Page 23 - Spring 2009

Fig. 3. Teatro Colon, Buenos Aires, Argentina (Beranek, 1996).
fixed source on the orchestra platform; (5) clarity, the strength of the initial sound along with early reflections arriving soon thereafter; and (6) warmth, the strength of the bass. Additional contributing factors that are not usually considered in these ratings include audience comfort, a low level of background noise, clear sightlines, and convenience. The absence of any of these qualities can offset the other positive factors.
Physical characteristics of successful halls
The features common to successful halls continue to be a subject of spirited debate, study, and technical measurement. Every rule seems to have at least one exception, but there is enough consensus that a very convincing case can be made.
The prominent feature of the most successful halls is their rectangular shape. Among the five best, only Teatro Colon varies—having the horseshoe shape of a classic European opera house. The best halls are narrow, usually less than 75 feet wide. Vereinssaal is only 65 feet, Symphony Hall 75 feet, Konzerthaus 68 feet, and Teatro Colon 80 feet. The exception is Concertgebouw at 91 feet. The narrow parallel
side walls in rectangular halls provide early reflections that increase clarity, along with later side reflections that sur- round or envelop the listener. The latter reflections include not only the first reflected sound but also additional multiple reflections extending out in time.
The best halls also have flat or gently sloping floors and elevated orchestra platforms. Most platforms are at or above the level of the last row of seats. This is in contrast to a legit- imate theater where the seating is raked for better sightlines. A typical stage height in a legitimate theater in the United States is 42 inches, based on the average seated eye height of about 44 inches. In Vereinssaal the orchestra platform is 39 inches, however, the main floor is flat except in the rear where the last few rows rise to about the level of the orches- tra. The platform height at Konzerthaus can vary between 31 and 95 inches, but the main floor seating plane is not raked. At Concertgebouw the platform is 59 inches and the floor is flat. At Symphony Hall the platform is 54 inches high and there are two seating configurations. In summer, for the “Pops” concerts, the floor is flat and patrons are seated around tables. In winter, a plywood floor is installed that rises at the rear to a height just above that of the orchestra. Teatro Colon has a gently raked floor and a pit filler that can be raised and lowered. The stage height is not given by Beranek (2004), but appears in photographs to be about 42 inches, consistent with its primary use as an opera house. The seats rise at the rear to about the same height as the stage.
Envelopment is enhanced when there are wall surfaces available to create multiple reflections in a plane just over the heads of the audience. This allows sound reflections to be sustained in this horizontal plane without being absorbed by the audience. When the seating is steeply raked or when the orchestra is seated on a low platform, side reflections are grounded out in the seating area after the first reflection. It can be observed that the most highly rated halls have reflec- tive surfaces located in the band above the first floor seating in the same horizontal plane as the orchestra. It is likely that this feature in Concertgebouw helps counteract the width of the room. The wall surfaces need not be smooth; indeed, scattering by diffuse objects can be helpful to envelopment. All of the best halls have significant side wall diffusion.
The reverberant character is quantified by the rever- beration time—the time it takes for a sound to die out. In large concert halls, when fully occupied, the reverberation time ranges from 1.8 to 2.2 seconds in the 500-1000 Hz octave bands. Shorter times are preferred for music in the classical style (Bach, Mozart and Haydn), which was origi- nally performed in smaller rooms. Longer times are pre- ferred for romantic music (Schubert, Mendelssohn, and Brahms). The reverberant character of these halls is gener- ally uniform. At Vereinssaal, Konzerthaus, and Concertgebouw the mid-frequency reverberation time is 2.0 seconds. Symphony Hall is 1.9 seconds. The exception is Teatro Colon at 1.6 seconds, where lower values are pre- ferred for better speech intelligibility.
Successful halls usually limit the number of seats to no more than 2400 and preferably to fewer than 2200.
22 Acoustics Today, April 2009