Sound Control

Sound-Dampening Wall Systems

To effectively sound-deaden a room often requires multiple strategies. The three main approaches are:

• decoupling the two wall surfaces from one another
• increasing the wall’s mass
• blocking the flanking path through which sound “leaks” from one side of the wall to the other

Insulation

Many builders use insulation to reduce sound transmission, but insulation is a relatively small part of an overall sound-control strategy. For example, adding fiberglass batts to a 2x4 partition with 1/2-in. drywall on either side increases the STC rating from 35 to only 38 (see figures for partition and block party walls, below).

Figure: Approximate STC Ratings for Partition Walls

Figure: Aproximate STC Ratings for Block Party Walls

Also, leaving just 6% of the wall uninsulated reduces the insulation’s sound-deadening properties by 35%. Therefore, batts should be carefully cut to fit around and behind wires, pipes, and outlet boxes. Specially labeled “sound-control” batts are only slightly more effective than ordinary batts.

Adding Mass

The higher the wall’s mass, the less sound it will transmit. A common approach is to upgrade from 1/2-in. to 5/8-in. drywall, or use a double layer. Doubling the drywall on one side of an insulated 2x4 wall provides an STC of 40. Doubling both sides provides an STC of 45.

Decoupling

The most effective way to reach high STC levels is to decouple two faces of a new wall. To save space, install 2x3 studs every 8 in. between a pair of 2x4 top and bottom plates, staggering the studs to line up with alternating sides of the plates. Alternatively, use 2x4 studs with 2x6 plates, placing the studs 8 in. or 12 in. apart for 16-in. o.c. or 24-in. o.c. nailing on each side of the wall. For even higher STC ratings, build two separate walls without a common plate.

To decouple the two faces from an existing wall, use resilient channels. These are fastened horizontally to the studs and absorb sound vibrations coming from the drywall. Drywall screws should be long enough to penetrate the drywall and the channel, but not long enough to reach the stud.

Doors

As with walls, heavier doors will transmit less sound, so a solid-core door will be quieter than a hollow-core model. Also, gaps around the door will pass at least as much sound as gaps in a wall. To reduce airborne sound transmission around a door, seal its edges with a flexible rubber gasket and install a weather-stripped threshold at the base (below).

Figure: Approximate STC Ratings for Doors

Sealing Holes

After hanging drywall, caulk any gaps around electrical boxes, heating registers, and plumbing fixtures. Use a polyurethane or acrylic caulk that will stay flexible over time. Installing electrical outlet plates over air leakage gaskets will also reduce sound transmission.

Sound-Dampening Floor Systems

STC measures airborne sound passing through a structure. But floors are also subject to impact noise, mostly from footsteps. A floor’s resistance to impact noise is expressed as its Impact Isolation Class, or IIC (below).

Figure: Approximate STC Ratings for Floor/Ceiling Assemblies

A padded carpet can boost a typical floor’s IIC rating from 34 to 56. Further improvements require either installing a layer of soundboard between the carpet underlayment and the subfloor or hanging the ceiling below on resilient channels.

Recommended IIC ratings are as follows:

• Bedroom under bedroom — 52
• Bedroom under living room — 57
• Bedroom under kitchen, family room or hall — 62