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How to Buy the Right Acoustic Panels: Part 1
by ct_acoustic_frontierJuly 06, 2022
How do you buy the right acoustic panels?
This blog article is part of a mini-series that should give you a good overview on how to buy the right acoustic panels for your home theater, home recording studio or listening room. I say partially because acoustics is a very complex topic, and as you read you'll start to understand the nuances and challenges of correctly implementing acoustic treatment.
What are your room issues?
Nearly everyone starts off with no knowledge of the importance of acoustics to final sound quality. Somehow they learn about it, whether from reading magazines or getting advice from their dealer. They learn that the room introduces a number of acoustic distortions which degrade sound quality. Yes, the sound quality you get is NOT just a result of what equipment you have. It is heavily influenced by the room. Some people get lucky and have a neutral-ish* room without any major acoustical distortions. Most, however, have some severe issues, particularly in the bass where the influence of room modes causes huge peaks and dips in frequency response leading to boomy bass and suckouts at other frequencies.
* no untreated room is truly neutral, and even heavily treated rooms often have areas where performance deviates from perfect.
[caption id="attachment_2569" align="aligncenter" width="700"] Peaks and dips due to room modes[/caption]
This next part is very important, so pay attention. The ONLY way to know what your issues are is to measure your room using some kind of acoustic measurement tool like Room EQ Wizard, Fuzzmeasure or XTZ Room Analyzer. Without measuring you have no idea what issues your room has and therefore where you should focus your energy to get maximum sound quality improvements for minimum dollar investment. It's like shooting in the dark - "did I hit the target?" and the answer is typically "No, you missed it completely". Far too many prospective clients call me saying that they have spent thousands of dollars on acoustic treatment and still have major sonic issues. Most have not measured their rooms...don't be one of those people! For more information, read this article I co-authored with Ethan Winer of RealTraps: Everything you need to measure your room. And this one: Four approaches to room acoustic measurement.
[caption id="attachment_2568" align="aligncenter" width="700"] Room EQ Wizard Spectrogram[/caption]
So, this means you cannot really fix your room's acoustic issues by going on a forum, describing your room through a couple of photos and a sketch and asking for answers. Nor can you get it through the 'free consulting services' that many acoustic treatment manufacturers offer in the hope that you will buy their products. The only way you can truly know what your issues are is to take in room acoustic measurements of things like frequency response and how sound decays at different frequencies. From there you compare the measurements to best practices, understand the root cause of your room's acoustical issues and figure out what changes are required to improve things. If your room is predictable (non-open plan, new build construction) and you use good modeling tools you can get 90% of the way there with computer simulations.
[caption id="attachment_2570" align="aligncenter" width="700"] Boundary Element Model study of frequency response across two rows in dedicated theater with six subwoofers[/caption]
It's a bit like a doctor when they are trying to figure out what is wrong with a patient. They will use all their senses as well as medical tests to determine the illness and from there they will devise a plan to cure the patient. They do not just (we hope) semi-randomly start prescribing drugs, trying one here and another there, in an attempt to fix the issue. Like a doctor, you should measure your room, figure out what the issues are and from there devise an acoustic treatment plan*.
* note that there are many ways to fix acoustical issues. Panels and bass traps are only one of them. You can also use, depending on the issue, room EQ, subwoofers, positional changes amongst other things.
What different kinds of acoustic panels are there?
The acoustical treatment market is confusing for the novice. There are a lot of manufacturers and each one has a lot of different products. It gets even more confusing when you start to realize that many products are hybrid products that combine different acoustical characteristics in one panel.
At a very high level there are only two different kinds of panels:
Absorbers are simple to understand. An absorber absorbs sound. There are really only two types of absorbers (although many combine both functions in one panel):
Porous (fiberglass or foam panels, curtains, carpet, etc). Sound moving though the absorber is converted to heat and sound energy is reduced.
Resonant (Helmholtz, membranes, microperforated, amongst others). Sound excites the mass of the absorber which resonates at some frequencies. The resonance is damped by porous absorption or through what are called viscous losses (think of this as air molecules moving against a part of the panel and in doing so losing energy).
Diffusers are a little bit more challenging to get your head around. They scatter sound. The video below does a good job of explaining the concept.
There are many types of diffusers. In fact anything that has a non flat structure has diffusion. Many architectural shapes such as columns, pyramids and the like are good diffusers. Now we have QRDs and many other products, typically called 'engineered' diffusers.
[caption id="attachment_2574" align="aligncenter" width="600"] RPG Harmonix K[/caption]
When picking acoustical panels it is very important to understand how absorption or diffusion varies at different frequencies. A 1" fiberglass panel, for example, will absorb well above about 1kHz and then decrease in effectiveness below that. Part 2 of this article will give you some tips on how to match acoustical panels to room issues. It gets complicated VERY quickly. However for now let's make a simple rule that if you use acoustic treatment then it should be 100% effective down to your room's transition frequency. For now let's use 250Hz. It's actually very hard to hit that rule, especially with diffusers, which have to be very deep to work all the way down to 250Hz. But if you aim for that you are on the right track.
[caption id="attachment_2572" align="aligncenter" width="600"] Absorption coefficient of Primacoustic Broadway panels in 1" (green), 2" (orange) and 3" (blue) thicknesses[/caption]
So now we have covered absorbers and diffusers. But like we said earlier there are a lot of hybrid products out there that have some absorptive properties and some diffusive properties. They are absorptive at some frequencies and diffusive at others, often with a transition region where they are both absorptive and diffusive. Confusing I know, but hybrid products are a very useful type of acoustical panel.
[caption id="attachment_2573" align="aligncenter" width="400"] RPG BAD panel, a classic hybrid absorber / diffuser. The transition from absorption to diffusion is managed by the drilled wooden template visible top right (the fabric that normally covers it has been cut away for this photo).[/caption]
In part 2 we'll go more in depth on how to match treatment to room acoustic issues. If you have any comments, please leave them below!
Much confusion still exists about what a room correction product does, what problems it can (and cannot) solve and therefore its 'place' in a modern high quality sound reproduction system. Part of the challenge of understanding room correction is that it requires a reasonable level of understanding of sound quality, acoustic science, acoustic measurement and psychoacoustics (how humans perceive sound). The majority of the articles I have read online or in print magazines do not cover the fundamentals in enough depth to allow the curious and committed reader a chance to understand room correction on anything more than a cursory level. By the end of this article I hope that you will have learnt enough to judge for yourself what room correction can and cannot do and how best to apply it in the context of a world class music or home theater system.