IIC vs STC

When it comes to sound, there are multiple ways to determine the movement of noise through assemblies. Sound travels in many different ways and as waves. Different ratings depend on the medium through which the sound waves travel. The emphasis tends to be placed on STC (sound transmission class) ratings, but others are just as important-or maybe more applicable-depending on the situation. This post will focus on the difference between IIC and STC-both very common ways to measure the transmission of sound through assemblies but approaching the source of sound differently. 

The STC rating is concerned with the travel of sound or noise (sound above a certain threshold for which it is considered unwanted) through the air. The STC measures how well an assembly of materials absorbs the transmission of airborne sound/noise. Assemblies not only include interior partition walls, ceilings/floors, and exterior walls, but also include windows, doors, and so on. 

However, sound waves do not just travel through the air, but are also transmitted through waves moving through materials. In this case, it may be more appropriate to focus on the IIC (impact insulation class) rating, which is more concerned with the travel of sound through materials and measures how well an assembly blocks impact noise. The IIC rating-although applied to assemblies-is more appropriate for floor/ceiling assemblies because that is where the impact noise will most likely occur. 

Depending on a variety of factors-but most likely which assembly (such as the wall, floor, or ceiling) is being rated and for which program-the employment of one rating over another may be better suited for measuring the impact of noise in a space and the design of assemblies. For example, in a dense, urban area, while there is a lot of noise traveling between walls, there may be more concern for the sound that is traveling through floors and ceilings within a building. In this case, it may be more appropriate to review IIC because that noise not only comes from side to side but also from above and below as well. In the case of offices, the clicking of high heels (always a common example when discussing sound) travels through structural members and flooring (typically wood in older construction), which may affect the space more so than airborne sounds. In this case, it would be more appropriate to be concerned with the IIC since it focuses on the sound transferred via materials through floors and ceilings. Conversely, STC may be more appropriate in isolating spaces from each other due to program-such as a wall between a quiet office and adjoining loud conference room, when the noise will most likely travel through the air. 

Not all sound is created equal, and when addressing acoustics, knowing the source is just as important as the rating because the sound comes from all over. Recognizing that and applying the ratings appropriately will allow for more successful strategies in alleviating the transfer of noise.