Footfall noise has become an increasing problem in houses, apartments and condos with the trend towards hardwood flooring.
What Is Footfall Noise?
Footfall noise, also know as impact noise can be in the form of someone walking, dragging a chair across the room, or dropping a hard object. Since hard floor surfaces don't have the ability to absorb this energy, it continues to propagate through the floor structure.
So What's Going On?
A foot striking a hard floor surface produces a significantly greater amount of energy per square inch than an instance of airborne noise. This energy is then transmitted into the floor assembly and exits through the ceiling below.
What's The Solution?
To eliminate footfall noise, you need to absorb the sound energy before it comes in contact with the floor assembly. This can be accomplished by installing a resilient acoustical rubber underlayment between the sub-floor and the finished floor surface. In order for an acoustical underlayment to be effective, it has to have the right combination of mass and resiliency. Some underlayments such as cork are too light, while others may have sufficient mass but are too stiff.
To determine if you are purchasing the correct acoustical underlayment, you need to look at the test data provided by the manufacturer. There are two numbers you need to be concerned with: STC, (Sound Transmission Class), and IIC, (Impact Insulation Class). STC is a standard developed for architects and acoustic professionals to rate a partition's ability to attenuate airborne sound. IIC is an acoustical rating used to quantify impact sound absorption. In a nutshell, a material with an STC or IIC of 55 will reduce sound transmission by 55 DBa. To put this in perspective, a reduction of 10 DBa is perceived by the human ear as about half as loud.
IIC ratings are assigned not only to the acoustical underlayment, but the entire floor assembly as well. Proper test data will reflect IIC ratings for different assemblies. You will want to select an acoustical underlayment with an STC and IIC of 50 or higher.
What If I've I've already Installed My Flooring Without An Acoustical Underlayment?
If you've already installed your flooring without taking in account the problem of footfall noise don't panic, you're not alone and there is an alternative.
If you have access to the ceiling below and can spare a few inches in wall height, you can add a layer of mass loaded vinyl over the existing ceiling to add mass, decouple the partition with sound isolation clips and standard furring channel, then finish off with an additional two layers of 5/8" drywall.