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Intro

IIC ratings cover a limited frequency range of 125 to 4,000 Hz. When footfall noise transfer issues are low in frequency, a high IIC product may not help. When someone steps or stands on a floor, it will deflect under their weight. This structural deflection will sound like a low frequency thump. There are a few ways to resolve this problem:

  1. Increase the stiffness of the floor/ceiling framing.
  2. Increase decoupling between the floor and the ceiling.
  3. Increase the structural damping.

Increase Stiffness (Stepped Blocking)

Stiffness increases by building the equivalent of another beam in the middle of the joist system (as presented in the drawing). Also, the additional load spreading will distribute the load among several joists. This alone may increase stiffness enough to resolve the structural deflection and low frequency footfall noise issues. We recommend stepped blocking as the first approach in resolving low frequency footfall noise transfer.

The blocking should be one size smaller than the framing, glued and nailed at each connection. Install blocking at the mid point for spans greater than 10′, at one third points for spans greater than 18′, and at one quarter points for spans greater than 25′.

structuraldeflectionblocking

Increase Decoupling

The most common way of decoupling, at least the one that has been around the longest is with resilient channel. Resilient channel can provide some improvement at high frequencies, but in low to mid frequencies where much of the footfall noise exists, the resilient channel does not provide much benefit. Resilient clips like the GenieClip, on the other hand, can provide significant improvement in all frequencies, including the low to mid end. If the stepped blocking is not an option for treating low frequency footfall noise, then the next best option is the installation of a resilient clip like the GenieClip. Combining both stepped blocking and resilient clips would maximize the isolation of low to mid frequencies in any ceiling.

Increase Damping

After utilizing the methods above, if any energy from the structural deflection does remain, it can be reduced further with a damping product like Green Glue Compound. Damping vibrations to reduce flanking in a structure is one of Green Glue Compound’s most underrated features. We strongly recommend including Green Glue Compound between layers of sub-floor (most effective placement for isolating footfall) or between layers of drywall on a ceiling (assuming installation of resilient clips or resilient channels).

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