How Many Acoustic Panels and "Bass" Traps Do I Need?
RT60? RDT? REW? How “live” should my room be?
Acoustic (absorption) panels control the reverberation time in the room: They determine how clear and full the sound is.*
This impact can be measured, and there are industry standards for what that measurement should look like in an accurate sounding room. Accurate in this case means you are hearing the audio in a room that sounds similar to the room in which it was created.
This post describes: 1) an easy and quick "rule of thumb" method for choosing the number of panels to put in a room, and then 2) a more complicated but objective method for treating a room, using tools that cost about $100.
THE RULE OF THUMB
If you just want a TLDR method that doesn't involve measurement, instead, that's simple and very effective, here you go.
Cover about 15% to 20% of your walls / ceiling with acoustic absorption panels made from fiberglass or rock wool insulation, that are at least 2" thick (ideally 4" thick), evenly spaced around the room — and enjoy. You are done!
Where do I get the panels?
GIK Acoustics is a solid choice: Custom Art Panels & Bass Traps - GIK Acoustics
Acoustic ART panels, Decorative Acoustic Art sound Panels - The Perfect ...
Other reliable companies include people like AcoustiMac, who sell ready to hand panels and also “kits” that are ready to assemble, reducing the cost but requiring some work on your part.
And of course there are very good DIY options for those who are so inclined: 'DIY Custom-Printed Movie Poster Acoustic Panels -...
Where do I put the panels?
Here is a separate post which covers that. Follow those steps and then you are done. Run your room EQ, and enjoy!
The OBJECTIVE APPROACH to Room Treatment
If you want to use measurements and standards to get it "just right," here is some info to get you started. “Right” in this context means measurably, objectively performing in a way similar to the room where the content was created.
This video does a nice job of talking about some of the judgment calls one must make in the real world:
So, let’s talk about what can be objectively measured, which helps us get into the ballpark of accurate sound. What is the measured? RT60 (or RDT for "reflected decay time”) is the usual and primary measure of how “live” (or reverberant) a room is. Note that REW (Room EQ Wizard) can measure T60M which is technically the best measure in a small room. But all these measures are similar enough for our purposes.**
Ok, so how do I measure the RT60 in my room?
FIRST:
You can measure the RT60 (RDT) in your room with a free piece of software (REW) and a hundred dollar calibrated microphone (like the UMIK 1).
How do you measure RT60 with REW? At present the steps are:
Open up an mdat with full range measurements of your L+R
Select a measurement on the left, then go to "RT60 Decay" window
The graphs will (probably) be empty
Hit the "Generate" button in the bottom left of the bottom graph
Hit the "Controls" gear
Hit "Calculate RT60 Model"
Now, go to the regular RT60 window. The measurement you used above will now have an RT60 graph in addition to T60M.
SECOND:
What is a good RT60? In the past people often talked about a single RT60 value for a room. This is a fudge. As you can see from the graph you created in REW, there is an RT60 value for every frequency you measure.
What's the target? Well, reasonable people can disagree. One can argue that the smaller a room, the lower the RT60 target will be. And there is some amount of taste that can come to bear. But if one's room is relatively linear from the transition frequency up to above 10k, and between .2 and .4, you are probably in the realm of "quite good."
Is my room’s RT60 good?
1. Is the RT60 graph for your room consistent from around 125hz (or a little higher) up to 10khz? If so, that is good.
2. Is that line roughly centered on .3 seconds (with .2 acceptable if the room is under 200 square feet, and .4 acceptable if the room is over 1000 square feet)?
If the answer to both those questions is YES then you are doing great. Set aside your measurement gear, stop adding acoustic panels, and enjoy your room.
Want to get even more precise in understanding the goal, how it shifts slightly in different sized rooms, and so on? Read on….
One of many examples of working through this topic can be found here. That's from 20 years ago, and is when a famous designer (Erskine) chimes in, he summarizes the goal with a single number. That’s good, as far as it goes, but the standard has been articulated in more detail, since.
In 2023 a lot of this was summarized and codified by the CTA / CEDIA.
Specific RT60/RDT Targets for Each Octave
Do you want to go further down the rabbit hole? OK!
If you want to try to follow a specific target, something like an industry standard, for critical listening, I find the Dolby specification for mastering suites to be a good goal. The Dolby Atmos Home Entertainment Studio Technical...
It is buried in their Room Design Tool:
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Adjusting RT60/RDT for rooms of different sizes
Further down the rabbit hole? Okay, yes, there is science about how the target should be adjusted a little based on room size, as well. This is derived from human psycho acoustics.
The ITU has a nuanced document about target RT60 numbers for rooms that is based on their size, discussed here which can be downloaded here:
https://www.itu.int/dms_pubrec/itu-r/rec/bs/R-REC-BS.1116-1-199710-S!!PDF-E.pdf
called: "Rec. ITU-R BS.1116-1 1. RECOMMENDATION ITU-R BS.1116-1*. METHODS FOR THE SUBJECTIVE ASSESSMENT OF SMALL IMPAIRMENTS IN AUDIO SYSTEMS INCLUDING MULTICHANNEL SOUND SYSTEMS"
which gets into how room size can make the targets a little different, to achieve the desired effect.
Decades of testing different size rooms and different amounts of decay with many many people, who were then asked what rooms sounded best, resulted in such consistent preferences that there is an equation to get it "right" or best. The ITU paper listed above is highly technical.
In 2023 the CTA / CEDIA published "just the targets" based on this room sizing approach:
Grimani talks about this (and other topics) in detail in this video from the AVS YouTube channel.
AVS Forum Tech Talk with Scott Wilkinson Episode 17: Anthony Grimani Acoustic Treatments VS. EQ
He is a trainer for CEDIA and other industry bodies, and if you want to learn more about the guidelines, watching some of his other videos is useful, as well. Here is an example of where some of those discussions go. (Note that the formula he discusses here is the same one that ends up in the CEDIA / CTA guidelines quoted above.)
And while we are watching YouTube, it's worth noting that Matt Poes makes some interesting points in his video, below. Among other things, he notes that the lower the RT60 time, the more you hear issues with your speakers, in the source material, and your setup, so ultimately measurements aren't the goal, but just a way to get in the ballpark. Then he does what most eperienced professionals do: He listens critically to a wide range of material, draws on years of experience in a multitude of state of the art -- and real world comprised -- rooms, and adjusts.
Ok, now you know whether you need more or fewer panels, but where should you place the acoustic panels? Here is a post about that.
Footnotes:
*They can also have an impact on some aspects of frequency response in some specific use cases, particularly in the bass, but that's a topic for another day. They are not the primary tool for that job. And they cannot meaningfully impact “soundproofing,” at all. There are other ways to improve sound isolation, described here.
**Note that I am using the term "RT60" as a catch all. Purists and pedants rightly call it RDT -- reflection decay time -- since RT60 makes more sense as a concept in a concert hall and not in a home theater. But the measurement and evaluation process is largely the same for what we are doing here.