Home Theaters and the Art of Acoustic Silence
By Christopher Klein
If it's true, as was once said, that "silence is the element in which great things fashion themselves," it's true for home theaters as well. A home theater that delivers a high-quality audio experience must be designed for silence as well as sound. It needs to be isolated from sounds coming from adjoining rooms, and from any electrical and mechanical systems running through the walls. It also needs to be acoustically correct, so that music and sounds are heard as intended.
As an acoustic professional, I'm often called by clients with questions about "sound-proofing." Though most are actually asking about "sound conditioning," which is very different, the issue of acoustic isolation is clearly on clients' minds.
Yet issues of isolation, sound proofing and sound conditioning are often overlooked in planning a home theater, which is unfortunate, since the planning stage is the best time to address them. This article will go into the need for acoustic isolation, differences between sound proofing and sound conditioning, and why it's important for home theater installers to understand these processes.
PROOFING VS. CONDITIONING
Sound proofing, simply put, is the process of trying to make a space free of sound from an adjoining space, while sound conditioning is the science of making a room acoustically correct so that listeners hear the audio as it was meant to sound when recorded.
Installation professionals approaching a home theater project should begin by considering the issue of sound proofing, or noise control. There are two key methods for measuring its effectiveness — the Sound Transmission Class (STC) rating, and the Noise Criteria (NC) rating.
The vast majority of home theaters employ stud construction, usually wood. A standard stud wall of 2x4-inch studs with sheet rock screwed directly to the studs provides an STC rating of 26 to 30. The STC rating reveals how well a partition or wall assembly reduces noise from one side to the other. It's obtained by averaging transmission loss from one room to another, measured at one-third octave bands from 125 Hz to 4,000 Hz. The higher the number, the better a wall's ability to reduce noise. While STC numbers are most effective at describing higher frequency damping, they are less effective with low-frequency noise levels, especially those in the subwoofer range. As a result, many consider NC ratings 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 surrounding partitions, NC level describes how quiet a space actually is. For example, an NC level of 20 rates as excellent; 30 is satisfactory.