Principles of Rigid Sound Attenuation

A Fun and Interactive Experience! (No really, you'll want some headphones)

IICSTC Test Results Cover Page

Why IIC and STC

While many people assume that IIC, HIIC and STC testing results across products are a fair comparison, the truth is that these tests can be manipulated to improve results, and quite often are. In this short interactive tour we’ll discuss:

  • How testing is performed
  • How results are measured (and manipulated)
  • Why monofilament technologies were a great idea in the 80’s (but not so much anymore)
  • Why you should always request the test results
  • Why rigid sound mats are the next logical leap in technology

Let us assure you that every dirty secret we share comes from an actual, real-life example. 

We Will Rock You (IIC and STC Explained)

IIC is an acronym for Impact Isolation Class and STC is an Acronym for Sound Transmission Class. These two tests determine how well a product reduces noise from impact to the floor assembly (things like high heels on a hard surface, dropping hammers and plates, and clogging in your kitchen) and noise from airborne sources (things like voices, TVS, singing in the shower, practicing hog calls and other things you swear your neighbors have done). You can click the words on the right to hear a great example of IIC and STC in pop culture that may help you to remember. The stomps are the IIC, and the claps are STC.

To complete the test, two rooms are built atop one another using the specified sub-floor assembly (more on that later) and the bare assembly is tested before applying the flooring treatments and testing again with these in place. The performance difference between these two tests is compared and a complex algorithm is applied to deliver a relative number that has nothing to do with decibels. It’s true. We’ll get there in a few minutes. 

Click above to hear an audible example of IIC and STC.
(Stop by clicking again)

Sound on Sound (Generating Test Tones)

Two different methods are used to measure the efficacy of the system. To test the IIC results, a tapping machine is utilized. The tapping machine is a small box containing a number of metal rods that are dropped repeatedly in rapid succession on the floor. The tapping machine is set on the floor of the top room in 4 different locations and the audio is analyzed in the room below, with levels taken for a specific band of frequencies. STC is tested by generating audio tones in the room above using an isolated speaker and then analyzing the audio on the same band of frequencies from the room below. 

Understanding Frequency

The chart below illustrates both the frequencies tested and a typical test result from each type of test. The highlighted portion on the graph shows the actual range of frequencies used to determine IIC and STC ratings. Clicking on the graph in a frequency range will allow you to hear samples of tones in that frequency.  As you can see, IIC tests will typically see increased results in lower frequencies while STC results will return higher numbers in higher frequencies.

Click on the graph to hear sample tones in each frequency range

Determining IIC and STC Ratings (And Reading the Results)

When ratings are determined, these test results are placed on a similar graph and a standard curve is applied and compared to the results from the tests of these 16 frequencies. This curve is adjusted up or down until a series of criteria are met and the ratings are applied. The issue here is that, because we are essentially averaging products performance, the end result can be two products that have the same IIC and STC ratings, but perform rather differently in the real world. The images below show two products with similar IIC, you can see that the 

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