Isolating Airborne Noise Through Floors
Treating sound problems at the source increases the value of the products used because it limits the flanking paths by treating the vibrations before reaching the framing. And treating soundproofing between floors is significantly less efficient than treating the ceiling below.
WHAT WILL WORK
In order from highest performing to lowest performing
Decoupling the floor is an option now that the Acoustic Sleeper exists. Other similar style products used to decouple floors are not successful because the material is too rigid and requires too much pressure to compress in wood structures. Acoustic Sleeper is well tested though to show high performance with a pretty minimal depth overall (about 1-1/8" plus finish floor). Green Glue Compound can be used in conjunction with the Acoustic Sleeper system by applying the Compound between the two layers of 7/16" plywood used in the Acoustic Sleeper system. View Acoustic Sleeper.
Comparing the many different types of underlayment that exist, rubber underlayment provides the most significant increase in isolation for airborne noise reduction. The performance of rubber is much higher and consistent when installed over or under concrete, but will perform well in wood structures. The same performance can be achieved in wood structures if the rubber can compress as intended. With concrete sub-floors, the rubber compresses properly because the concrete has no give. Over wood, the rubber does not typically compress as intended because the wood will allow some give in the structure. Adding about seven pounds per square foot of mass below or above the rubber resolves this issue by stiffening the structure, allowing the rubber to compress. Read Maximizing Rubber Underlayment Performance. An alternative to adding mass over the rubber would be the use of a thicker rubber underlayment product or to decouple the ceiling in the assembly. View GenieMat® RST rubber underlayment.
Green Glue Compound in the floor will provide minimal benefit without decoupling the assembly ceiling (resilient sound clips or resilient channel). If decoupling the ceiling, then adding the Compound to the floor between layers of rigid mass can gain as much as 6 STC points and 7 IIC points. View Green Glue Compound tubes or Green Glue Compound pails.
WHAT WILL NOT WORK
In no particular order
Layers of mass over a resilient material like GenieMat® RST is extremely beneficial in a wood structure. However, additional layers of mass over existing mass without the added resilient layer will provide little benefit in reducing airborne noise.
Cork floors, unless incredibly thick (1” or thicker), will not significantly improve the IIC rating of a floor. Cork soundproofing tests show performance of only about a third as much as rubber underlayment of the same thickness.
These types of underlayment products are often available through local flooring supply stores. The airborne isolation ratings advertised by the manufacturers of these products are often well above 60 or even 70. These ratings are not realistic. Foam underlayment will help soften the cushion a little resulting in minimal isolation gains for impact footfall noise, but no actual gains in isolating airborne noise.