What Are STC Ratings In Soundproofing?
Jul 11, 2022So you may have come across the term STC when researching how to build a soundproof studio. In this article we will go over what STC means, how it can be misleading and how it is useful.
What Does STC Stand For?
STC stands for Sound Transmission Class and it was created back in the 1960's by the American Society For Testing and Materials (ASTM). They created the standard to compare sound isolation assemblies (aka walls, doors, floors, windows and ceilings) between each other with the purpose of reducing human speech. STC was created for use with homes and offices not with recording studios.
The Problem With STC Ratings
Because STC was not intended for recording studios it only calculates and assemblies ability to reduce sound down to 125hz and up to 4khz. That is the spectrum of the human voice. Now in a recording studio there are sounds well below 125 hz and well above 4 khz. This means we cannot solely rely on STC ratings when evaluating how soundproof a wall may be.
The Basics of STC Ratings
STC ratings are pretty straightforward. A high STC rating is better than a low one. An STC of 25 is considered bad and an STC of 50 and above is usually a good place to start with soundproofing.
You can see in this chart from Acoustical Solutions that each STC number correlates to the volume of speech heard through a wall. Notice at 60+ good soundproofing is achieved.
In this chart by Indow, a maker of window inserts for sound reduction, they also show that 65+ is soundproof and that an STC of 50 has loud music barely being heard.
STC Ratings Help Us Understand What Types Of Wall Construction Are Best
In the diagram below adapted from Rod Gervais's book, "Home Recording Studio: Build It Like The Pros" we can see that a wall with an STC of 40 has two adjacent walls built side by side. You would think this would give the optimal amount of soundproofing, but lab tests prove that two walls each with an extra layer of soundproofing on the outside actually provides superior results. Now you get an STC of 63.
Conclusions
Number one, don't rely on STC alone, but use it as a tool. Number two, know that STC does not represent low frequencies below 125hz. Lastly, use good soundproofing techniques that are tried and true and don't fall prey to a new material claiming to have high STC ratings.
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