Sound Transmission in Subflooring

STC—Sound Transmission Class (STC) is a value given to a system of materials based on how well it can control airborne sound. In the USA, it is commonly used to rate interior partitions, ceilings and floors, doors, windows, and exterior wall configurations that create the system. It is classified using the test methods defined by ASTM E413 and E90.

To put it into terms we know, the STC rating gives you a rough idea of how much sound a floor, for example, might stop that is coming from the radio, vacuum cleaner, and noisy teenager. This would include how the floor is constructed and all the parts of the floor covering that is placed on it.  The higher the rating the better the control. A ceramic tile installation over a concrete slab will have a much lower STC than a carpet with a pad over the same concrete slab. STC is the most common sound reduction measurement in use. STC ratings are quite limited however and should be part of total soundproofing requirements and expectations.

IIC – Impact Insulation Class is the value given to a system of materials on how well a system can control impact sounds.  In the USA, it is commonly used to rate interior floors. It is classified using the test methods defined by ASTM E989-06.

To put it into terms we know, the IIC rating gives you a rough idea of how much sound a floor might stop that is coming from the dancing teenager using a jackhammer to escape her bedroom. 

The IIC test is done by using a tapping machine with steel-faced hammers. These hammers strike a test floor material generating sounds between 125 to 4,000 Hz. The impact creates vibrations that travel through the floor to the room below. Then they will compare the results of each tap to the results of a reference assembly.  This will determine the IIC rating. A higher number shows better performance. 

The IIC ratings for basic concrete sub-floor alone is around 28 to 35 IIC. The IIC ratings for a basic wood subfloor alone are around 40 to 45 IIC. Flooring material and sound impact material added to the assembly will increase or decrease the total IIC rating. An IIC rating of 50 and above is most common in building codes and some housing requirements.

DELTA Δ – The Delta IIC rating shows what the product adds to the assembly in terms of isolating impact noise. The Delta IIC test starts with testing a full assembly, typically six to eight inches of concrete, with nothing above or below the concrete. Then they will install an underlayment like Schönox TS to the concrete, and the same test repeats. The Delta IIC rating is the difference in the gain between the first and second tests. A higher number the better performance.

The Delta IIC rating is the best rating to consider when comparing the performance of different types of underlayment. It keeps the manufacturers of underlayments from giving misleading information. They do this by using materials or methods of isolation that the average assembly would not use. You can see this when a manufacturer will claim an IIC of 60 or above using materials less than 1/2” thick. They achieve their rating with assemblies not used in real-world applications but will then claim their underlayment is what achieved that IIC rating. You should always ask to see a Delta IIC rating. If they cannot provide a Delta IIC rating, then we cannot compare our material to the others in a fair manner.