Resilient Channels – How Metal Absorbs Sound Waves
Anyone who’s ever shared a wall with someone else has probably wondered if the walls were intentionally thin, or the neighbors intentionally loud. Often the assumption is that the walls between dwellings simply need to be thicker to muffle sound. The fact is that the design of the wall, not the size, can be a contributing factor in transmitting sound from one side to the other.
Consider how walls are constructed in most multi-family housing units. Basically, they are made of gypsum board firmly attached to both sides of a wood frame. When sound waves hit one side of the wall it causes the gypsum board on that side to vibrate. Since the gypsum board is rigidly connected to the frame, the vibration is transmitted right through the framing to the gypsum board on the other side. Those same vibrations traveling through the wall frame can also send sound throughout adjacent floors and ceilings. Noises will radiate through the structure because there’s almost nothing there to cushion or absorb the sound waves.
In order to dampen those approaching sound waves, resilient furring channels can be inserted between one of the gypsum walls and the frame. The resilient channel acts as a shock absorber in this system, muffling vibrations coming from either side of the wall. Resilient channels are routinely used to improve the sound ratings for walls and are especially effective in floor/ceiling constructions.
Resilient channels typically add 3 to 5 Sound Transmission Class (STC) points to an otherwise identical wall or ceiling. This can often be enough to meet the STC and Impact Insulation Class (IIC) ratings required by the project design goals or local codes. In California, the building code specifies minimum lab tested ratings of STC 50 and IIC 50 for partitions in multi-family dwellings.
In fact, the IIC 50 rating required by the building code almost mandates the use of resilient channels to hang the ceiling in a multi-family project. Unless the floors are completely carpeted (which is rare for kitchens and bathrooms), it is very difficult to achieve IIC 50 ratings without using resilient channels and batt insulation in the floor-ceiling construction.
It is important to distinguish acoustically effective resilient channels from hat channels, z-channels, and other lightweight metal furring systems. These other systems may resemble resilient channels, but they afford no movement and are simply too rigid to be effective. Only resilient channels have any acoustical benefit.
As can be seen in the diagram above, resilient channels have two flanges, one wide and the other narrow, with a slotted portion in the middle. These slots create the flexibility which allow the resilient channel to act as an isolator. Every manufacturer has a different design for resilient channels. Assuming they are installed correctly, the ones with the least amount of material connecting the wide and narrow flanges will (in theory) transmit the least amount of sound.
Regardless of which brand of resilient channel is used, it is extremely important to install them correctly. Improper installation will nullify any advantage you would have gained from using them in the first place. On a recent project, it took one unfortunate contractor two tries before he finally got it right. On his first attempt, he mistakenly put in a hat channel system. The second time, he correctly installed three different kinds of resilient channels, none of which was the brand specified in the design. The third time around everything went as planned. Luckily, both of the contractor’s mistakes were caught before the gypsum board had been hung.
There are a few simple procedures that need to be followed when using resilient channels. On walls, the channel should be mounted perpendicular to the framing with the narrow flange along the bottom. This allows the gypsum board’s weight to draw itself away from the framing. For ceilings, the flanges should all be pointing in one direction. This keeps the channels from fighting each other.
When fastening the gypsum boards, the screws must be connected to the channels between the studs or joists. It is critical not to “short out” the resilient channels by screwing into the wood behind them. This inflexible connection destroys the benefit of the channels, and you have now spent a lot of time and money to create a complex rigid attachment.
The resilient channels should be held back from intersecting surfaces about an inch on the side edges, and about 2 to 3 inches at the top and bottom of the wall. It does little good to carefully attach the channels in the middle of the wall when the baseboard screws connect the entire bottom edge to the sill plates. Similarly, it is easy to short out the resilient channels at the top of the wall by screwing into headers. The gypsum boards attached to the channels also need to be held back one-quarter inch from similar intersecting surfaces. If the panels are jammed against the edges, then they will be rendered ineffective. The gaps should then be sealed airtight with a specified acoustic caulking. For a more aesthetic effect, the caulked edges can be painted or covered by a trim.
When resilient channels are properly installed, it should be possible to slightly flex the wall or ceiling surface. A lack of flex indicates that the channels are shorted out by screws fastened into the wood framing. Also, it usually does not matter which side of the wall is resiliently suspended. This makes it easier to accommodate shear plywood and wall mounted cabinets–simply put the resilient channels on the opposite side of the wall in the affected rooms.
Keep in mind that this article only outlines the benefits of correctly installed resilient channels. If you feel that this technology would be useful in an upcoming project, it is advisable to seek help from an experienced consultant. One particular contractor already knows how much easier it is to get things right the first time around.
Sound Transmission Class (STC)–Describes how much sound a wall or floor/ceiling construction will block from one room to the next. STC is applied to situations where speech or office noise constitutes the main sound problem. To determine the rating, an active loudspeaker is placed on one side of the partition, and sound levels are measured on both sides. The difference in levels shows how much sound can be blocked by the partition. The higher the rating, the better the sound insulation properties.
Impact Insulation Class (IIC)–Measures the noise created by foot-falls/impact on a floor through a ceiling. A really expensive but standardized testing device generates the impact sound by dropping 5 hammers which impart a known energy into the floor/ceiling construction. In the receiving room below, the resulting sound pressure level is measured in frequency bandwidths comparable to those used in sound transmission loss measurements. Increasing IIC values correspond to improved impact noise dampening qualities.
A hat channel (top) and a resilient channel (bottom) with side views-for picture