Sound Abatement 101 – Understanding IIC and STC Ratings
It’s finally here! Our most requested topic for discussion has finally made it to the top of the to-do list. Sound abatement is a complicated matter, the most common misconception being that the results are coming from a specific flooring product and not the entire system. There is no such thing as an STC or IIC rating for flooring or underlayment alone. STC and IIC come as the result of an entire system, top to bottom. To further discuss this subject, we met with Daniel Mohler. He is the project lead at Intertek in York, Pennsylvania. He works at their acoustical chamber studios, where they have done around 20,000 sound tests. Intertek’s doors are always open. They love giving tours if you are ever in the area.
Let’s start with the basics. STC is airborne sound, which is a sound that can come from your upstairs neighbor blasting a TV or radio or talking very loudly at each other. However, most buildings already have decent STC performance on their own. The most common noise complaints will come from IIC, which stands for Impact Isolation Class. This is the sound that will come from your upstairs neighbor dragging furniture, walking, or stomping.
If you want to get super technical, STC falls in the category of ASTM E90, which is a method that gives us STC results in sound transmission loss. While IIC is measured with ASTM E492, this being an impact test.
These tests are usually run in conjunction. The flooring and or underlayment is installed in the system being tested. And while the testing process only takes about 15 minutes, it takes weeks to prepare.
But what do these 15 minutes entail? Daniel does two background measurements where he has no sound turned on. This helps with measuring the level in the acoustical chamber. Their lab is a very well-isolated building that may look like a square. But it is a half-billion-dollar construction. This construction is equipped with top-of-the-line technology to run all these tests in a very accurate way.
For STC, he blasts lots of sounds in the upstairs chamber, or as they call it, their second story. He blasts around 120 decibels of sound, which would be equivocal to two jet engines running simultaneously. He then measures how loud it is upstairs and how loud it is in the lower chamber while these jets are running. The more sound the system stops, the better the rating will be.
For IIC, he performs the same two background measurements. He then uses a tapping machine, a calibrated device that drops five impact hammers from 40 millimeters high. He follows the ASTM standard by moving the machine around and running it in different placements. When testing the IIC, the point of concern becomes the sound at the lower chamber. The quieter it is at the lower chamber when the tapping machine runs, the higher the score will be.
Some consumers are often confused by these ratings. Usually what is provided to the consumer is just a single number with no explanation of what the system was in the testing. This can be very misleading. Daniel recommends that you reach out for the installation specifics. Instead of providing just an IIC of 54, he recommends transitioning to using terminology like IIC of 54 on a six-inch slab.
This is where Delta IIC testing can provide much better comparisons for consumers. Delta IIC immediately puts ratings on equal standing and helps eliminate some of the “marketing games” that are played.
Daniel explains further,
“If I started with an IIC 30 on my bare concrete slab, and then I add your flooring, and […] I get an IIC 50, your Delta would be around 20. […] That number is that improvement of the IIC result from a bare concrete slab. Let’s say another lab gets their concrete slab built a little differently, or it’s kind of fancy, whatever it might be. Their base slab starts at 33. So, they are three points better than me. But then, when they do their IIC tests, they get a 53. That’s still a 20-point improvement. Delta IIC, to me, is the singular most comparable […] result, and it takes away the ability to do any system you want.”
Delta IIC numbers take away a lot of the number playing, standardizing the performance.
Now that you have a better understanding of what the tests and ratings mean, it’s also important to understand that every flooring type has different acoustical properties right from the start that will affect how it’s going to perform. Per Daniel, the worst flooring type that you can have for acoustics, assuming that all the parameters are equal, is a sheet vinyl or 2 mm glue-down vinyl. That would give you very little acoustical benefit. The softer a flooring material is the more resilience it will have. And the better they’re going to perform. If they can absorb sound, they will do a little bit better.
Acoustics are not the only concern when it comes to flooring. Durability and waterproofing are also important. And at times, things that will help with sound will hurt durability. Sound is affected by many factors, even if the floor is glued down or floated. It being installed in different directions also makes a difference. There is a lot that comes into play.
In conclusion, I think you will agree that sound abatement is a complicated topic, and this is only the beginning of it. If you’d like to hear more of our interview with Daniel Mohler from Intertek, please check out our What the Floor podcast episode by using the link below and stay tuned for the sequel, Sound Abatement 201!