Achieving Excellent Auditorium Acoustics
By: William Elliott
From an acoustical perspective, the school auditorium presents a unique design challenge. The two primary uses of the auditorium, dramatic and musical, each have inherent acoustical requirements. Dramatic performances require a high level of speech intelligibility; musical performances require an acoustical environment with the necessary reverberation.
Aside from the primary uses of the school auditorium, lectures, assemblies, study halls, and a myriad of other events often find home in this space. Although the gymnasium is frequently used for functions that require ample seating capacity, it is the auditorium that is truly the crucible for academics, the fine arts, and oratory brought alive through active participation.
To function well, the auditorium must be conducive to its intended use(s). Although the multi-purpose nature of the auditorium may appear to place many constraints on the acoustical environment, there is a set of design principles that, if followed, will produce interior acoustics that always complement the sound source, whatever (or wherever) it may be. An auditorium with proper acoustics encourages the audience to contemplate the acoustical content (be it oratorical, dramatic, or musical) of the sound source. An auditorium with poor acoustics is immediately apparent and detracts from the presentation.
Creating the desired acoustical environment within the auditorium is akin to the approach the architect takes when designing the structure itself. The scale of the interior space, the use of various finish materials, and the physical articulation of the space are the means through which the building itself it realized. To the same end, a successful acoustical environment depends on the shape of the space and how it is related to the chosen finish materials. The architect juxtaposes interior finishes (e.g., wood, concrete, glass, etc.) to achieve the desired aesthetic; the acoustician examines the acoustical performance of the same materials to achieve the desired acoustical environment.
Auditorium Design Criteria
Successful acoustics in the school auditorium are built on four pillars: 1) the background noise level must be low enough so as not to interfere with the perception of the desired sound, 2) the desired sound(s) must be sufficiently loud, 3) the sound within the auditorium should be distributed with considerable uniformity (this statement implies the avoidance of focusing, echoes, and areas of deficient sound level when compared with other positions in the room), and 4) the reverberation time should be well-suited to the intended use of the space.
In the case of the school auditorium, which “wears many hats,” this translates into a reverberation time that is short enough to maintain speech intelligibility, but is long enough to provide blending for musical performances.
1. Background Noise
A common metric used to characterize background noise is known as the noise criterion rating (NC rating). This is a single number rating given to the measured background sound levels within a space; the lower the NC rating, the quieter the space. An NC-25 to NC-30 is recommended for school auditoria. The background sound, which contributes to the NC rating, can originate from any number of sources, including the air handling equipment, noise in adjacent rooms, nearby traffic, lighting dimmers, etc.
When designing to achieve the acceptable NC rating for an auditorium, knowledge of another acoustical metric is required. The sound transmission class (STC) rating for a wall or roof indicates the ability of that composite construction to resist the transmission of sound from one side of the construction to the other. A high STC rating indicates the composite construction in question (wall or roof) functions well in preventing undesired sounds from entering the auditorium. An auditorium with high STC walls limits sound generated in adjacent spaces from entering the auditorium, and, conversely, high STC walls limit sound generated in the auditorium from entering adjacent spaces.
2. Reinforcement of the Desired Sound
Although sound reinforcement is typically a phrase used to describe the electronic equipment that accomplishes this task, it is important to understand that the successful auditorium itself functions to reinforce the desired sound. This reinforcement is accomplished through properly placed hard reflecting surfaces. These surfaces provide early reflections, which reinforce the direct sound arriving at the listeners' ears.
3. Distribution of Sound
The distribution of sound within the school auditorium is not surprisingly related to how the space is articulated, that is both the room's shape and the room's interior finish materials. The diffusion of sound energy (i.e., the redirection of sound reflections) is especially desirable for musical uses. Sound diffusion may be accomplished in tandem with the natural reinforcement of the desired sound by “breaking up” otherwise monolithic surfaces.
The distribution of sound over the audience area is also dependent on the efficiency by which sound travels from the stage house to the main space of the auditorium. In the case of a musical performing ensemble, a stage enclosure is essential for assisting in the projection of sound into the auditorium. Such an enclosure also serves a second, equally important purpose of encouraging good on-stage communication and intelligibility. These qualities are crucial for successful rehearsals and performances.
4. Reverberation Time
The design of a school auditorium with the correct reverberation time is perhaps the most delicate of the four acoustic design concepts presented here. The reverberation time of a given spaces is, roughly defined, the time it takes for sound to decay from a given level to the threshold of audibility. The space must be reverberant enough to meet expectations for musical performances, but not too reverberant so as to compromise speech intelligibility. Experience with relatively reverberant spaces reveals that good speech acoustics may be achieved if the ratio between the direct sound energy and the reverberant energy is high. Also, if the space is sufficiently large enough so as to require an integrated speech reinforcement system, highly directional loudspeakers may be employed to distribute sound to the audience with a minimum excitation of the reverberant room. Typical reverberation times for school auditoria range from 1.2 to 1.5 seconds. Because the reverberation time measured in an auditorium is related to the amount of sound-absorbing material present, it is influenced by the number of people sitting in the audience. To maintain a certain degree of reverberation time invariability with respect to audience size, it is important to choose upholstered seats; upholstered seats most closely resemble the absorption characteristics of seated audience members.
Divisible Auditoria
The aspect of school auditorium design that poses the greatest challenge from an acoustical standpoint is not the multi-purpose functionality of the space, but rather the divisible auditorium concept.
The divisible auditorium may not only be used for large assemblies but for more intimate teaching purposes when the operable walls are deployed (the subdivided spaces being akin to intimate college lecture halls). The design criteria for good speech intelligibility still apply in the smaller spaces, but the challenge becomes properly designing for adequate isolation between the subdivided spaces. Also, the suppression of background noise may need to be reevaluated since some background noise will help to mask the sound entering one subdivision of the auditorium from an adjacent space. Other non-acoustical issues that may impact the room shape and thus need be considered include addressing the sloping floor/operable wall interface and storage for the operable walls.
Acoustical treatments and an excellent listening environment are so much more than simple wall-mounted panels. Room shape, interior finish materials (type, quantity and placement) are as integral to the visual design as to the acoustical performance.
The highest probability for a successful auditorium, then, comes from understanding the criteria that must be met and asking questions early in the design process. This is true for both new construction and renovation.
It is important to establish the acoustical design criteria early (in conjunction with discussions on the intended use for the space); doing so avoids costly substitutions/redesigns during the construction process. Be proactive in the design process with aesthetics and acoustics; the auditorium is the most versatile space within a school, and, executed well, it is potentially its most useful educational environment.
Bill Elliot is a senior consultant with Cavanaugh Tocci Associates Inc. At Cavanaugh Tocci Associates Inc., his work includes projects in architectural acoustics and noise control. He is an associate member of the Acoustical Society of America , and a recipient of the 2003 Robert Bradford Newman Award in Architectural Acoustics.
