Many Architects Are Baffled by the Problems of Acoustic Design

By Roger K. Lewis

Saturday, December 15, 2001

The great architect Le Corbusier said "architecture is the magnificent play of forms in light." Few would disagree that pleasing the eye is a primary mission of architecture. But pleasing the ear is also a mission.

The quality of nonvisual sensory experience in architecture, especially auditory experience, is not a trivial issue. An aesthetically satisfying design may fail acoustically. Recall how often you have been in acoustically compromised spaces -- too much intrusive noise, too loud, too echoing. Sometimes it's too difficult to hear unamplified sound; sometimes there's too much sound amplification. Perhaps you work in an open office environment with little or no privacy, where multiple conversations nearby are both audible and distracting.

Acoustics is something of a stepchild in the world of architecture and interior design. It is given scant attention in architectural schools and often just cursory attention in practice. We know a lot about the science of sound and how to control it, yet we frequently fail to deal with sound rigorously when we create structures and spaces. Acoustic consultants typically are on the scene when we design theaters, concert halls, auditoriums or recording studios, but rarely for other projects.

Basic acoustic objectives in architecture fall into three broad areas. For any occupied space in a building, we can identify those objectives relative to desired levels of acoustic performance.

• Sound transmission between spaces. Whether an opera house, a classroom or a corridor, some spaces must be free of noise from elsewhere. At the same time, noises generated within a space must not escape into other nearby spaces. This requires controlling sound transmission between spaces or between interconnected parts of buildings.
• Reverberation quality within a space. Depending on their use and desired ambience, some spaces need to be quiet, hushed and acoustically muffled, while others need to be acoustically "live" and reverberant. The acoustic quality suitable for an intimate bistro or your living room may not be desirable for a tavern or beauty salon. Reverberation is a function of a room's size and volume, shape and materials.
• Sound enhancement. In spaces used for assembly, public speech and musical or theatrical performances, the geometry and finishes ideally enable the entire audience to hear sounds from the stage that are distinct and comprehensible, sufficiently loud and, in the case of music performed by ensembles, well blended.

Achieving the right mixture of these objectives requires understanding the many possible sources of sound, how sounds are transmitted and various construction strategies and tactics for controlling sound.

Sound energy readily moves through and between air and solid matter. You don't have to be a rocket scientist, or an acoustician, to know that most noise is generated by human voices, by playing or reproducing music, by traffic, by planes or trains or by machines both inside and outside of buildings. Short of eliminating noise sources, the best way to keep noise out is to build an enclosing structure through which sound cannot easily travel.

Four basic construction tactics prevent noise transmission into a space: closing and tightly sealing all openings, even the tiniest, through which sound can pass; making walls, floors and roofs sufficiently massive; acoustically separating abutting structural components; and preventing noise from arriving through ducts and other mechanical components.

The smallest cracks or air passages permit sound to intrude. For example, if you seal gaps around light switches and electrical receptacles, subtle noises from adjacent rooms will decrease. Your house will be noticeably quieter if equipped with tightly fitting, solid-core doors and well-crafted windows with high-quality seals, gaskets and double-pane insulating glass.

Heavy masonry or concrete walls impede sound more effectively than partitions framed with wood or metal studs. However, walls that are double partitions, structurally separated and insulated, can be almost as effective as a massive wall because sound impinging on the outer partition is not transmitted to the inner partition. Additional layers of gypsum board on walls, or layers of dense, sound-deadening board installed within floors and ceilings, further reduce sound transmission.

There is still another way for noise to sneak into rooms: through ducts leading to and from fans. Therefore, thoughtful acoustic design entails locating mechanical equipment as far as possible from noise-sensitive spaces, installing acoustic isolators and insulation in ductwork and providing acoustic insulation around mechanical rooms.

Noise caused by mechanical equipment vibrations also can be diminished by placing machines on resilient mounts that acoustically isolate the machines and prevent vibrations from being transmitted through structural components.

Carpet, drapery, upholstered furniture, wall fabrics, clothing, fibrous insulation and those ubiquitous suspended ceilings dampen sound within spaces. All are acoustically absorbent materials affecting reverberation within a space, but take note: These materials do not block the transmission of sound.

In musical or theatrical spaces, the art of room acoustics is most challenging. To make a room perform, designers must create just the right combination of "fuzzy," acoustically absorbent interior surfaces and hard, acoustically reflective surfaces fitting the volume and geometry of the space. Sound must be bounced off ceilings, suspended overhead surfaces and flanking walls, both scattering and mixing to reach the audience with adequate amplitude and clarity. At the same time, the room must absorb decaying sounds so that lingering echoes do not obscure the fresh sounds that follow.

Not only must acousticians consider the architecture of the room, but they also must take into account sound absorption attributable to the audience. For this reason, in spaces with fixed, upholstered seating, the sound absorption characteristics of an empty seat are typically comparable to those of the seat when occupied, ensuring that the room's acoustic qualities do not depend on the size of the audience.

Acoustics is both an art and a science. Designers know what is needed to hear or perform music or to engage in comfortable conversation. They know what frequencies and intensity of background noise are desirable or undesirable, and what types of sounds and sound levels are harmful physiologically or psychologically. And most know at least something about how to design spaces pleasing to the ear as well as to the eye.

But sometimes they may not be listening.

Roger K. Lewis is a practicing architect and a professor of architecture at the University of Maryland.

© 2001 The Washington Post Company

Source: http://www.washingtonpost.com/ac2/wp-dyn?pagename=article&node=&contentId=A44985-2001Dec14

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