STC vs. CAC: Decode the Jargon of Soundproofing

Searching for effective ways to manage sound in your building projects? You’ve probably heard of STC and CAC, but what do these acronyms really mean?

Dive into our comprehensive guide to understand the differences between Sound Transmission Class (STC) and Ceiling Attenuation Class (CAC) and learn which one you need to focus on for walls, ceilings, or individual products. Your journey to acoustically optimized spaces starts here!

What Is Sound Transmission Class (STC)?

So, you’ve likely heard of STC, or Sound Transmission Class. It’s the granddaddy of acoustics in the building industry, especially when it comes to walls and windows.

STC is essentially a number that tells you how good a product is at keeping sound from traveling from one side to the other. The metric was first introduced in 1961 and has since become the standard metric to describe sound blocking materials.

STC is measured through a set of standardized tests. We’re talking about controlled environments where sound is emitted at one end and measured at the other, across a range of frequencies. The STC rating then gives you a ballpark figure of how effective your product will be at blocking noise.

What Is Ceiling Attenuation Class (CAC)?

Now, let’s talk about CAC or Ceiling Attenuation Class. While STC focuses on walls, CAC is all about that roof over your head — or more accurately, the suspended ceiling above you.

Think of it as the measure of how good your ceiling is at not letting sound transfer between rooms separated by a common airspace (also known as a plenum).

Unlike the single-pass test for STC, CAC utilizes a double-pass test. It’s because sound moving from one room to another has to get through not one but two ceiling barriers. The test measures the attenuation of the sound as it goes up through one ceiling, bounces off the structural slab or deck above, and then comes back down through the ceiling of the adjacent room.

The CAC and STC Scale: Measurement Parameters

STC ratings range from 25 to 65. The higher the rating, the less noise passes through. A rating of 25 means there is no soundproofing, while a rating of 65 indicates complete soundproofing.

For example, a standard single-pane window usually has an STC rating of 25, while double-pane windows typically have ratings around 30. Most products designed for soundproofing fall within an STC range of 35 to 55.

Refer to the following table for additional examples.

STC RatingLevel of SoundproofingTypical Applications
25No soundproofingStandard single-pane windows
26-30Minimal soundproofingDouble-paned windows, some interior doors
31-35Moderate soundproofingBasic office partitions, residential walls
36-40Good soundproofingSpecialized office partitions, better residential walls
41-45Very good soundproofingHigh-quality doors, studio walls
46-50Excellent soundproofingCommercial construction, recording studios
51-55Near complete soundproofingHigh-grade recording studios, home theaters
56-60Exceptional soundproofingIndustrial applications, high-security settings
61-65Complete soundproofingSpecialized applications, extreme sound isolation

Like STC, CAC also has its own range of ratings, where higher numbers indicate better sound attenuation. Materials with ratings below 30 generally provide minimal noise reduction. You’ll start to notice significant sound attenuation at a rating of 35. For instance, a suspended ceiling with acoustic tiles usually has a CAC rating around 35. For superior sound control, aim for a CAC rating of 40 or higher.

See the table below for further guidance.

CAC RatingLevel of Sound AttenuationComments
25-29PoorNot effective for reducing noise
30-34MinimalSlightly better but still not very effective
35-39ModerateStarts to show effectiveness in noise reduction
40-44GoodSuitable for most residential and office settings
45-50ExcellentSuperior results, used in specialized settings

It’s important to note that STC and CAC ratings are not static; they can change over time as materials wear down or if other modifications are made to the structure.

Making the Right Acoustic Choices for Your Space

Navigating the maze of acoustic metrics can be challenging, but understanding the difference between STC and CAC can make all the difference. Whether you’re partitioning an office, soundproofing a home, or setting up a recording studio, knowing these scales can guide you to better acoustic solutions.

We hope this guide has helped clear up any confusion and empowered you to make soundproofing decisions with confidence.

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