The difference between underlayment sound rating test


STC – Sound Transmission Class

One of the most common sound rating tests used in North America. Covering the most common range of noises we hear including speech, television and other airborne noises. The STC test are done by completing 16 different tests done at the following frequencies (125Hz, 160Hz, 200Hz, 250Hz, 315Hz, 400Hz, 500Hz, 630Hz, 800Hz, 1000Hz, 1250Hz, 1600Hz, 2000Hz, 2500Hz, 3150Hz & 4000Hz). Each number from the 16 tests are then averaged and you get the STC Rating. Reviewing the sound test data should show the STC contour line of the transmission loss graph. The number will show if the STC contour line is consistent with the frequencies.

The testing is very complex and the numbers are measured across a mathematical equation based on the average of sound rating test across frequencies.

IIC – Impact Insulation Class

An IIC sound rating test is completed by placing a tapping machine with steel faced hammers hitting a test floor. The impact sounds are between the 125Hz to 4,000 Hz. The vibrations created by the machine travel to the other room. The sound engineer then plots the tap on a graph to compare the results. With concrete sub-floors and no underlayment, the average sound rating is 28 to 35 IIC. Wood sub-floors average rating of 40-45 IIC. Most building and HOA requires a rating of 50 IIC and above.

Delta IIC Rating

The Delta IIC rating is the sound test homeowners should be looking at. Basically, this rating is what the product adds in terms of absorbing impact noise. The sound test is only done on six to eight inches of concrete slab with nothing above or below the concrete. One test is done without the underlayment and the other test is done with the underlayment. Whatever the difference is the number. The higher the number, the better performance in terms of sound absorption.

These tests are done only with concrete slabs and they don’t perform well in wood structures.

The ratings keep manufacturers from displaying misleading results to homeowners.

Multi-Family Dwellings

If you are in a multi-family housing or HOA, we recommend you to look at STC and IIC ratings.


If you are a regular homeowner, we recommend you to look at Delta IIC ratings.

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