Soundproofing a Floor

Techniques for soundproofing a floor: Three common scenarios.

In soundproofing a floor, the techniques to consider depend on what kind of noise problem you're trying to solve.
    flooring being laid
    Image courtesy of US Department of Labor
  • Do you want to reduce the noise traveling from an upper floor to the room(s) below it, or to reduce the noise entering a room from beneath, through the floor?

  • Is the problem impact noise, or airborne noise, or both?

Different situations call for different approaches to soundproofing a floor, each using one or more of the four basic elements of soundproofing. Let's look at the most common scenarios.

Reducing Impact Noise from Above

This is when you can hear impact noise coming from the room above you, such as footsteps, furniture dragging, and items dropping. If the room above you belongs to someone else and you can't make changes to the floor above, you'll have to do the best you can by treating your own ceiling. But it's always best to reduce the noise as close to the source as possible, so if the upstairs is part of your own house or is under your control, that's definitely the first place to put the effort.

Carpet and Pad

By far the most effective way to reduce impact noise from above is to lay carpet with thick padding underneath — the thicker, the better. This dissipates the impact energy before it has a chance to enter the building structure as sound. Carpeting gives superb results and often resolves the problem of impact noise. Note, however, that it does little to mitigate airborne sound such as voices, stereo, or TV, so if those noise sources are part of the problem as well, you'll need to use some of the techniques below.

Resilient Underlayment with Damping Compound

If you have impact noise but carpeting is not feasible in your case, or is something you'd prefer to avoid, then you'll want to put down layers of flooring that include resilient underlayment and damping compound. The resilient underlay introduces a decoupling effect, and the damping compound further dissipates the sound energy, and is especially important at lower frequencies where underlayment does not help.

You will also have the opportunity to choose materials that provide additional mass, introducing a third soundproofing element that is especially important if airborne noise (such as voices or music) is also a concern. (If your existing subfloor is already substantially massive, such as a concrete slab, adding additional mass will not accomplish much and won't be necessary.)

Resilient Underlayment:
There are several varieties of resilient underlayment material available, including foam, fiber, cork, and rubber. A shredded rubber mat is an excellent choice because it is heavier than foam or cork, so it gives you the extra soundproofing benefit of added mass.

Damping Compound:
The damping compound (typically Green Glue Noiseproofing Compound) can often be applied directly onto the existing subfloor layer, if the subfloor is not thick and massive, and it is of a material that can be screwed into. Since the damping compound needs to be between two stiff surfaces to do its job, a rigid layer needs to be placed above it. You can use cement board, or if total thickness is not a concern and you want the extra mass for more sound isolation, you can use thicker sheets of medium-density fiberboard (MDF) or oriented strand board (OSB). Any sheets that have damping compound between them should then be screwed together.

Install your choice of finished flooring (such as hardwood, wood laminate, or tile) on top of the underlayment as a floating raft — in other words, do not attach it to the layer beneath with nails, screws, or adhesive. Also, leave a gap between the edge of the flooring and each wall, and fill the gap with acoustic sealant. This gap allows for expansion of the flooring, and also acts as a decoupler, helping reduce the sound energy that is transmitted through the walls into the building structure (the "flanking noise" problem).

The elements above can be combined in a number of different configurations for soundproofing a floor. Here is one example:

diagram of soundproof flooring layers

Reducing Airborne Noise from Above

If impact noise is not a problem but you can hear voices or music coming from the story above, again the best approach is to treat the floor above if you have access to it.

In this scenario, mass and damping will be the key soundproofing elements to use. In fact, if impact noise is not a problem, you'll want to avoid using resilient underlayment, because any compressible layer will introduce a resonance cavity (the "triple leaf" effect). For impact noise, this is worth the tradeoff because the decoupling element is so effective, but it's unnecessary and undesirable in soundproofing a floor if airborne noise is the only concern.

For mass, a standard technique when soundproofing a floor is to put down one or more layers of MDF or OSB. To add damping, apply Green Glue Noiseproofing Compound between the mass layers, which should then be screwed together. If the existing subfloor is of similar thickness and stiffness as the panel you are laying over it, Green Glue can also be applied directly to the subfloor. The finished flooring goes on top of the sandwiched layers.

Reducing Noise from Below

This is when you have noise coming from the story below you, entering through the floor of the room. Following the general principle that a noise problem should be treated as close to the source as possible, it would be best to address this by treating the ceiling of the story below. But if that's not feasible, or if you need still more noise reduction, soundproofing the floor above will provide some noise isolation. (Some sound will still be conducted through the walls.)

In this case you are dealing mainly with airborne noise, so you can follow the same procedures outlined in the previous section, adding mass and damping (and avoiding resilient underlayment).

A Note about Flanking Noise

Noise takes whatever path is available. Some portion of the noise will enter the structure of the building and will travel up or down via the walls; this is called flanking noise. How much noise escapes this way will depend on the details of your building's structure. If you have or are planning to have tile or wood floors, a noise reduction plan may need to include soundproofing the walls as well as adding soundproof flooring and/or ceiling in order to get the quietest results.

Read more: Materials used in soundproofing floors

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