Impact noise, the sound generated by one object directly striking another, is one of the most difficult sounds to control. Cork flooring has been a traditional and reasonably successful sound control medium. Some ways of laying down a cork floor work better than others in terms of sound control, and in all cases you should consider the advantages and shortcomings of cork floors when considering them as a sound-control medium.
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Stopping Impact Noise
Sound is a form of energy. Once it has been generated at the point of impact it quickly radiates throughout the structure, passing from one material to another with greater or lesser efficiency, depending on the material. Unfortunately, most building materials are very efficient sound transmitters. About the only cost-effective way you can control impact noise in a residential structure is to stop it before it starts, by introducing a material at the point of impact which is soft enough or pliable enough to respond to the input energy, such as a high-heel strike or a dropped object, by physical compression rather than by transmission through the floor and beyond. Cork floors have traditionally been used as a good sound-absorbing medium.
In many ways, cork floors are a good solution to the impact noise control problem. They are relatively inexpensive; squares of cork are available for as little as a dollar per square foot (as of Spring, 2010). Better grades can cost up to five or six dollars a foot, which is no longer inexpensive, but about the same as many other flooring choices. You need to get a cork flooring tile that is thick enough to do the job; 1/2" is a minimum, but 3/4" or 1" is better--at which point it is no longer a very inexpensive floor.
Cork flooring works reasonably well for impact sound control when it is newly installed, but the floor does not wear particularly well. You may be tempted to improve durability by applying a protective coating like urethane. In doing this you're also reducing the ability of the cork to absorb sound without transmitting it. Cork has an organic open cell structure. This is why it's spongy when you press it, and also why it absorbs sound well. When you put a coating on it like urethane, the liquid, unless the cork is very dense (and therefore very expensive) tends to seep down through the interstices of the cork, both stiffening it and filling the open spaces with what dries to a durable, hard substance. Sellers of cork for sound control purposes will dispute this, but experimental results confirm the lessening of impact sound control. Even wax, which hardens the surface of the cork, reduces its sound-absorbing capabilities, although not as much as urethanes, paints and shellacs.
Bypassing the Durability Problem
Here is the dilemma: either the cork absorbs sound well but isn't very durable, or you treat it in some way to increase durability, which diminishes its efficiency as a sound-absorbing medium. One solution is to put the cork under the final flooring. For a typical installation of this kind you would have an underlayment, something like 3/4" plywood over the floor joists, on top of which you would install a 1/2" or, better, a 3/4" or 1" cork underlayment, and over that you would install the final flooring, a hardwood or similar finish floor This works reasonably well, because the open cellular structure of the cork is intact--it's still a very good sound absorbing medium--and while the hardwood floor on top of it is a very good transmitter of sound, the transmission is directly onto the cork, which absorbs it.
How Much Sound Can a Cork Floor Stop?
The advantages of thicker cork are greater than you may think. A 1/2" cork tile has an NRC (Noise Reduction Coefficient) rating of .08 to .12; a 1" thick tile has an NRC rating of .3 to .7. This is a threefold to a sevenfold difference. For sound absorption purposes, the often-recommended 1/8" cork (used as an underlayment) or 1/4" cork is virtually useless. The 1" tile used as finish flooring is a reasonably good impact noise absorber, provided it isn't treated in some way that defeats its purpose. As a sound-absorbing underlayment, thicker cork tiles are also good, but need to be cost-compared with similar sound-absorbing substances like high-density polyurethane foams.
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