Resource Center

Working with ASTM Test Numbers

The STC, FSTC, IIC, ∆IIC, and FIIC test results are expressed as a single dB (decibel) value.  The value is the result of calculations across a full range of sound frequencies starting at 63 hertz up to 5,000 hertz.  Higher dB values indicate better sound reduction.  Since these are logarithmic calculations, increases from 40dB to 45dB are easier to achieve than increases from 50dB to 55dB, or 60dB to 65dB.

The Sound Transmission Class (STC) value of a floor assembly is primarily a function of mass.  For example, an eight inch concrete slab will be better than a six inch concrete slab for reducing sound transmission.

The Impact sound transmitted by a floor is a function of how well it absorbs sound vibration caused by an impact.  This is measured as the Impact Insulation Class or IIC.  Carpeting is the best impact absorber, wood less, and ceramic tile or marble the least absorbing.  Think about dropping an object on the finished floor like a wine glass.

The Delta IIC measures the Impact Class improvement for a specific “total” floor assembly.  This test is only performed on a six inch concrete slab without a ceiling assembly.  The result for the tested floor assembly (underlayment materials and the finished floor surface) is compared with the “bare” six inch concrete slab IIC value.  In order to understand the IIC improvement, you need to know the finished flooring surface as well as the sound underlayment and adhesives used in the floor assembly tested.  Most important, Delta IIC values are NOT additive.  For example, if the underlayment material claims a ∆IIC of 15dB and a thin set or adhesive manufacturer claims a ∆IIC of 10dB, having both in the same floor assembly will not result in an IIC improvement of 25dB.  Make sure you know how the product was testing, including the finished flooring and other floor assembly components.

Field testing is an excellent way to establish the Sound Transmission Class (FSTC) or Impact Insolation Class (FIIC) of a specific floor assembly and building.  The result is specific to the tested room and should not be used to grade or measure a particular component (underlayment, adhesive, thin set, etc.).  Laboratory sound testing is conducted in a controlled environment and field testing is not.  As a result, field test results can be misleading.  The specific construction, temperature, humidity, or room content can influence the result.  For example, was a perimeter isolation barrier material used to isolate the room?  Was the air conditioning on or off?  Was anything in the room like drapes or furniture, etc.