General Design Considerations

Although there are design features unique to the type of room, there are also general guidelines for large mixed-use facilities that apply to all of these rooms.

1. The source should be elevated above the seated audience so that sight lines are clear of obstruction.

2. A sound system capable of reproducing the full program frequency range must be incorporated into the design. The loudspeaker layout should be arranged so that the talker is the perceived origination point.

3. Where unamplified music is a part of the program, the singers or musicians should be aided by nearby reflective surfaces both overhead and on either side. These can be integrated into the design of the structure, suspended above the players, or built into a moveable shell. Musical instruments supporting the singers should be located near them, so there is no great time difference between the two.

4. The floor should be raked to provide every other row sight lines of the APS.

5. The room volume and absorption should be controlled to achieve a reverberation time consistent with the program. Padded seating must be used to minimize the differences between the empty and occupied conditions. Room reverberation must be controlled to limit the loudness of large groups.

6. Background noise levels should be limited to NC 25 in small (< 500 seats) auditoria and to NC 20 in opera houses and drama theaters. In large concert halls it should be limited to NC 15.

Program

Architectural designs begin with a program, listing the various uses contemplated for a space and the percentage distribution of each. Program definition allows the architect and acoustician to know whether to steer the design compromise toward a speech-like or music-like solution. Not infrequently, in the design of performance spaces, there may be disagreements as to the proper direction, not only within the design team, but also among the members of the client team. It is always best to sort this out in writing early in the process so that the direction is clear.

Changes to the program, particularly late in the process, can have profound consequences. Hidden agendas are difficult to uncover. This is particularly true when there is an expectation that the room will serve a purpose different from its stated use. For example, a church may be designed as a worship space, but may also be expected to serve as a theater, performing arts center, and television studio where there are significant additional lighting, rigging, audio, and HVAC requirements. A well-crafted program can allow these issues to be sorted out and can help establish a clear design direction.

Room Shape

Floor plans are laid out on the basis of the program, which drives the seating configuration, volume, sight lines, circulation, and aesthetics. In rooms designed for speech it is good practice to bring the audience close to the talker. A fan-shaped room, particularly one with balconies, allows a larger number of people to be close to the stage, than a rectangular room. For unamplified music, where strong side reflections are important, a narrow rectangular room with a high ceiling has traditionally yielded the best result. For rooms of mixed use a compromise between the speech and music requirements must be crafted.

Small general-purpose auditoria are typically rectangular, with the walls near the front of the room angled out from the stage. For lecture halls more of a fan shape is preferred, with the maximum angle between opposing walls no more than 140°. In the case of mixed use spaces the included angle falls between 40° and 80°, with smaller values being best for music. Examples are given in Fig. 20.1. The side walls at the rear of the room are either parallel or flared out at a shallower angle than in front. This scheme provides useful early reflections from the walls near the stage or platform, while allowing regular aisles and constant width seating sections at the rear.

In large multiuse theaters it becomes increasingly difficult to accommodate the audience with good sight lines. A fan shape is the usual choice, truncated on the sides, with one or more balconies. The distance to the farthest viewer depends on the type of performance. The human eye can perceive an object as small as one minute of arc (Burris-Meyer and Cole, 1975) or about 0.35" (9 mm) at 100 ft (30 m). As the last row is pushed back to 135 feet (41 m), the smallest resolvable dimension is increased to 0.5" (13 mm) and a raised eyebrow or small gesture is lost. Thus if the subtle facial expressions of a theatrical performance are to be appreciated, the farthest patron should be seated no more than about 80 feet away.

Design of Multipurpose AuditoRia and Sanctuaries 699 Figure 20.1 Side Wall Design Near the Stage

Rectangular seating plan - Poor side wall reflections Potential flutter echo on the thrust stage
Different Shapes Bank Floor Plan

This is a more restrictive requirement than seating for orchestral performances, and limits first floor seating to around 26 rows or 1100 seats for three banks of fourteen seats.

In very large spaces the seating fan is forced to a greater angle often with four seating banks with a center aisle or three banks with a half bank on each outside wall. In the Radio City Music Hall in New York for example there are five seating banks. The remainder of the audience is seated in the balconies, which must not cause shadowing through excessive overlap of the seats below. It is difficult to design auditoria for intimate theatrical productions larger than about 1500 seats. In large houses the types of performances must be adjusted to the scale of the space.

The shape of worship spaces is determined primarily by liturgical requirements. Churches require a focal point either at the front or in the center. A raised platform helps acoustics as well as sight lines, but if it is too high there may be unshielded sound paths to the side or end walls, leading to long-delayed reflections and flutter echo. A cruciform shape may be highly desirable in some denominations. Medium-sized congregations, in the 500 to 1000 range, can be accommodated in a wide variety of room shapes, but care must be exercised not to let the room volumes get out of hand. Very large churches in the 2500 to 10,000 seat range are fan-shaped for the practical reason that this is the only way to accommodate the necessary seating. To maintain sight lines the included fan angle should not exceed 160°. These large venues rely on electronic sound reinforcement and video projection systems to transmit their message, so the sight lines to these sources are also important.

Religious and theatrical facilities are sometimes designed with circular or multifaceted hexagonal or octagonal floor plans. Such shapes should be avoided wherever possible since they focus sound and create localized regions of high level. Churches and theaters in the round, even with diffusive or absorptive walls, are also poor acoustical choices. At best they result in fair-to-mediocre sight and sound quality in spite of the expenditure of great effort and treasure.

Worship spaces have higher ceilings than auditoria and a stately architectural form, often with historical roots. Ceilings of religious structures can be dome-shaped, which generate focused reflections. When long delayed reflections arrive back at the talker at an elevated level, the effect can be particularly unpleasant. There is a combination of level and delay time (around 100-150 msec) so disconcerting to the talker, that, in the worst case, it can act as a severe impediment to normal speech. Large concave shapes should be avoided, particularly when the talker and listener are near the center of curvature. A calculation of the curvature gain can identify problem conditions. Curved walls can be treated with absorption, however since the normal-incidence absorption coefficients are lower than random-incidence values, there may still be reflected levels high enough to be of concern

Seating

Circulation and building code requirements influence the shape of the floor plan. In the conventional seating plan US building codes require that there be no more than 6 seats between a seated patron and an aisle, permitting a maximum of 14 seats in a section bounded by two aisles. Normal row-to-row spacing is 36 to 38" (0.92 to 0.97 m). In continental seating the larger distance between rows ( 40-42" or 1.01-1.07 m) allows them to be considered cross aisles and the six-seat limit no longer applies. Opera chairs are available in various widths ranging from 19 to 23" (0.48 to 0.58 m). Normal seats are about 22" wide (0.56 m) with some at 21" (0.53 m) to accommodate the stagger. A full 14-seat bank occupies 25.5 ft (7.8 m) plus the aisles.

The decision on whether to have a center aisle is influenced by the program. In religious structures a center aisle is the usual choice to provide for a processional march in traditional wedding ceremonies. In a theater the best seats are located in the center so an even number of aisles is a better alternative. Building codes in the United States specify minimum 36" (0.9 m) single-sided and 42" double-sided aisles, which widen 1.5" (38 mm) for every five feet of seating as they approach the nearest exit.

Handicapped seating requirements call for wheelchair spaces along with adjacent companion seats within the space. Recent changes to the Uniform Building Code to accommodate handicapped access can limit floor slopes to 1:12—less than ideal for every other row sight lines depending on the height of the APS. Requirements vary and enforcement of these codes differs. It is always best to get clear directions from the local building officials.

Table 20.1 Range of Volume per Seat by Type of Auditorium (Doelle, 1972)

Volume per Seat cu ft (cu m)

Table 20.1 Range of Volume per Seat by Type of Auditorium (Doelle, 1972)

Volume per Seat cu ft (cu m)

Type of Auditorium

Min

Mid

Max

Rooms for Speech

80 (2.3)

110 (3.1)

150 (4.3)

Concert Halls

220 (6.2)

275 (7.8)

380 (10.8)

Opera Houses

160 (4.5)

200 (5.7)

260 (7.4)

Churches/Synagogues

180 (5.1)

255 (7.2)

320 (9.1)

Multipurpose Auditoriums

180 (5.1)

250 (7.1)

300 (8.5)

Motion-picture Theaters

100 (2.8)

125 (3.5)

Room volumes are selected using the volume per seat as a guideline. Since the ideal reverberation time for speech is low, the preferred volume per seat for speech is also low, ranging from 80 to 150 cu ft (2.3 to 4.3 cu m), with smaller auditoria having the larger volume per seat. For unamplified music it can vary from 160 to 400 cu ft (4.5 to 11.3 cu m), again with smaller halls having the greater volume per seat. In rooms designed for mixed uses, the volumes fall between these values, 180 to 300 cu ft (5.1 to 8.5 cu m). Table 20.1 shows typical ranges by room use.

Reverberation Time

From the program, the designer can establish the reverberation time and a preferred volume per seat for the space. The seating capacity will then fix the overall volume of the room and the circulation and other functions will lead to a preliminary floor plan. Figure 17.10 shows reverberation time versus room volume values for various types of spaces. Figure 17.11 shows the preferred behavior of reverberation time with frequency for music. In small auditoria and churches the overall volumes are rarely high enough to raise the bass response to the level shown in this figure and the alternative, which is using heavy plaster or multiple layers of drywall, is cost prohibitive except in pure concert halls. As rooms grow larger a high volume can be used with applied absorption to control the mid-frequency reverberation. The reverberation time falls off naturally at high frequencies due to air absorption and thin absorbent materials. By using wood slats over absorptive panels it is possible to offset these effects and raise the high-frequency reverberation time. This provides a favorable frequency balance as long as the absorption is needed anyway at other frequencies.

Absorption

In large auditoria and religious structures the biggest absorbing surface is the seated audience. It provides about 85% of the total absorption in concert halls and somewhat less in auditoria where applied absorptive material may be used. Since it is desirable to maintain a consistent acoustical environment no matter the number of people, it is most important to use seats, whose absorption characteristics closely resemble that of a seated occupant. This requires that the chairs or pews have thick padding on both the seats and backs. Beranek (1996) observed that the absorptive properties of a given number of seats depends not on the number of seats, but on the area they cover. Thus reverberation calculations should be based on an absorption coefficient per unit area of seating, not on the number of sabins per seat.

As sound propagates over the seats of a theater it is found that there is an extra attenuation above that expected from distance and grazing losses. This extra attenuation is centered on 125 Hz and appears as a deep dip in the attenuation vs frequency response curve. The phenomenon has been discussed in earlier chapters and measured data are shown in Fig. 7.44. Ando (1985) analyzed the effect and related it to the quarter-wave well impedance caused by the depth of the seats. He proposed various solutions including cavities in the floor and several geometrical alternatives. Unfortunately, there is little measured data available to determine the effectiveness of his suggestions.

When the natural acoustics of a room is augmented by a sound system and design constraints allow the use of absorbent surface materials, the room volume and surface orientations are less critical than in concert hall environments. In these cases absorptive materials can be added to control unwanted reflections and reverberation since the level generated by the sound system can make up for the energy lost through absorption. In rooms accommodating both music and speech, the ideal solution is to design to the reverberation time appropriate for the music, and to manage the intelligibility using the reinforcement system. Where unamplified speech must be supported, variable absorption can be a good alternative.

Balconies allow more of the audience to be seated close to the stage and limit the slope of the orchestra seating. Balcony floors have steeper rakes than the orchestra floor, but the slope can be calculated using the same equations. The slope should not be greater than 30° (Ramsey and Sleeper, 1970) to 26° (Egan, 1988), and the top of the balcony should not be more than 65 feet above the stage to avoid vertigo (Egan, 1988). Figure 20.2 shows a typical balcony configuration.

Balcony overhangs must be controlled to allow the reverberant sound to reach the seats beneath them. Beranek suggests a depth-to-opening ratio of one for concert halls and two for opera houses. For speech, balcony overhangs can be deeper without undue degradation. A two-to-one depth-to-opening ratio is acceptable.

A slightly convex under-balcony ceiling can help redirect sound into the shielded area. Likewise a rising leading edge at the front of the balcony is also helpful. Where the

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