Underlayments and Sound Definitions (IIC – STC – Delta IIC)

One of the most well-known reasons for using flooring underlayment is its acoustic benefits, often referred to as underlayment sound definitions. This means reducing unwanted noise from traveling to other rooms, which is especially important in multifamily housing where units are stacked. Although there’s no universal test to measure noise reduction within a room, several ASTM lab tests measure unwanted noise caused by impacts and airborne sounds, focusing on how much sound transfers through a floor-to-ceiling assembly into living spaces below.

Let’s review the common industry acronyms related to sound ratings:

STC – Sound Transmission Class – This score measures airborne sound transmission, such as noise from TVs, stereos, and voices. The higher the STC rating, the better the material or assembly at reducing airborne sound transmission.

IIC – Impact Isolation Class – This score measures the effectiveness of a floor or ceiling assembly in isolating vibrations and reducing impact sounds, such as footsteps or objects hitting the floor. A higher rating means better impact sound isolation.

Delta IIC: This rating measures the added impact sound isolation from underlayment. It reflects the difference between the IIC of the untreated assembly and the IIC achieved once underlayment is added. It’s useful because it isolates the performance of the underlayment itself, rather than the entire assembly.

Delta IIC is especially valuable for comparing the performance of different materials, as it avoids misleading results. Understanding these ratings will help you choose the best underlayment to reduce noise.

If you’d like to view sound rating tests for each underlayment, click here. Additionally, if you’re unsure of which underlayment to choose for your floor, try using our underlayment selector tool here.