Soundproofing a Wall
Soundproofing a wall describes the PROCESS of how Sonic-Shield arrives at the individual soundproofing solution that each of our customers require.
In previous application notes, Sound Transmission Through a Wall, and Flanking Noise Through a Wall, we discussed how sound is transmitted directly through the structure of the wall and indirectly though noise flanking paths. In our application note, Decibels and Combining Them, we discussed the importance of identifying the loudest noise source(s) in order to develop an effective soundproofing strategy.
This application note describes the process of identifying noise transmission paths through a wall and describes alternative means by which this noise can be reduced or eliminated. There are numerous ways to soundproof a wall, and it is not the intent of this article to provide a review of these various soundproofing techniques, materials and products. With that said, Sonic-Shield stocks, fabricates, supplies and recommends the most effective sound proofing materials and products available on the market.
The first step in soundproofing a wall is to determine the source(s) of the noise and how it gets into the room (i.e., the noise transmission paths). Figure 1 shows the various noise transmission paths through and around the wall.
Noise source identification and characterization is typically accomplished using a sound level meter (SLM), which can be a professional grade instrument, or there are many apps available for mobile devices that can be used (see our application note on Sound Level Meters). Once the source is identified, it is important to determine the frequency content of the noise, since certain soundproofing treatments are effective in specific frequency ranges. This information can now be used to develop a soundproofing strategy, as follows:
- Windows – If the offending noise source is external to the house or office, such as traffic noise or aircraft noise, the primary noise paths are usually the windows. Most window manufacturers offer “higher STC” windows, where they will use a thicker glass. These windows, however, will only marginally increase the STC from about 28 to about 33. It will normally be necessary to increase the STC of the window to be equal to or greater than the STC of the wall (in the high 30s) to obtain an appreciable amount of noise reduction. The windows can be replaced with “acoustic windows”, which include a double- or triple-pane glass, sometimes used in conjunction with an inert gas between panes. These windows can provide STCs in the 50s and higher, however, they tend to be expensive. Other affordable means for reducing noise transmission through windows include the use of acoustical curtains (which contain a thin layer of acoustical barrier material), double glazing (which involves the installation of a sheet of clear plastic on the inside of the window), or window “plugs” (which include both sound barrier and sound absorption material) and are placed inside the window opening.
- Doors –Interior doors have STC values in the high teens or low-20s, because they are typically hollow with air gaps at the threshold and around the edges through which noise passes. Typical soundproofing measures might involve replacing the hollow core doors with solid core doors, which can increase the STC to the high-20s. “Acoustical doors” are also available, which are typically made out of metal and will include a significant amount of damping and absorption material inside of them. If it is not desirable to replace the hollow core door, some benefit may be gained by adding an acoustical panel, consisting of an acoustical barrier material together with an acoustical absorption material, rigidly fastened to the inside of the door. On the door jamb, we also recommend use of an acoustical seal together with an acoustical door sweep at the threshold.
- Ducts – Air conditioning ducts typically provide an unimpeded air path from one room to another through which sound can pass unabated. Duct noise can be solved with a number of approaches, including the use of a duct liner inside of the duct, acoustical plenums and splitters inside of the duct that do not impede air flow, or acoustical baffles that can be placed at the inlet or outlet grills. In some cases, noise within the duct (from fans, for instance) can pass directly through thin metallic ducts, which can be addressed by wrapping the duct with sound barrier and absorption materials.
- Electrical Outlets – Sound can readily pass through electrical outlets in the walls, light boxes and recessed lights in the ceiling. For large electrical fixtures (such as recessed fluorescent lights and “can” lights), we recommend the use of soundproofing covers, consisting of sound absorption and sound barrier materials. Because of the potential for heat build-up, these are usually constructed using fire- and heat-resistant materials. Depending on the ability of the ceiling to support the additional weight of these covers, the covers themselves may be supported independently of the ceiling. For smaller fixtures, such as electrical outlets, we recommend the use of “putty pads”, which are fire- and heat-resistant pads that can be molded onto the back of outlet boxes to reduce noise transmission through them.
- Ceiling – One of the most neglected sound paths includes flanking paths through the ceiling. Walls are typically built only to the height of the ceiling, which enables noise from adjoining rooms to pass into the space above the ceiling and down through the ceiling into other rooms. For such situations, we recommend that the wall above the ceiling be extended up to the deck of the floor above the room, and lined with acoustic barrier and absorption materials. If it is not possible to extend the wall above the ceiling, we recommend that soundproofing treatment be installed on or within the ceiling of the room.
- Floor – The sound of footfalls from adjacent rooms can pass through the floor underneath the walls into other rooms. In these cases, we recommend the use of a resilient floor pad with a high Impact Insulation Class (IIC). This will create a “cushion” with the hard floor surface and reduce the magnitude of footfall noise. Additional benefits may also be obtained by adding a high STC noise barrier to the floor.
- Existing Walls – The addition of soundproofing materials to reduce noise transmission through the walls is typically the first area investigated by consumers, when it should actually be the last, considering all of the various sound flanking paths. Once it is determined that it is necessary to increase the STC of the wall, there are numerous alternatives that can be considered. If it is desired to keep the current walls in place, we recommend the use of an additional layer of drywall, installed over the existing wall using a viscoelastic “caulk”. These caulks do not harden, and enable the outside wall layer to behave independently of the inside wall layer and attenuate noise. Noise barrier sheets that are bonded directly to the walls using a “rigid” adhesive (such as carpet adhesive) are generally not effective, since the noise barrier cannot move independently of the wall.
- New Walls – If it is possible to remove the drywall or in situations involving new construction, many more soundproofing alternatives are available. On the base or footer of the wall, vibration-absorbing materials can be used to reduce the possibility of floor vibrations from traveling up the wall, transforming into sound and re-radiating into the room. For applications, such as theaters or gymnasiums where very high sound levels are expected on one side of the wall, we recommend a staggered stud arrangement, where the sheet of drywall on one side of the wall is isolated from the sheet of drywall on the other side of the wall (see Figures 2 and 3). Additional sound attenuation can be obtained by adding resilient channels to the drywall sheets and adding a layers of sound barrier material to one or both sides of the wall, such as MLV and sound absorber material in the air gap between the walls.
Sonic-Shield has an engineering staff with advanced technical capabilities who can assist individuals and building owners in identifying noise sources and noise transmission paths through walls. We can also develop, recommend and supply the appropriate sound abatement materials and products to significantly reduce the passage of noise through walls.