Design for Acoustics
Sound systems are often used in rooms where contemporary music is being played. While we would like the natural acoustics of the room to provide the necessary sound quality, these sound systems are often set at such high sound levels that they negate the effect of any architectural designs.
Artificial reverberation can also be set in these sound systems. It is therefore advisable to have as much absorption as possible in rooms where contemporary amplified music will be played.
A Concert Hall with Flat Walls
Spivey Hall, located on the Morrow, Georgia campus of Clayton College and State University, which opened in 1991, was designed purely for music — though a wide variety of music, from organ to piano. The hall seats approximately 400 and is basically rectangular, about 50 feet (15 meters) tall at its highest point, about 100 feet (30 meters) long and 56 feet (17 meters) wide.
Although this may sound like a shoebox design, that was hardly the case. To maximize acoustic quality, the ceiling is pitched in a cathedral style, the rear wall is convex to diffuse sound, and the walls are stepped and sloped.
The only permanent surface in the hall that is absorptive is the seating area. The seats are padded and all other surfaces are reflective to maximize the reverberation time in the room for organ recitals. The floor is wood, the ceiling is gypsum board, and the walls are part plaster on masonry and part gypsum board on randomly spaced framing.
One of the many unique aspects of this hall is the potential to adjust the reverberation time. This can be accomplished using the retractable curtains on the side and rear walls, which can be opened and closed at the touch of a button. Depending on the exposure of the curtains, the mid-frequency reverberation times range from 1.7 to 2.4 seconds.
The open-plan office design has become very popular through the latter half of the 20th century and into the 21st. This design scheme saves money, promotes teamwork, and improves flexibility for future renovations.
Many employees, however, view this design as a series of compromises in terms of space, prestige, and (most of all) privacy. As employees consider changing from the traditional closed office to open-plan cubicles, they often have concerns about their abilities to work productively in what they anticipate to be a noisier, more distracting workplace.
The overwhelmingly largest complaint about the open-plan office design is the lack of acoustic privacy. There are techniques, however, for maximizing acoustic privacy in these environments.
The MIT Career Services Center
The Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts, renovated its Career Services Center in 1995. One of their most significant concerns was acoustic privacy between stations in their interview area. The configuration was a series of closed offices, each containing two to three stations. The stations were separated by thin partitions and the side walls were painted drywall. There was little if any privacy between these stations.
The new design was completely open plan. To provide some privacy between workstations in this design, it was necessary to incorporate generous use of absorptive treatments. The floors were carpeted to minimize impact and footfall noise, the ceiling was covered with a suspended tile having a minimum noise reduction coefficient (NRC) of 0.90 and the walls in each cubicle were treated with absorptive panels.
The partitions between workstations are absorptive themselves, with NRC values of more than 0.75. In order to maximize the absorption of the ceiling, no lighting fixtures were built into the suspended panels. Instead, lighting was provided by hanging fixtures and lighting built into soffits.
The overall layout of the area considered blocking any line-of-sight between occupants of cubicles to maximize the noise reduction of the partitions. The high amount of absorption and blockage of line-of-sight provided the best acoustic conditions possible with this arrangement; however, the background sound level was too low to ensure adequate privacy.
The final step in the design, then, was to raise the background level using a sound masking system having loudspeakers hidden above the suspended ceiling. In this case, loudspeakers for the masking system were placed behind unducted ventilation grills.
The result of this process is adequate normal speech privacy in these interviewing stations, a condition that did not exist in the former closed-office setup. When one walks through this room, conversations can only be heard when passing open sections of cubicles.
James Cowan is a board-certified noise control engineer with the Cambridge, Massachusetts-based multidisciplinary acoustical consulting firm Acentech. He is also author of Handbook of Environmental Acoustics and Architectural Acoustics, a multimedia CD-ROM that teaches architects and designers the principles of acoustics.
This article is excerpted from Architectural Acoustics Design Guide copyright © 2000, by The McGraw-Hill Companies, available from McGraw-Hill and Amazon.com.