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July 09, 2004

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Palo Alto Online

Publication Date: Friday, July 09, 2004

Can you hear me now? Can you hear me now? (July 09, 2004)

Soundproofing has improved in recent years

by Lynn Comeskey

Q You wrote an article a while ago about soundproofing and I can't find the copy I saved. Could you send me a copy? I have a similar problem: My son has just learned about rock music.

A I wrote that some time ago; I have learned more about sound attenuation and I want to describe a new product that seems to work. I will write about various techniques to reduce sound transmission and indicate the STC rating for each approach. Sound Transmission Class is a method of measuring acoustic retardance as determined by the ASTM (American Society and Testing Materials). All numbers are approximate.

A wall with an STC rating between 50 and 60 should keep most adjacent noise down to a tolerable level. If you have a serious problem, you should consult an acoustical engineer.

Walls and floors between living space (whether it's between rooms in your house or rooms in an apartment) are usually constructed of wood as small as 2" x 4" and as large as 2" x 12" with 1/2" thick Sheetrock and/or 3/4" plywood on each side. This combination provides a minimal amount of sound reduction (STC 30). If you are remodeling or building a new structure and you can use steel studs instead of wood, you would obtain an STC of 41 instead of 30. Adding insulation will add 3 to 4 to the STC rating.

The most commonly used technique to reduce sound transmission is the application of resilient channels. Metal channels are screwed to the wood studs at right angles. The Sheetrock is screwed to the metal channel with attention not to put any of the latter screws into the wood. The drywall is thus isolated from the studs and transmits less noise and vibration from one side of the wall to the other. With the resilient channel over wood, the STC increases by 3 or 4 points.

Four other techniques follow (assumes modifying a standard wood wall):

** Doubling the amount of drywall on both sides of the wall adds 3 to 4 to the STC.

** The Homasote Company produces a board named "Sound Barrier." This product installed under drywall on both sides would add 11 to the STC.

** Installing the studs in a staggered fashion, so that every other stud is in contact with the drywall on either side of the wall, adds 3 to 5 to STC.

** Framing a double wall with a gap between the two "walls" adds 7 to 9 to STC.

As you can see, none of the above approaches will provide a very high STC rating. You can try doing several of the techniques to increase the STC, but they don't necessarily add mathematically.

A relatively new product is being developed by a Sunnyvale company using constrained layer damping, which is the joining of two relatively inelastic layers with a very elastic binder. The product consists of two pieces of drywall laminated to a thin steel panel. The three pieces are held together by a thick layer of "visco elastic" glue. The product is heavy and not inexpensive, but the company reports some pretty impressive results.

These panels are produced in three thicknesses and can either be applied over existing drywall or over new studs. A layer of the special glue is used to install the panel to existing drywall thus providing an additional sound and vibration barrier. Installing the middle thickness product on one side of an existing wall provides an STC of 51. Installing the panels on both sides of an existing wall provides an STC of 53 (not much of an improvement). This is a good example of how the values of STCs do not necessarily add. The STC for the product installed directly over studs isn't as high because there are two fewer layers of drywall and two fewer layers of the glue (STC 46).

I have had the product installed on a wall between two apartments and the reduction in the level of sound transmission was noticeable. I have no way of quantifying the level of reduction but it was impressive. I have to add that once one means of sound transmission is controlled, other means become more noticeable. In this case, the sound transmitted through the common plywood floor (between the two apartments) became more apparent! This is called in the sound biz, a flanking path of sound transmission. It is important to consider the flanking paths of sound transmission when planning a sound-reduction project.

I have presented this information not so much for its precision, but to provide you with the relative effects of the various techniques to reduce sound transmission. Again, if you have a serious problem, consult an acoustical engineer.

Lynn Comeskey continues to work with Mac & Lou Construction Co. in Palo Alto. Readers can write Comeskey care of the Palo Alto Weekly, P.O. Box 1610, Palo Alto, CA 94301.

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