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When considering noiseproofing materials, you’ll likely come across a rating system called STC, which stands for Sound Transmission Class. The STC rating system is the most commonly recognized sound isolation standard in North America, and it is heavily used by marketers for promoting soundproofing products. The problem is that the standard is outdated and not always very useful, and the ratings don’t tell you much about how effectively a material works.

The STC standard dates back to 1961 and ranks walls over the frequency range of 125 to 4,000 Hz. And that’s the main problem—the standard doesn’t consider noise below 125 Hz. If you’re looking for a sound isolation solution, chances are it involves low frequency noise. Most of the sonic energy generated by the average home theater, for example, falls below 125 Hz. So does a lot of noise from traffic, airplanes, music, and machinery. The STC standard is not useful for calculating noise isolation for applications at these frequencies.

The STC calculations also assume that noise generally has an even energy dispersion. But this is not how noise behaves in most real-world situations. For these reasons consumers are often better off relying on more modern rating systems such as the Outdoor–Indoor Transmission Class (OITC), which considers frequencies down to 80 Hz.

For more about the STC ratings and how they are calculated, click here.