Soundproofing Plus: How to Reduce Echo: Measuring Acoustic Absorption

How to Reduce Echo: Measuring Acoustic Absorption

At Audimute, we answer many questions concerning noise, reflection, and how to reduce echo. The answer is simple—sound absorption—but how can you tell if a material is going to absorb sound?

The NRC—or noise reduction coefficient—tells you how. To reduce echo, an absorptive material needs to capture sound, preventing it from bouncing around a room. The NRC is an easy reference number for comparing the effectiveness of sound absorbing products.

It’s simple: The scale is from zero to one, with an NRC of one being “perfectly absorptive.” Sound-absorbing materials are exposed in the lab to six frequencies, between 125Hz and 4KHz. The NRC is an average of the absorption over those frequencies.

This narrow spectrum and the perfect lab testing environment are a couple of the reasons that NRC is a good way to compare materials, but not the final answer, if you’re learning how to reduce echo.
Why not? First, NRC itself can be tricky. Some materials may show a NRC greater than one, even though the scale ends there. How is this possible?

During testing, sound is absorbed by every part of a material, including the edges. Yet the NRC calculation doesn’t count these contoured parts as part of the material. The material appears to absorb more than it should—more than the maximum of a perfect “one”— because the calculation “thinks” the material is smaller than it actually is.

Also, consider the untested sound: NRC doesn’t tell you how well a material absorbs (or doesn’t absorb) very high and very low frequencies.

In environments with substantial low-frequency sound—home theaters, for example—using a large amount of high-NRC material may create a bass-heavy room by absorbing everything except the low end. For determining how to reduce echo, balance is key: instead of relying solely on NRC, target all frequencies of interest and get expert advice in how to control them.

We’ve explained the NRC concept further in our video, cleverly entitled “Measuring the Effectiveness of Acoustic Materials." Feel free to call us at 866-505-MUTE (6883) for more expert discussion on NRC or any other sound-enhancement concepts.

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