Designing Auditoriums

This Weatherford, Texas, High School auditorium was designed by VLK Architects; the acoustic consultant was Wrightson Johnson Haddon & Williams.

Picture the auditoriums from your K-12 days. Chances are one was a large, box-shaped, concrete block room with a stage on one end, mobile risers and a metal ceiling. It was used for everything from band concerts to basketball tournaments. Chances are also good that you had to strain to hear the lead actress in the class play, but your ears rang after the band recital.

In the past, too little thought was given to acoustics during the design and construction of school auditoriums. If trained acousticians were involved at all, it was after construction when they were asked to solve sound issues that could have easily been avoided in the design phase.

Today, new school construction is generally abandoning the multipurpose gymnasium/auditorium/cafeteria approach in favor of more suitable, individual spaces with acousticians consulted during the initial design stage. Because the conflicting acoustical objectives of an auditorium (to reinforce sound from a single location) and that of a gymnasium (to suppress noise from many sources) are impossible to resolve, the combination of the two spaces is now generally avoided. Although budget is always a consideration, the cost of building one space with the variable systems necessary to make it acoustically acceptable for both purposes often equals the cost of building two acoustically accurate spaces.

Russ Berger, president of the architectural acoustics firm Russ Berger Design Group, knows well the challenges associated with blending architectural styling with acoustical considerations. "There are three basic acoustical ingredients that architects and acousticians have to work with in building a good sounding room – the volume of the space, shape of the space, and the architectural/acoustical finishes. Usually the finishes are the one component that the designer has the most control over and oftentimes is the only component that can be modified," says Berger.

Reverberation Control

The most common acoustical challenge is excessive reverberation caused by the large physical volume and hard surfaces. Excessive reverberation results in poor speech intelligibility and distorted musical performances.

The proper amount of reverberation depends on the type of performance. Dramatic performances and lectures require very little reverberation to ensure clarity of speech. Music performances, on the other hand, usually require some amount of reverberation.

Auditorium flexibility is a reality most of the time and effective acoustical finish treatments can help alleviate, if not solve many of the problems, advises Berger.

Parallel walls are a major culprit of excessive reverberation and echoes. Good design avoids parallel surfaces, or breaks up surfaces with irregularities or angles. Parallel surfaces are minimized to avoid flutter echoes and redirect sound back toward the audience.

The rear wall should be shaped to reflect energy down to the audience, or incorporate acoustic absorbers to "trap" the sound. A concave-shaped rear wall should be avoided because it will cause disturbing echoes for those on stage. The use of more absorbent building materials, such as glass fiber panels, as opposed to bare gypsum wallboard and concrete will also help to cut down reflections.

The ceiling above the stage should be angled down to reflect sound to the audience. The ceiling above the audience should also have sections that are angled to spread reflections throughout the space.

Overhead reflectors that raise, lower, or change the ceiling configuration and portable stage enclosures are available to alter the way sound is reflected based upon performance type. While these options can be very effective, the cost and labor involved can be prohibitive.

Adding Absorption

In addition to construction features that enhance acoustics, materials can be introduced that provide absorption. A primary source of absorption in an auditorium is the audience. Auditorium seating should be upholstered to provide approximately the same amount of absorption as a seated person, thereby keeping the acoustical properties of the room constant regardless of whether it’s a packed house or an empty audition.

Carpet, curtains and certain types of ceiling tiles are also popular ways to provide absorption. Retractable banners and draperies that slide horizontally or rise vertically are another way to add absorption when necessary and remove it when unnecessary.

The most cost-effective way to incorporate absorption in a space is to install acoustical absorptive panels made of porous mineral fiber, e.g. glass, and covered by acoustically transparent fabric.

For adequate absorption, at least 10 percent to 20 percent of available wall space should be treated with acoustical absorbers. But note that absorption can be overdone, causing the reflected sound from the stage to be absorbed before it reaches the listener. Some reverberation is necessary to create intimacy, a feeling of musical envelopment by the audience. This can be achieved through diffusion and redirection of sound energy. Acoustical diffusers use highly engineered, complex surfaces to evenly distribute sound and provide the same degree of high quality sound to every person in the auditorium. This is often referred to as widening the "sweet spot" or eliminating "hot spots" and "nulls."

Ensuring accurate listening conditions in school auditoriums is not costly, nor time consuming. It is a simple matter of involving a trained consultant in the design of the space that will provide quality performances and increased learning.

Jeff D. Szymanski, PE is the chief acoustical engineer for Auralex Acoustics Inc. His experience covers many acoustics areas including a concentration on architectural acoustics. He can be reached at (317) 842-2600 or savant@auralex.com.

National Council of Acoustical Consultants: www.ncac.com