IN CONCRETE STRUCTURES
Concrete structures have lower IIC ratings (approximately 28 to 35 IIC) than wood structures because they are more rigid and massive. For those same reasons, concrete structures also respond best to resilient sound isolation products like rubber underlayment because the added resilience provides something sorely lacking from the basic concrete structure. When combined with the extreme mass of the concrete, the resilient rubber underlayment adds high levels of isolation for footfall impact noise transfer.
The easiest way to increase the performance of the rubber underlayment over a concrete sub-floor is to install a thicker rubber or a double layer of rubber to create double deflection.
IN WOOD STRUCTURES
Basic wood structures start with higher IIC ratings (approximately 40 to 45 IIC) than concrete structures because they are lightweight and will flex. Installing rubber directly to a wood sub-floor will achieve less dramatic IIC gains than in concrete structures. This is because when someone steps on the rubber, the wood beneath will also give. As opposed to concrete structures, which have little or no give. The two best ways to resolve the flex in wood structures is through stepped blocking between the joists or layering mass over the sub-floor.
Fixing structural deflection can be resolved with stepped blocking. Stepped blocking is the process of increasing structural stiffness by adding framing within the joist cavity running perpendicular to the joists. This stepped blocking is the equivalent of another beam in the middle of the joist system. The rigidity created with stepped blocking takes the ‘give’ out of the wood structure allowing the rubber underlayment to deflect (compress) properly.
Layer significant mass, approximately 7 pounds per square foot, directly to the sub-floor or over the top of the rubber underlayment. Any heavy material will work, i.e. plywood, OSB, MDF, drywall, lightweight concrete (best option of course), or a cementitious board like HardiBacker.
Layering the mass directly to the sub-floor will help most with isolating low frequencies for both airborne and footfall impact noise transfer. Most severe sound issues are low in frequency. Layering the mass over the rubber underlayment will lose some performance in low frequency isolation, but gain some performance in the mid to high frequency range.