Acoustical Treatments: When Selecting Panels, Look to Absorption

When comparing 20 Hz and 20 kHz (the top and bottom limit of human hearing), the waveform for 20 Hz is near 56 feet long, while a 20 kHz wave is only 0.6 inches long.

Acoustical Treatments: When Selecting Panels, Look to Absorption
A depiction of an anechoic chamber. Anechoic chambers are rooms in which there are ideally no reflections.
Credit: Trevor Cox

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Acoustical Treatments: When Selecting Panels, Look to Absorption

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When selecting panels, you are going to see two different ratings. These numbers might not mean much at first, but they are actually very important because they characterize the absorber. The first is the Absorption Coefficient. This is found in sabines, and represents the amount of absorption per frequency. For reference, 1 SQFT of open window is the same as 1 Sabin. The second number you will see is a Noise Reduction Coefficient or NRC. The NRC is calculated by by averaging its sound absorption coefficients at 250Hz, 500Hz, 1000Hz, and 2000Hz.  Because absorption coefficients can be over 1, it is possible for an NRC to be larger than 1.  For example, this means that if a 2-inch panel of plain Owens Corning 703 on a wall has an NRC of .7, that means that the absorption coefficients averaged from 250Hz, 500Hz, 1000Hz, and 2000Hz is equal to .7. 

The second type of Absorbers, Membrane Absorbers, are also known as diaphragmatic absorbers and are best to be used for low frequency absorption. In Membrane Absorbers, the sound wave travels and hits the membrane and the membrane reacts at less intensity than the actual wave. This lower intensity wave passes through and hits the fibrous material to be dissipated as heat. The wave reflects back and is then dissipated more from the fibrous material, and dampened again by the membrane.

The final type of Absorber is my favorite. Known as a Resonance Absorber or Helmholtz Resonator, it was first developed by Hermann von Helmholtz in the early 1860s. Not only did he develop this style of absorption, but also developed many substantial ideas in other fields such as optics, nerve physiology, and electromagnetism. If you get some time look him up!

The same effect caused by blowing over the top of a Coke bottle is what the Helmholtz Resonator employs. That tone, instead of being annoying, would actually be the tone that is absorbed. The math is based on several variables, but you can design a target frequency with a specific Q. This can actually be decently wide or extremely narrow, depending on construction.


More About Tom Noble
Tom Noble received his Bachelor of Science in Acoustics from Columbia College in Chicago. During college, he served as a researcher for the Army Corps of Engineers with a specific focus on Low-Frequency Propagation. After college, he owned his own company working with churches and other AV clients. One of his favorite jobs during that time was being able to design and build a recording studio in downtown Nashville. Shortly after, he worked for an integrator, doing work all over the country, specializing in DSP programming and tuning of rooms for many churches and large corporate clients. He is now the head AV design engineer for Lifeway Christian Resources in Nashville. He is married to his beautiful wife with an amazing son and beautiful little daughter.
Get in Touch: tom.noble@lifeway.com    More by Tom Noble

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Worship Facilities Magazine, March-April 2018
The March-April 2018 issue of Worship Facilities Magazine offers articles about how to prepare, prevent and respond to church violence, a look into what church management software can do for your church community, and a piece on how a once popular nightclub venue was transitioned to become Shoreline Church's new home.


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