Page B1.2 . 21 September 2005                     
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    QUIZ

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    Sounding Cinematic

    continued

    Sound Isolation

    It's widely but mistakenly believed that soundproofing can make a space completely free of external noise. In fact, soundproofing can only achieve noise reduction. Architects and installation professionals approaching a home theater project should consider two key methods for measuring the effectiveness of noise control the sound transmission class (STC) rating, and the noise criteria (NC) rating.

    Most home theaters are of wood stud construction. A standard interior wall of 2x4 studs with sheet rock screwed directly to the studs provides an STC rating of around 26-30. These numbers are obtained by averaging the transmission loss (decibel difference between adjacent rooms), as measured at 250, 500,1000, and 2000 Hz. The higher the STC rating, the better is a wall's ability to reduce noise.

    STC numbers are more effective at describing the damping of high-frequency sounds than low-frequency. This is especially true for low frequencies in the subwoofer range. As a result, many acoustics professionals consider the NC rating better for determining how well a home theater is acoustically isolated.

    NC ratings look at specific low-frequency levels to see if the sound transmission loss in a room is appropriate. Based partly on the transmission loss level of all surrounding partitions, the NC level describes how quiet a space actually is. In this context, an NC level of 20 is excellent; 30 is satisfactory.

    Sound Conditioning

    Whereas the enemy of soundproofing is external noise, including plumbing, heating, and air conditioning systems, the enemy of sound conditioning is the envelope itself. Either overly absorptive or overly reflective surfaces of the room's walls, floors, and ceilings can wreak havoc on the acoustical quality of any audio system.

    The key to sound conditioning is bass response. Almost all home theaters tend to be over damped in the mid- and high-frequency ranges, while the low range is barely addressed. Applying one- or two-inch- (2.5- or 5-centimeter-) thick panels of fiberglass onto the walls cures some acoustic anomalies, such as flutter echo the springy sound heard when clapping hands in a room and helps stereo imaging by taming first-order sonic reflections. These panels also lower the reverberation time in the range of 300 Hz and above.

    But these panels won't help with low-frequency bass, which often causes a home theater's thorniest acoustic problems. This is because the room dimension ratios recommended for surround-sound systems would require ceiling heights that are greater than those in normal residential construction. As a result, sound conditioning becomes more critical in assuring proper bass response, which is much more difficult than high-frequency control.

    A good place to start is by applying "bass traps." A bass trap is any device that reduces the strength of a low frequency "standing wave" or resonance. Several companies make effective models, including Acoustic Sciences Corp. in Oregon, Echo Busters in New York, and RPG in Maryland. These are usually standalone products that can be installed in the corners of a room. Bass traps can also be built into walls using conventional construction materials, but these would require very thick walls full of insulation.

    Bass can also be managed through the use of a membrane or diaphragmatic bass absorber, such as our own system at Acoustic Room Systems, a division of CinemaTech in Dallas, Texas. This system incorporates a reversible panel that's soft on one side to control high frequencies, and hard on the other side to absorb low frequencies. The entire system, a patented product developed by Owens Corning, is an inch-and-a-quarter (3.2 centimeters) thick. To get similar results with a simple fiberglass panel system would take material over three feet (90 centimeters) thick.

    A great variety of products, styles, and prices are available to control noise and tame room acoustics. With every system offering benefits and drawbacks, it can be challenging to know what's best for each application. But the rewards of planning a home theater to include acoustic components for noise control and sound conditioning will be immediately apparent in client satisfaction.

    Discuss this article in the Architecture Forum...

    Christopher Klein is the director of acoustics with CinemaTech. He is former co-owner of Acoustic Room Systems and former market development manager for Owens Corning's Acoustic System Business. He has been a professional musician for over 25 years.

     

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    The El Dorado Home Theaters showroom in El Dorado Hills, California is fitted with CAT loudspeakers.
    Photo: Lee Wege Photography

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    Blocking details for the walls of home theaters.
    Image: Acoustic Room Systems

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    Midwall detail.
    Image: Acoustic Room Systems

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    Compressed insulation being installed in the Velvet Lounge recording studio, which has acoustic requirements similar to those of home theaters.
    Photo: Keith Schneider

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    Construction of the Velvet Lounge was detailed to control reflections of high-frequency sounds and to absorb base frequencies.
    Photo: Keith Schneider

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    In the Audio Video Showroom in Santa Rosa, California, fabric-covered walls, floors, and furnishings contribute to the acoustic design.
    Photo: Lee Wege Photography

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    Installation by Definitive Audio in Bellevue, Washington.
    Photo: Lee Wege Photography

     

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