Measuring a diffuser in an anechoic chamber

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.

When I tell people that I do acoustics for a living, they nearly always tell me how much they love the guitar. This was always funny to me, but it pointed out that the study of acoustics is a broad topic that very few people ever consider.

The first thing we must understand is that sound is a waveform.

The study of acoustics can be vast and deep. It can be difficult to talk in brevity on the subject, because there are so many nuances that make up the field. Having said that, I am going to give an overview of some types of acoustic treatments, what they are, and how they work.

Before that, though, I want to impart a rule that I was taught that has really stuck with me from the beginning of my career. I do not know who coined the phrase originally, but a mentor of mine always told us a fundamental truth: nothing is large, and nothing is small it's all relative to waveform.

The first thing we must understand is that sound is a waveform. This waveform can be complex, but in its purest form, a sound wave is a perfect sine wave.

The second thing that we need to understand is that all waveforms are different.

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.

Bringing it back around to the idea that nothing is large and nothing is small, if you have a seat cushion, that is going to be huge for a 20 kHz waveform, but not even noticed by a 20 Hz wave. To look at it another way, let's say you have a standard wood and gypsum wall in your room. That ½-inch thick drywall is huge to 9 kHz and up, but to anything lower, it is much less of an obstacle. When implementing this idea in your room, it might make you think a little more about the necessary thickness of material that one needs, to actually absorb frequencies you target.

But how do we achieve this thickness?

What types of treatments are there?

The first and most common is absorption. There are really three main types of absorbers, and the first is the most common of the most common. These Porous Absorbers are typically applied with a rigid fiberglass board. These are the easiest to find online, and some of the most common are 1-inch and 2-inch panels found in a 2-foot wide by 4-foot section.

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.

All of these, though, have only addressed absorption. Direct reflections (sometimes called slapback, or echo) are harder to handle. To deal with these direct reflections, you can absorb them, but you can also diffuse them. A Diffuser (named by the guy that named the Toaster) will take the direct sound and scatter/diffuse it, so that the direct reflection is not significant.

The most common type of Diffuser is the Quadratic Residue Diffuser (QRD). They are commonly found two ways. They may have long slats at varying heights, or they may be what people often refer to as a skyline diffuser. This will have many wells at varying heights that resemble a city skyline.

Diffusion is an incredibly useful tool when used appropriately. Overabsorption can take the life out of a room, but by properly diffusing the sound in combination with proper absorption, you can remove the problem areas while keeping the room live. However, this nontechnical summary just scratches the surface on this subject.

Nothing is large and nothing is small… it's all relative to waveform. This puts treatment into perspective.

Simply buying something to put on the wall may work, but it may not as well. Not every treatment is going to work in solving the problem. While there are tools like membrane absorbers and helmholtz absorbers that can target specific areas, they may not also be the right areas.

Absorption can be one of the most beneficial or most detrimental choices you make in a room. So do the research, call the experts, who can help significantly in having it done right.


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