Concert Standard materials may be used in concert hall construction, but for acoustical reasons they are often handled in special ways. Halls by David McCandless Specifying for Sound Performance To some designers and specification separated. One discipline is the design writers it may seem strange to think of the auditorium itself: the volume, of all concert hall construction shaping, finishes, and furnishings materials as “acoustical” materials, necessary to achieve the best but such is the case. All interior wall, acoustical environment for the natural floor, and ceiling surfaces, and all (unamplified) sounds of musical finishes and furnishings have an instruments. This discipline is called effect on the acoustical environment “room acoustics.” in which concerts are performed. In If it is a place for listening, a big addition, the way the hall is con- room's acoustical environment is its structed and the design of the HVAC hallmark. For a concert hall, “good system play a part in eliminating acoustics” involves noise disturbance during a concert. • good distribution of sounds to all Standard materials may be used, but the seats, which depends on proper for acoustical reasons they are often shaping and finishes of all interior handled in special ways. surfaces In concert hall acoustics design • natural sound diffusion and there are two basic disciplines—room envelopment acoustics and noise control—whose • a sense of intimacy for the audi- needs and solutions are different, ence and a sense of ensemble for both though they must often be achieved in musicians and audience the same wall, floor, or ceiling detail. • proper reverberation times Sometimes the same materials are throughout all frequencies, which used for different reasons, and their depend on the room's volume and the materials' specifications are similarly total sound absorption of all materials The two acoustical disciplines: room acoustics and noise control. • freedom from the acoustical acoustics have been established. The that shape will be hard, sound- faults of echoes, flutter, and focus floor, wall, and ceiling structures reflective materials, individually • freedom from disturbing noises enclosing that envelope must then be described further in the specifications. Many halls in which classical designed to achieve the necessary Some typical hard materials and music is performed have proscenium noise control. The design process relevant room acoustics concerns are stages, lofts, and backstage areas used must recognize the two acoustical described below. when other types of programs are disciplines throughout the project's Plaster. Plaster is a traditional presented. Symphony concerts in creative and construction document material for ceilings and walls in these halls usually require special phases. concert halls. It is used in large areas orchestra shells designed to put the The degree of noise control re- and on broad surfaces, as well as in orchestra in the same room with the quired is usually set when the room specially shaped details. Plaster is audience, eliminating sound energy acoustics criteria are established. usually used with extra thickness—up loss and late arrival of sounds from Typically, the allowable background to 1½ to 2 inches—because the the back of the stage. Halls planned (ambient) noise levels are set at Noise stiffness and mass are necessary to for concert programs only, such as Criterion 25 (NC-25), a single resist panel vibration, which causes Boston's Symphony Hall, among number rating representing a series of low frequency absorption, and to many others, have a platform for the allowable levels across eight octaves achieve good reflections at all orchestra in the same room with the of the hearing range. Sometimes an frequencies. Since the necessary audience, without any backstage, loft, even lower Noise Criterion is used, stiffness requires more mass than is or proscenium arch. especially where sound recording is typical in most of today's building “Noise control,” the elimination of anticipated. While the NC levels are details (gypsum board, for example), distracting sounds from the concert used most specifically for controlling a stronger supporting structure may hall, is the other basic discipline in noise from the HVAC systems, they also need to be specified. auditorium acoustics. The typical are also used as a basis for controlling The acoustics of a concert hall require sources of disturbing noises are other other transmitted noises. These very diffuse reflections throughout activities within the building; the controls must be developed in relation the room. To enhance the reverber- building's mechanical systems; and to each other so that the total of ance and give a sense of sound airplanes, trains, and other traffic transmitted noises through the room's envelopment, the side walls and outside the building. envelope and through the HVAC sometimes the ceiling are often When the proper “envelope” (the system together do not exceed the shaped to scatter sounds. Plaster is room's shape and volume) has been design criteria. often used to create these shapes, designed, with the proper interior which may be large architectural finishes and furnishings, the room “Hard” Acoustical Finishes and forms or smaller decorative and Furnishings functional forms. Many of the DAVID McCANDLESS, AIA, is direc- The volume and shape of a concert architectural shapes used in concert tor of architectural acoustics for Jack hall are established in the drawings hall interior surfaces are repeated Evans & Associates, Inc., an acousti- and details in a project's contract enough to permit precasting. This cal consulting firm in Austin, Texas. documents. Most of the surfaces of casting is often done in fiberglass- Reprinted by permission The Construction Specifier, April 1990 The two disciplines in a typical detail. reinforced gypsum (FRG). When framing, they are stressed so that they large, flat surfaces of FRG are cannot vibrate diaphragmatically. So designed, they must be specified to be bent, they are as stiff as thick plaster, stiff like thick plaster. and will not exhibit much low Gypsum board. Gypsum board is frequency absorption unless the seldom used in concert hall design whole curved system is not rigidly concern. Small glass decorative because it has too much low fre- supported. elements might actually provide some quency absorption resulting from Masonry. Masonry materials such useful sound diffusion. natural panel vibration in response to as stone, brick, block, and concrete sound pressure pulses. When gypsum usually are thick and massive enough “Soft” Acoustical Finishes and Furnishings board is used in walls and ceilings, as to reflect low frequencies. Some in civic auditoriums and most porous, precast masonry units, The people attending a concert churches, the designer should specify however, absorb middle and upper probably constitute the most absorp- double layers of ⅝-inch-thick gypsum frequencies due to their porosity; if tive surface in a concert hall. The board, laminated for stiffness and used over large areas these materials other absorptive surfaces will be the installed on a furring system designed will significantly affect the reverbera- treatments for controlling echoes, and with extra stiffness. tion times of the room. The porosity sound foci from rear walls, balcony Wood and wood paneling. can be sealed and painted, of course, faces, and similar surfaces. These Wood, often used in concert hall but only on the recommendation of an areas must be limited, however, or design for aesthetic reasons, must be acoustical consultant who has else the room could become too installed with care. Thin wood assessed all finishes and furnishings “dead”—not “live” enough to give the paneling with air space behind it (i.e., in the room. music the desired reverberance. on furring) will allow unwanted low Metals and metal paneling. In Since the audience is the major frequency absorption through panel addition to absorbing low frequencies, sound absorber, comparing the room vibration—much as thin plaster and diaphragmatic movement of metal volume to the number of seats gypsum board do. Such paneling must materials might also cause rattles, so planned is useful. The reverberation be backed with solid construction they, too, should be laminated to solid time in the room is related directly to such as plaster or thick gypsum board, core material. the volume of the room and, in- to which it can be laminated. Large Glass. Glass is seldom used in versely, to the total sound absorption areas—and small surfaces repeated concert halls, since these rooms are in the room. The volume/seat ratio enough to be acoustically signifi- usually internal spaces surrounded by that has been found to be appropriate, cant—require this treatment. Solid public hallways and functional through comparison of many good wood used in moldings and other spaces, and because windows could symphony halls, can range from 250 details usually is not a concern; in be a path of unwanted noise transmis- to 350 cubic feet per seat. This is only fact, it may make the paneling stiffer. sion. Glass might be used for a rule of thumb for room size, When plywood and gypsum board decorative design and lighting, but however. Proper sound distribution, are bent over curved wall or ceiling only large, flat areas would be of diffusion, envelopment, intimacy, and Reprinted by permission The Construction Specifier, April 1990 Sound Transmission Class (STC). A single-number rating system for barrier constructions for full-frequency performance in laboratory testing. Two stud walls shown. the right reverberation times must still be achieved, along with freedom from noise disturbance, echoes, focus, and flutter. Some of the typical, soft (absorptive) materials used, and their relevant concerns in concert halls, are described below. Seating. Seating absorption should match the absorption of people so that when people are absent, as in rehearsals and low attendance, there will not be a noticeable change in the reverberation times of the hall. Designers should specify seats upholstered on the cushion and back rest, and possibly on the rear of the seat back.