The advent of high quality audio systems and home theaters has changed the building environment completely. Where hotels, motels, homes, condos, and apartments previously only needed to deal with isolating a neighbor’s voice, we must now contend with loud movies coming from next door, even when the sound is simply coming from a good television. Given today’s need for high quality transmission loss in walls, it is important to rethink the original standards of Sound Transmission Class (STC) 34 walls and start thinking about raising the transmission loss by 20dB – 30dB or more, to provide a high quality living environment for all.
Over the years, many techniques have been used to enhance the sound transmission loss through residential and commercial walls. Many of these methods require unusual or difficult construction techniques that may not be followed accurately by an installer. As technology has given way to new materials, the more exotic construction techniques can revert back to standard STC ratings for single wood or steel stud construction. Furthermore, new technology allows easy, fast, and low cost retrofitting in existing structures (including homes and offices) where the existing installed drywall does not have to be removed to achieve large increases in Sound Transmission Class (STC) ratings. Most importantly, this guide draws from actual lab and field testing on complete assemblies–it is important to prove how well a specific wall assembly performs. Serious Materials is committed to industry standard STC testing on complete assemblies and references independent lab tests throughout this guide.
Interior Walls Using Traditional Technologies
Walls with higher STC values have been achieved for years using a variety of construction techniques.
A commonly used technique since the 1960s, on both wood and metal studs, is called resilient channel (RC). These are metal channels that are placed at right angles to the studs. The drywall is then screwed into the channel, being very careful that no screws touch a stud directly. In this way, the drywall is isolated from the studs, thus carrying less noise and vibration to the outer wall.
When installed correctly, RC can typically improve STC ratings by 8-14 points, depending on construction. An important note here is that resilient channel is easily “short-circuited”, or in other words, the isolated sheet of drywall can lose some of its isolation due to mechanical connections between the drywall and the frame or adjacent wall/floor/ceiling. For this reason, very careful construction techniques must be followed. For instance, no screws can ever touch a stud; drywall must not touch floors or adjacent walls or ceilings; no pictures or shelves can be hung on the wall where the fastener screws into the studs, etc. If there is one failure, it can ruin any gain that would have been had. Very careful construction and review by the architect or acoustic engineer is required.
Other stud arrangements, such as staggered-stud and double-stud increase STC points as well (covered below), but consume valuable space and can nearly double the labor costs in both wood and steel stud construction. For retrofit in existing construction, there have been few choices that do not require demolition. It should be noted that adding resilient channels to an existing wall (over the existing gypsum) will not increase STC values. There is not enough space between the old and new gypsum for RC to work effectively.
Another technique that is similar to resilient channel is the use of “sound clips”. These work as well as RC, and in some cases produce better STC results than RC. These clips use a hat channel (a metal channel) attached to the clips which are attached to the studs. While this consumes more floor space than RC and requires more labor, the results can be better. The cost of the clips, channel, and labor can add $3-$4/sq ft to a standard wall, according to a leading manufacturer of clips. This method also relies on panel isolation (similar to RC) and requires essentially the same careful construction methods as RC. It can also fail the same way RC fails (that is the wall must be completely floating and not touch adjacent surfaces).
While using resilient channels is clearly appealing from a material cost perspective (ignoring litigation risk), homeowners and others may choose this and use it successfully by being very careful of the failure mechanisms. Below is a list compiled by acoustic engineers that people should be aware of before beginning a project:
Dead on arrival. RC channels are thin and prone to damage from shipping or on-the-job storage. Any bend in the channel can cause shorting. We have multiple reports of damaged RC channels that are deployed because by the time the damage is perceived, it is too late to re-order.
The RC channels are placed too close together. If this happens, the composite stiffness of the wall will be too high and will result in reduced sound insulation.
The RC channel extends too far and touches an adjoining wall. This error causes a short circuit in the wall, resulting in radically degraded sound insulation.
Drywall is not installed properly. If the subcontractor adds drywall that is beyond spec (e.g. adding a layer of Type X to meet fire code), the resulting structure can sag, and the weight of the drywall on the resilient channel can cause the wall to touch the floor, causing a short circuit in the wall, resulting in poor sound insulation.
Electrical junction boxes attached to the stud and to the wall. This common error causes a short circuit in the wall and results in poor sound insulation. This mistake is easy to make with the faceplate, which must also be isolated, or by not cutting enough of the drywall away around the junction box. The same principle applies to ceiling attachments, such as lighting and fans.
Resilient ceiling. If the ceiling is also resilient, the walls and the ceiling cannot touch each other. To achieve this, it is recommended the walls be put up before the ceiling. This is counter to standard drywall installation practice. Actions of other subcontractors. When RC channels are used in floor/ceiling assemblies involving stuffing materials into the open truss, the risk is magnified. Plumbing, HVAC and electrical materials are routinely attached inside the small cavities in ways that guarantee short-circuiting the RC channel.
Moisture and humidity warping. In high-humidity areas (such as the Eastern seaboard), humidity can bow and buckle drywall, 1/4″ to 1/2″ in many cases. This distortion can bring the RC channel in contact with other elements and cause a short circuit.
Interior Walls Using QuietRock
The QuietRock line of engineered products is a sound isolation system designed to replace standard drywall in any wall (or ceiling) construction including wood or steel studs. QuietRock is a multi-layer laminated gypsum wall product from Serious Materials, Inc.
One major advantage of QuietRock is the ability to use standard construction techniques and achieve high STCs, without the limitations of standard materials– eliminating the need for expensive, difficult nonstandard construction techniques. The wall need-not be “floating”. Simply hang it like standard drywall. All too often, an architect or acoustical consultant designs a high STC wall, only to discover that a contractor did not carefully follow the exact (and sometimes difficult) installation instructions, hence not achieving the desired result. QuietRock is the first technology for walls that cannot be “short-circuited”, thus reducing litigation concerns post construction.
You can use QuietRock panels just like any other gypsum or drywall product. The panels can be cut and attached to the wall similar to drywall. The only difference is that a QuietRock panel is an “internally damped” product which uses constrained-layer damping in several layers inside the panels. QuietRock is available in several performance grades and various sizes (see product specs for exact data), yet is available as thin as 5/8″(like regular drywall) and weighs about the same.
Conclusion
Consumers and office workers are happier with quiet environments. Interior walls with STCs in the 30s are no longer acceptable for most situations, especially home theater, home recording, and multi-family. As a minimum, setting an STC target in the 50s or ideally the 60s results in happier homeowners, apartment dwellers, and office workers. With new technologies, such as QuietRock, it is becoming easier than ever to raise STC ratings with less labor, lower cost, minimal effort, low risk, lower liability, and a high confidence of success.
Resource Link: How To Make Walls Quiet
