A teen whose musical tastes run to head-pounding heavy metal classics? Uncle Ted, whose snoring would wake the dead? Who knows what’s giving Robert Car pause for thought. Whatever it is, he wants to build in some effective sound-proofing.
“I’m aware of using more than one layer of drywall,” he writes in Q&A post, “but is that the only way?”
Increase the mass of the wall
Car is reluctant to add a second layer of gypsum drywall mainly because it will, however slightly, reduce the usable space in the room.
But increased mass is part of an effective strategy for reducing sound transmission.
“Here are some good rules of thumb,” writes Robert Riversong. “When the mass of a barrier is doubled, the isolation quality (or STC rating) increases by approximately 5 dB, which is clearly noticeable. Installing insulation within a wall or floor/ceiling cavity will improve the STC rating by about 4 to 6 dB. An airspace of 1-1/2 in. will improve the STC by approximately 3 dB. An airspace of 3 in. will improve the STC by approximately 6 dB.”
A second layer of drywall, often 5/8 in. rather than 1/2 in. thick, along with resilient channel to isolate the drywall from the wall framing, increases mass and reduces sound transmission. Andy Ault, however, suggests three other options.
“We practically make this our [standard operating procedure] in walls between master baths and nurseries/adjoining bedrooms,” Ault writes of SoundStop. “We bond the SoundStop to the framing using an acoustical sealant running horizontal to the framing. Then we tape the joints with butyl tape. Next, we layer the gyp board over that in a vertical orientation. It’s quick, easy, and relatively cheap and gets very high marks from our clients.”
Pricey options are unnecessary
Some of the products Ault has used are expensive in comparison with conventional drywall, and the added cost may not be worth it, says Ted White.
He calls SoundStop “another generic ‘sound board.’ Soundboard does not decouple, has very little mass, and is too dense for proper absorption,” White says. “Also, the very limited if nonexistent acoustic data makes this product all but abandoned by the acoustic community.”
In apparent reference to Ault’s next step up, QuietRock, White says that any “pre-damped” drywall will perform well, and at a lower cost. “You will always be able to field-assemble your own board using standard 5/8-in. drywall and a damping compound,” he says. “The resulting wall will be half the cost, and much higher performance.”
Finally, the massloaded vinyl (which can cost as much as $3.25/sq. ft). “is simply mass,” White adds. “Drywall is mass, plywood is mass, etc. Mass is mass. At $1-plus a square foot, you’re better off using standard drywall from Home Depot.”
And if you’re going to add a second layer of drywall, he adds, don’t forget to use a product called Green Glue to further dampen sound transmission.
Insulation helps, too
GBA Advisor Michael Chandler’s approach is to cover one side of the wall with a 1/8-in. layer of structural cardboard sheathing called ThermoPly and spray the back with an inch of open-cell polyurethane foam.
“We generally do this on the ‘noisy side’ of the wall, especially in laundry rooms and bathrooms,” Chandler says. “The foam is a lousy sound emitter, and it helps seal up electrical openings and flanking paths.”
He repeats advice given by Riversong, that potential pathways for sound transmission under doors and through ductwork and electrical outlets must be sealed. This is what he means by “flanking.”
“Insulation in the stud bays definitely improves sound containment,” writes Thomas Jefferson. “Denser is better, but even a standard fiberglass batt works.” Denim batts, mentioned as a possible option by Car, is a good choice as well, he says.
“I think the highest-performing material would be mineral wool,” says Jefferson, “e.g., Roxul or Thermafiber. Dense-packed cellulose would also work well and has the best green cred (lowest energy embodied, highest recycled content).”
White, however, says insulation in a single-stud wall does a good but not great job, mainly because sound transmission through the wall framing limits its effectiveness. He suggests blown-in cellulose or fiberglass if the walls have already been drywalled, and R-13 fiberglass batts if they are still open.
“Why? Because it works best in the low frequencies,” says White. “Mineral fiber has a slight edge in the upper frequencies, but that wall will not have high-frequency issues. It will have low frequencies as the only significant frequency issue. Since fiberglass has the edge there, we use it.”
Our expert’s opinion
Here’s the take of GBA technical director Peter Yost:
As with all areas of performance, managing sound requires the right blend of design, materials, and construction. You can’t purchase quiet; you need to get the science right first.
Sound is energy expressed as pressure variations in air. When sound strikes a material, it can be transmitted, reflected, or absorbed. To reduce sound transmission through building assemblies, there are three options:
1. Reduce transmission by reducing air leaks (flanking paths).
2. Reduce transmission by absorbing the vibrations with materials and/or spaces in the assemblies.
3. Capture or trap air vibrations with variegated surfaces on the noise-generating side of the assembly (to trap or capture means to absorb rather than reflect or transmit).
To seriously reduce sound transmission from room to room, you need to use all three options.
Flanking paths are addressed the same way we address them for energy efficiency (by air-sealing all gaps and installing weatherstripping on doors). But if the rooms have forced-air heating or cooling, configuration of duct runs may come into play.
Mass dissipates vibrational energy, but different materials can respond to different frequencies.
Variegated surfaces are materials such as corkboard, acoustical ceiling tiles, or conical foam cushion.
How you combine these techniques depends on the quality and the quantity of the sound transmission you are trying to block. If it’s a teenage rock band, you might have to move them to the basement to eliminate at least one plane of transmission, the floor.
I have never done any acoustical testing of building assemblies, so I rely on resources such as Architectural Acoustics: Principles and Design or the simpler and free “Classroom Acoustics”, offered by the Noise Pollution Clearinghouse. Both include building assemblies with verified STC ratings.
The practical advice offered by White, Riversong, Chandler, and Ault seems reasonable. Just remember that individual materials don’t manage sound transmission, whole assemblies (wall, ceiling, and floor) do.