Sound transmission through a wall
Sound transmission through a wall is just one of the ways that sound is transmitted. Soundproofing fundamentally involves the means by which we can control the propagation of sound. This application note describes sound transmission through a wall, and how a combination of structural design and soundproofing materials can be used to reduce the amount of sound transmission through a wall or walls. Background material, describing some of the physics of how sound interacts with materials can be found in a previous application note, entitled Acoustical Interactions.
In a typical building structure, the walls consist of 2”x 4” studs, spaced a distance of 16” or 24” on center, with a 5/8” sheet of gypsum drywall on each side (Figure 1).
If a sound source is placed on one side of the wall, some of the incident acoustic energy is reflected back off the wall, some of it is absorbed by the wall, and some of it is transmitted through the wall. In general, sound is reflected off of surfaces because the different media have different densities (or more precisely, different impedances), requiring transformation of the sound waves. Those waves that are not transformed are reflected back through the original medium.
In between studs, the sound transmission through a wall that is not reflected or absorbed on the front surface of the drywall becomes structure borne noise which travels through the thickness of the drywall, contributing to sound transmission through a wall. When the sound encounters the space between the studs, it must again be transformed into airborne noise. As described above, when sound needs to be transformed from one medium to another, some of the acoustical energy is lost in the transformation, and some of it is reflected back into the original medium. Generally, the amount of acoustical energy that is reflected or dissipated at the boundaries of two media is dependent on the magnitude of the impedance mismatch between the two media. That is, the larger the impedance mismatch, between two media, the smaller the amount of acoustic energy will transmit across the boundary. Since an air gap will have a large impedance mismatch with drywall, it is an effective means of reducing the amount of sound transmitted from one side of the wall to the other. At the next drywall layer, the airborne noise within the gap must transform once again into structure borne noise through the drywall until it reaches the outside layer of the drywall where it is transforms back into airborne noise and radiates into the next room.
At the stud locations, the incident sound wave is transformed into structure borne noise, which passes through the drywall into the stud then through the second drywall layer. The acoustic impedance between drywall and stud is not as great as that between drywall and air, and thus a significant amount of acoustical energy will be transmitted through the wall at the stud locations. Since the surface area of the studs is small, the magnitude of the noise passing through the studs may not be significant compared to the magnitude of the noise passing through the drywall. Most wall assemblies in newer construction use steel studs that have a smaller cross-sectional area than wood studs, and therefore transmit less noise from one side of the wall to the other.
The ability to reduce sound transmission through a wall requires an understanding of the mechanisms by which sound is transmitted through the wall. This will determine the appropriate mix of sound barrier and sound absorption materials in an effective soundproofing strategy. Our team of acoustic experts can help you identify noise transmission paths and assist you in selecting the appropriate mix of soundproofing materials.