Sound isolation involves six basic principles: decoupling, damping, mass, absorption, resonance, and conduction. Taking each of these principles into account will maximize the performance of your sound isolation project.
Basic Sound Isolation Concepts
The basic definition of decouple is to separate objects. Resilient clips, resilient channel, double stud and staggered stud framing are the different ways to decouple a wall or ceiling. Sound transfers through vibrations and if two materials are not touching or have minimal contact, then sound transmission will be seriously reduced.
Decoupling will always be the best way to isolate both walls and ceilings from the lowest to highest frequencies. Decoupling with resilient clips is also the key way to isolate impact footfall isolation in ceilings. Read Concept of Decoupling.
Many things can be dampened including walls, ceilings, floors, pipes, sheet metal, doors, isolation boxes, basically anything with low mass that will vibrate from sound transfer. Most damping between layers of drywall or wood is accomplished with a damping compound such as Green Glue Compound. While our VibraDamp will dampen lightweight plastic or metal surfaces including pipes, sheet metal, and the inside of automobiles. The concept of damping is to convert sound vibrations to non-perceptible amounts of heat.
Damping will help isolate across all frequencies with the most significant gains in the mid to high frequency range. When applied to the floor or ceiling, damping will also help with isolating impact footfall sound transfer, assuming the assembly is resilient, i.e. resilient clips, resilient channel, or independent joists. Read Concept of Damping.
Sound is transferred through vibration. With this, a heavier partition is more difficult to vibrate while a light weight partition is easy to vibrate. Minimal increases in mass will not lead to significant gains in isolation. If your wall has a layer of drywall on each side, then you will need to add two more layers on either side to increase the isolation by five STC points. To increase isolation in the same wall by another five STC points, you will now need to add four more layers on either side of the wall. Basically doubling the mass in the assembly each time to achieve significant gains.
Increasing mass will help with low, mid, and high frequency sound transfer. Low frequency isolation will benefit the most from adding mass to assemblies with very little mass.
Insulation is the material of choice for meeting the absorption requirements of a wall, ceiling, or floor. Only the size of the insulation is crucial and not the type of insulation. We recommend R-13 for 2x4 walls, and a maximum of R-19 for 2x6 or resilient clipped walls. R-19 is recommended for any depth of ceiling joist cavity.
Absorption will help increase isolation in the mid to high frequency ranges, but will not help to isolate low frequencies and footfall impact noise. The value of absorption is increased significantly when the assembly is decoupled.
Issues with resonance are most often created when attempting to decouple or when using the same type of material in multiple layers. Resolving resonance issues when decoupling is simple. Simply avoid creating more than one air space in any assembly and insulate that one air space with the proper amount of insulation. If you create additional air spaces, e.g. installing resilient clips over existing drywall or building a new wall in front of an existing wall, then you can reduce resonance issues by insulating this newly created air space. Regardless, we recommend only one air space per wall or ceiling cavity in any sound isolation assembly.
In regards to using the same type of material in multiple layers, this can be avoided by using varying thicknesses of drywall or plywood in an assembly. Each building material will have a specific resonance point. This point will vary depending on the type of product and the mass of that product. The resonance point is the weakest frequency point for that material in terms of sound transmission loss. Having two materials of the same type in the assembly will exacerbate the weak performance of that material at that frequency point. So, if one side of the assembly has 1/2” drywall, use 5/8” drywall on the other side. If you are adding multiple layers of drywall or wood to one side of the assembly then use a different thickness for each layer. Including a damping compound like Green Glue Compound in the layering will reduce these resonance issues enough that multiple layers of the same material thickness are not a problem.
This sound principle is often referred to as flanking. Essentially vibrations from sound transmitting through conductive materials into other conductive materials, e.g. wood framing, plumbing pipes, and sheet metal. This sound transmission is indirect, so the loss from the transmission is difficult to predict, but can be significant. You can break the transmission of sound through conduction by either decoupling the materials or damping the materials. Both of these approaches can be accomplished by a number of products available on our site. Read Explanation of Flanking.