The Development of Room Acoustical Demands on Operas Within the Last 50 Years

The development of room acoustical demands on operas within the last 50 years Gerhard Mtiller and Helmut A, Mtiller Mtiller-BBM GmbH, Robert-Koch-Str. 11, D-82 152 Planegg, Germany Abstract: Using the experiences gained by eliminating the room acoustical complaints about an opera house, it will be shown that the room acoustical design criteria had to be slightly modified and extended, It turned out that speech intelligibility is less important than a merging sound. Furthermore, the acoustical contact among musicians and their feedback to the auditorium has to be taken very seriously, In addition, a flawless acoustical localization of the sources on the stage by the audience is of importance. DEVELOPMENT FROM ROYAL TO MODERN OPERAS When designing the Festspielhaus Bayreuth, Richard Wagner departed from the old-fashioned horseshoe- shaped royal opera and tried to create an auditorium where all of the visitors could see and hear equally well. When rebuilding the destroyed theatres after World War 11,many architects took up this idea. In addition, the acousticians of that time believed in the results of simple opinion polls which questioned audience and musicians and which seemed to indicate that in operas only the sound transmission from the stage, resp. the or- ches~a pit to the auditorium and a high speech intelligibility are of importance. Therefore, all the sound energy produced by the sources was directed to the auditorium, and the reflections were used for compensating the decreasing sound pressure with distance. Late reflections were eliminated or at least weakened by sound absorbing linings so that a low reverberation time and a high speech intelligibility were obtained. TEATRO REGIO TORINO, A MODERN OPERA In the Teatro Regio Torino (designed in the sixties by Arch. C. Mollino and the acoustical consultants Prof. G. Sacerdote / Ing. R. Pisani [I]) this goal was achieved in such a perfect way that the ffuctuation of the sound pressure level within the auditorium area did not exceed 2 dB. For getting a high speech intelligibility, the first order reflections reached the listeners within 50 ms afier the direct sound. Later arriving reflections were weakened by sound absorbing claddings. Thus a relatively low reverberation time of approx. 1.4 s was provided at midfrequencies [1]. Surprisingly, the acoustics of the Teatro Regio received negative reviews. The artists, singers on stage, and orchestra members in the pit claimed having insufficient acoustical contact between each other and to the auditorium as well as a lack of melting of the sound. Furthermore, the audience criticized the latter and, addition- ally, the dependence of the sound transmission on the position of the sound sources on stage and of the listener in the auditorium. ~, .,,=;<f’+~::::.::;:.- ..*, ,/ ,f: <<=:;=::~..-...,’ ‘-’?-< *:#$\ h:- i ~I i?wy~- “1,:: + < 0 5 10 15m FIGURE 1. Teatro Regio, Cross and longitudinal section (black lines and grey hatching indicate changes) MEAS~ES FOR ELIMINAT~G T~ COMPLAINTS Similar to the experiences with the Wurttembergisches Staatstheater Stuttgart [2], it turned out that it was partly the acoustical measures carried out to achieve the goals mentioned above that caused the deficiencies: the shape of the proscenium with its lack of reflections back to the musicians, the high sound absorption of walls, floors, chairs. Apart from this, the focusing shape of the side walls and of the rear part of the ceiling (Figure 1) were acoustically problematic. 327 Taking into account the fact that this theatre had already become a protected monument, an additional proscenium was installed, the sound absorbing carpets on the floor replaced by a wooden floor and the absorbing carpets at the walls as well as the absorbing cushions in front of the balconies removed. For avoiding an increased focusing effect of the walls after removing the carpet, diffise reflecting and inclined polycylinders were installed. In addition, the tremendous width of the auditorium close to the stage was reduced and the open area of the orchestra pit enlarged (Figure 1). The sound absorption of the chahs was reduced by removing the cloth from the rear sides of the backrests and replacing it by wooden plates. RESULTS The room acoustical changes are highly appreciated by the audience and the musicians. The fact that part of the useful sound energy is no longer reflected to the audience but used for improving the conditions for the musicians is more than compensated by their increased self-confidence and relaxed, precise playing. The changes measured unoccupied with open playing curtain and sound absorbing stage decoration led to the following values (using ~SSA measurement equipment with omnidkectional sound source and omnidirectional microphones and a program for calculating the strength indices and the cross correlation coefficients): Acoustical contact: To describe the improvement of the acoustical contact among musicians, the changes of the average strength coefficients integrated over 80 ms are used. Within the orchestra pit G80 increased by 2-3 dB and between orchestra and stage by 1-2 dB. Since the reflections from the wails of the pit are essen- tially weakened by the musicians, the differences become even higher in case the orchestra pit is occupied. Merging sound: For indicating the change into a merging sound as well as the increased feedback from the auditorium to the musicians, the measured reverberation time can be used (see Figure 2). The reverberation time was increased from 1.4 to 1.9 s at midfrequencies with sound absorbing stage decor and from 1.5 to 2.0 s with iron curtain closed. In both cases, the frequency response of the reverberation was changed to a brighter sound, 2 absorbingstagedecor afier modification T [S] ::-<;, , -,:, ,:.**‘:’ “!:~2, 1.5 -<” ‘,,%~-- -- ‘ ----- iron curtain closed f %- beforemodification 1’ FIGURE 2. Reverberation time, auditorium unoccupied m125250500 1 k Hz 4 k frequency Acoustical orientation: For checking the acoustical orientation, the cross correlation factor was measured with two omnidirectional microphones at a distance of 20 cm and the perpendicular line to the connecting line between the microphones directed against the source. In Figure 3, the length of the arrows represent the maximum cross correlation coefficient. Their direction indicates where the maximum points relative to the direction of the source. As can be seen, due to the measures taken the orientation has been improved. The de- viation on receiving point E6 is due to the fact that strong restrictions in changing the architectural shape did not allow to modify the side walls and the respective ceiling, source lzk :&:g> :$3fi&~@ @lccc ‘$i,> &d Wb ‘u FIGURE 3. Maximum of the lCCC and its angle relative to the source position q afier 8 before CONCLUSIONS It follows that the most important changes in the acoustical design of operas within the last 50 years are that greater value is placed on the acoustical contact among musicians and to the feedback of the auditorium as well as to the merging of sound supported by a longer reverberation time even with a slightly reduced speech intelligibility. The acoustical orientation of sound sources on the stage (singers) is taken more seriously than in earlier times. REFERENCES 1. Ii nuovo Teatro Regio di Torino, Sot. d. ingeneri e d. architetti 9-10, Sett.-Ott, 1973, p. 17 ff, 99 ff 2. Muller, H.A. and Muller, Kh,, Fortschritfe der Akus/ik - DAGA 85, Bad Honnef, DPG, p. 409 ff 328.

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