How To Acoustic Room Treatment Fun & Interesting

I like listening to music – really listening to music. Sit back, clear my mind of all of the various thoughts that are tumbling through it, free myself of all the usual distractions and just listen. And it’s always been a dream of mine to have a place to do that – a “listening” room where I can escape reality and truly relax.

The room in my basement isn’t ideal. It’s 14′ x 14′ and the ceiling is just 7′ high. Square rooms are particularly bad for room modes (frequencies where the sound is amplified) because there are two at the same frequency: front to back and side to side. This gives the room that “boomy” sound when those frequencies are excited.
One advantage that the room I’m working with has is that it’s inside a bigger room – the entire basement – so the walls that divide the space are not much of a barrier for the low frequency energy that’s the biggest problem in a smaller room. The room is surrounded on three sides by utility areas where it doesn’t matter if sound escapes to.

But that advantage is just a short head start on the marathon race that is room treatment. There’s still a lot left to do and it can be very expensive. I should point out that what I’m doing isn’t “soundproofing”. That’s an entirely different thing. Acoustic treatment addresses the way the sound interacts with the room, reducing those room modes I mentioned above and dealing with reflections. We’ve all been inside a room that echoes – room treatment cancels that in a very systematic way.

There are basically two methods to reduce problems with excess sound energy in a room: velocity and pressure. Velocity slows the sound by passing it through an absorber material, like foam. This is effective for a broad range of frequencies depending on the thickness. And while it does work to reduce some low frequency energy, it’s more effective for the mid to high frequency sound. Most of the commercially available acoustic panels are this type – velocity traps.

Pressure traps react to the sound energy by vibrating – moving in response to the sound energy, much like you can feel the floor vibrating when loud, bass-heavy dance music is playing. Since it takes energy to move the diaphragm of the pressure trap, some of the energy is consumed and therefore reduced. This type of treatment isn’t easy to build, though, because it has to be located where the problems exist in the room and it has to be “tuned” to the problem frequency. For example, my room has two very strong low frequency modes – 37Hz and 40Hz – and it would take a lot of velocity absorbers to reduce the strength of those modes to the point where they are less of a problem. However, targeted pressure traps if placed correctly can significantly reduce those modes because they are designed to react to that specific and very narrow frequency range.

That is “kind of” what I did. I say kind of, because I really couldn’t say for certain how well it would work when I started. But I didn’t have anything to lose by trying, either.
Before I started working on the room, I had a definite vision of how I wanted it to look. And the look is plywood wall panels in a semi-industrial inside-a-truck-trailer exposed fastener type style that probably won’t appeal to the average person. The average person that would want to make the room look like a movie theater, that is.
So since the walls would be done in this panel style, I figured I could make it so that the panels could flex and in flexing would absorb some of the low frequency energy in the room. That was the theory.

So I finished the first wall with the panels. Unfortunately, I didn’t get a room measurement before the panels to compare it. That would have told me immediately whether it had an effect and how strong the effect is. Still kicking myself over that one…

The wall has the biggest panel right in the middle and the panels themselves have a damping material on the back. I used self adhesive ice and water shield that’s used for roofing. There are products made specifically for this, but are basically the same thing and cost a lot more. The damping also adds mass to the panel and that lowers its resonant frequency. That resonant frequency is where the panel will vibrate the most when excited.

In an ideal world I’d have the engineering background to calculate that frequency based on the panel thickness and the weight of the added mass. Instead I did what I usually do: guessed with my fingers crossed and hoped for the best. I did set up a way to measure the panel vibrations, but the results were difficult to interpret and somewhat unreliable. Instead I figured I’d measure the before and after when I do the opposite side of the room. That wall will be treated exactly the same way.

Here’s the test setup. I used a pair of smaller speakers on top of a pair of bigger speaker to lift them up to the normal height and placed them where the real speakers will be in the finished room. By “real” I mean the ones that I’ll make especially to go in this room. I’m using a program called Room Equalization Wizard (REW) and a calibrated test microphone to do ten measurements throughout the room:

I marked an “X” in each location to be able to put the mic back in the same place when I run another series of measurements. I made measurements after each change I made in the room to track the progress.

The next wall is the front of the room. This started as a wall made from 2×6, so 5-1/2″ deep. I increased the depth on the sides and across the bottom to create bigger bass traps and put a layer of 6″ rockwool inside the two vertical ones in the corners:

I then filled the rest with regular pink fiberglass:

And made holey diffuser panels to cover the bass traps. I made a video showing how I did that the hard way, without a CNC:

The inside of the diffuser panels were covered with a tightly woven cloth to keep the fiberglass from escaping:

And two days of drilling and thousands of holes later…

The diffusers allow the low frequencies to go right through, but reflect the higher frequencies back into the room. The idea is to make the room sound more “live”. If you have too much absorption it can kill the high end and make the room sound dead.

Next I made the quadratic diffusers. These disperse the reflected sound and can make the room seem larger than it actually is:

The cloth covered box on top is another bass trap, but deeper still. Of all the walls in the room I put the most effort into the front. I figured it’s the one I’ll be looking at, so it deserves the most attention.
But it’s also function. This wall will have the TV screen and had to be built to accommodate that. The deep bass traps also had a major impact of the room response, reducing reverb time in the low end significantly.

The next major step was the ceiling. It was open joist before and I filled those with fiberglass and then covered that with more black cloth and plywood strips:

The strips are 2″, 4″ and 6″ wide with 1″ gaps between. I should mention that this really isn’t the “scientific” way to do this type of diffusion. The slats are wider than idea and the placement doesn’t follow a specific pattern. I see this more as a way to add some diffusion, but primarily provide enough reflective surface to restore the high end frequency response – that “liveness”.

Next, the wall opposite to the first. I filled it with fiberglass to add more stuff in the path of the sound. Even though this insulation will be behind the plywood, some bass energy will go through that and through the fiberglass, slowing it down as it passes:

And this wall has a door that leads into my electronics room (or what eventually will be my electronics room… it’s just an overfilled closet right now). I wanted to make the door so that it looks like the wall panels and blended in. Also I don’t want an area on the wall where there would be an odd reflection – sound loves symmetry!

I made a quick and dirty frame from recycled lumber that is the right size for the opening:

And then put two of the panels on before hanging it. This was a fit and file operation to get this lined up in a house where level, plumb and square are unknown qualities.

To look invisible the fit had to be good and that involved a fair bit of hand plane work to scribe the panels tight to the corner:

Took most of a full day to build the door and get it fitting properly, but I think the results are worth it. The door disappears in the finished wall.

The rest of the wall was done exactly like the first wall, with panels that have patches of ice and water shield on the back. The biggest one:

The panels are fastened to plywood furring strips that are screwed to the wall studs. This frame leaves the middle of the panel open to flex and hopefully absorb some of the excess low frequency energy:

Then I finished the rest of the panels in the same way, fitting each one individually and spraying on two coats of water based polyurethane. All of the panels are screwed on so they can be removed easily if I need to change something. In fact the whole room is done this way:


A view from the back wall looking forward:

The floor isn’t concrete – it’s OSB that has been painted that grey colour. I’ll cover this with thin carpet tiles for the time being, but eventually change that to hardwood with an area rug, if needed. In general terms, the floor in a room like this is left untreated, mainly because it’s very expensive to do anything effective with it. Plus any treatment that does work will eat into the headroom, and this ceiling is already low.

The room isn’t finished, yet, but let’s take a look at the progress as far as measurements go. Like I said, I’ve been taking measurements after I finish a section and tracking the improvements. It’s interesting to see these acoustic changes as they happen, and what makes the biggest jump in performance.

Here’s the first room measurement taken from the approximate listening position:

This is what’s known as a waterfall plot and it shows how quickly the sound dies out over the entire frequency range. The numbers marching across the sides are the time – 0 to 800 milliseconds. It takes around half a second for the 37 and 40Hz frequencies to die down by 30 db. That means they are bouncing around the room longer than that to fully dissipate.
But, this isn’t bad to start with. Now, like I said, the room has a head start being inside a larger room, and this was taken after the first wall was finished. And I’ve learned since then that the panels on that wall are actually effective at absorbing some of the low frequency energy. I used an accelerometer to measure the panel vibrations and that plus the room measurements tell me that the panels are working.

In this video I make the preamp for the accelerometer and use it to test the panels for the first time:

I have all the data from the 12 separate measurements I took that show the gradual improvement over the course of treating the room, but let’s skip ahead to the latest measurement. I made this after the third wall was finished:

I think it’s easy to see the change, especially in the low end. And it’s also extremely easy to hear the difference in the room.

Another way to look at the data is with the spectrogram. This is like looking at the waterfall plot from above, so you can see exactly how far out the reverb extends. This is the first measurement again but is limited to the first 1000 Hz of the test signal to show the low frequency problem areas in more detail.

Easy to see the 37 and 40Hz modes that are the width and length of the room. And then the harmonics of those at 80Hz and 12oHz. These are hanging around for a long time and will cause music to sound muddy in the bass region.

And here’s that same plot for the last measurement:

This shows a major reduction is reverb time in the low end and smoother response across the frequency range. This last measurement does show an increase in reverb time in the higher frequencies, but that’s because the two side walls are now hard reflective surfaces that will need some velocity absorbers to reduce the reflections. That comes after the room is finished and I can do the final tweaks.

In this video I show the room as it is now (July 2021) and give a brief update on the progress that was made:

I’ll check back in with updates to this as they happen and it’s my goal to have this room finished before winter sets in. Could be just the place to hide out away from nasty weather!