from the design data are probably due to channels that are not independent at the lowest frequencies. The improvements in the impulse response, can be seen in Fig. 10. The musicians, 1:he conductor and the audience have reported that the use of the MCR system improves the acoustical quality of the hall. The subjective effect of the system is greater than one would expect from what has been measured. This may partly be due to the increased sound level. Tht: orchestra has better contact with the audience in the hall since we have improved the coupling between the stage and the audience.

CONCLUSIONS By means of our computer technique for sound ray tracing, we could predict in advance a satisfactory sound energy distribution, thereby convincing the building committee and the architect of the necessary unconventional redesign of the old hall. We had to accept a marginal

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volume per seat and the weight restrictions forced us to use too many lightweight constructions. We therefore could not obtain a satisfactor~ reverberation time at all frequencies. A multi-channel reverberation (MCR) system was designed especially for this hall. This increased the bass reverberation times by 50 per cent and the reverberation times at the higher frequencies by 20 per cent. In addition, the s o ~ d level has been increased. The MCR system has given noticeable improvement of the hall's acoustical quality. The hall now has satisfactory acoustics for symphonic music, according to reports from musicians and audience members. With the system shut off, the hal! has good acoustics for speech, shows and cinematic presentations. A MCR system therefore gives a practical way of varying the hall acoustics and is at present being included in other hall projects in which the laboratory is involved.

REFERENCES I. S. Strata, H. Dahi, A. Krokstad and E. Eknes, Acoustical design of the Grieg Memorial Hall in Bergen. Applied Acous:ics, 18 (1985), P!~. 127-42. 2. S. Strom, Concert hall acoustics. Room acoustical parameters with high correlation to subjective preference. Technical report STF44 A82006, 1982. 3. N. V. Franssen, S,r ramplification des champ~ acoustiques. Acustica, 20 (1968), pp. 3! 5-23. 4. M. R. ,~hroeder, Die statistischen Parameter de,, Frequenzkurven yon grossen R'.qumen, Acustica, 4 (1954), pp. 549-600. 5. M. R. Schroeder and K. H. Kuttruff, On frequency response curves in rooms. Journal of the Acoustical Soc. of America, 34 (' 962), pp. 76-80. 6. A. Krokstad, Electroacoustic means of controlling auditorium acoustics, Applied Acoustics (in press). 7. R. Thiele, Richtungsverteilung und Zeitfolgen der Schallriickwiirfen in Raumen, Acustica, 3 (1953), pp. 291-302. 8. R. Kilter, Zur Gewinnen yon Einzahlkriterien bei lmpuismessungen in der Raumakustik, Acustica, 21 (1969), pp. 370-2.