DIY Acoustic Mixing Room Treatment
Recently, I decided to build acoustic treatment for my home studio, where I produce and listen to music. The entire endeavor came with a learning curve that helped to discover unknown unknowns and turn these into proper learnings. Let me tell you my story, how it went, and about the outcome.
Knowing next to nothing about physics of acoustics and hearing everyone talking about „put foam on your walls, and you be good” I thought myself:
- Acoustic room treatment is crazy-expensive.
- Let’s build it myself. How hard can it be?
I quickly recognized my fallacy.
Learning 1: There is a ton of content out there on the internet from people that talk about stuff they do not understand. It’s too easy to fall for it.
Learning 2: There is content to some degree that explains how to build proper acoustic treatment.
Learning 3 (the most important): There is some content that talks about acoustic mechanics, the effect of room to sound, and how all this interacts.
Several videos on YouTube talk about porous acoustic treatment by using Rockwool. That was also my starting point. I quickly realized that Rockwool isn’t the most healthy material to work with so I went for an alternative, which is sheep wool. I wanted to build six absorber panels and two bass traps. Until here I learned that:
Learning 4: Putting a speaker into a room causes sound to resonate with itself, and most of resonance pressure occurs in corners.
Taming bass frequencies in the corners seemed a safe place to begin with. So I got myself some supplies:
- Wooden boards
- Sheep wool
- Acoustic foam panels
- Fabric (so the filling does not fall out on either side of the frame)
- Aviary wire
- Wooden circles
- Stapler
And then, construction work started.
First, cut all the boards into pieces that form a rectangle, then screw them together and cover the back with fabric cloth.
Next goes the wool material, which is intended for mid and mid-high frequencies. The wool gets glued with spray adhesive to the rear fabric and the sides.
Once that is done, then a tiny layer of foam comes on top to tame higher frequencies. The last step for a panel is covering the entire panel with decorative fabric so that they look nice once they get mounted on the walls/ceiling.
During the construction phase, I kept educating myself on acoustic treatment and why this is even necessary. There is an excellent presentation by Anthony Grimany on optimizing listening rooms. Many things go on when sound is played in a room, such as resonances that amplify or attenuate specific frequencies or the fact that most of the sound we hear is through reflection and only little is direct sound.
Room geometry has a direct impact on reflections and resonances. Each room has a fundamental resonance frequency in the range between 20 Hz and 200 Hz. Those frequencies and their harmonics are what cause dips and spikes in volumes when moving around a room. Their effect can be up to 20 dB SPL in either direction.
If we compare that effect to say cables or frequency response of a speaker, that are typically in the range of +/- 3dB SPL, addressing room modes has the highest impact and requires the most attention. The biggest impact can be made by changing room geometry. As that is rarely possible unless you’re about to build a dedicated room, finding the ideal place for speakers and the listening position has the second-highest impact on sound reception. Taking care of the fundamental resonance frequency has by far the highest impact on the sound. It also affects the impact of harmonic resonances as the fundamental frequency repeats in the higher frequency spectrum.
A way to deal with low-frequency buildup is installing bass traps in each corner. Bass frequencies behave differently than higher frequencies. Bass frequencies manifest as pressure that flows along room boundaries. Higher frequencies behave like rays of light so they can be reflected from walls like light from mirrors. For bass frequencies, we need heavy material to absorb it by turning pressure energy into heat. That’s more difficult than it sounds like. Absorbers should have roughly a thickness of 25% of the wavelength of the frequency to absorb. 30Hz has in Air a wavelength of approximately 11 meters. That is the reason why bass absorption is difficult. Often we see various types of absorbers (diaphragmatic, membrane, and helmholtz) to tame bass frequencies.
Back to my bass traps. I decided to go for room-tall absorbers using aviary wire to build the body with multiple sections.
The wire construction is almost acoustic transparent, and little sound gets reflected. The inner part of the cylinder is filled with sheep wool that does the most absorption. The outside got wrapped into fabric for aesthetics.
I wasn’t able to find measurements for the sound absorption coefficient. Judging from Rockwool, the coefficient ranges from 0.2 at 125Hz up to 1.0 at roundabout 500 Hz. Right now, that doesn’t help me as I’m not able to apply these numbers.
Simply by putting both of these into corners, the subjective impression was already pretty convincing. The placement of the remaining absorbers is a bit more predictable, even with little knowledge. Since sound reflects, a hack is to sit at the listening position and have someone else holding a (small) mirror on the wall until you can see your speaker. That primary point of reflection marks the ideal spot for an absorber. The same goes for the ceiling as the ceiling is a reflective area, too.
Learning 5: Absorber panel effectivity grows with their thickness. Leaving an air gap between the absorber and the wall increases effectivity without further cost or construction. However, there are various ways to construct panels.
At this point I didn’t even mention diffusers. They help to scatter sound to break up linear reflection paths. The net effect of installing absorbers and bass traps.
Now comes the fun part. I made a few measurements: Sound recordings before/after and measurements. The measurement software is free for download at: https://www.roomeqwizard.com/
Untreated
Treated
Combined
The graphs show the frequency response in the room. That is, how much volume can be perceived for a given frequency played by the speakers. The ideal frequency response is a flat line. The green line shows the response before treatment, the red one after. There is some difference between both and what we can see is that the differences are peaks and dips are less extreme in the red one (treated) than the green (untreated).
This graph is pretty hard to interpret as it shows the cumulative effect. Another way to view the effect of room treatment is measuring reverberation time (RT60). It’s the time of an impulse to get down to 1/1000th of its original sound pressure — a reduction by 60 dB.
The ideal time for mixing rooms lies at about 0.3 seconds. The following graph shows RT60 measurement across frequencies. In the untreated room, 60 Hz and around 120 Hz have problematic peaks. The reverberation time improves significantly beginning from 500 Hz upward.
Now let’s have a look at a measurement after treatment. Frequencies in the lows (40 up to 100 Hz) are a bit problematic. In any case, that’s already a huge improvement. From 250 Hz on, RT60 gets into the ballpark.
Dips and peaks hint here again at the remaining room modes. These are rooted in the fundamental frequency at roughly 32 Hz and its harmonics multiples of 32 Hz). Some frequencies are amplified or attenuated.
Here are also two audio samples that give you an impression of the treatment impact:
Acoustic treatment is a pretty extensive topic. By no means, I have grasped sufficient knowledge to continue beyond this point properly. I learned a lot and this post summarizes my learnings.
Learning 6: Even when I realized that I didn’t know anything and just built something, I learned that it was worth it. The room isn’t perfect yet, but given the budget, it’s an okay solution.
At my listening position, the sound improved by orders of magnitude. There are still modes that create a bit of trouble but those ones are addressed by Sonarworks Reference that takes room frequency response measurements and uses EQ to flatten the response curve. So one of the next steps will likely result in building rear wall diffusers and diaphragm absorbers.
By far the most theoretical knowledge is available from AcousticFields YouTube channel. The majority of videos are enjoyable, others tend to be somewhat strange. In any case, a great resource.
