DIY Sound-absorber panels

A while ago I built new crossovers for my floorstanders and while performing the measurements I noticed some nasty behaviour between 100-200Hz.

Frequency response with the old crossovers, 1/12 smoothing

I was suggested that it was probably because of something called SBIR (Speaker Boundary Interference Response). In short that means that the lower frequencies start wrapping around the speaker hitting the rear wall and then being reflected with a 180 degree phase shift and then interfering with the signals coming directly from the driver. The linked article from GIK Acoustics mentions treating the wall behind the speaker to try to absorb the sound that wraps around the speaker so you get less reflected sound coming back to interfere with the orginal signals from the speaker.

I had a look at buying absorber panels from different companies but they were all so expensive so I figured I could probably do something myself for much cheaper (and it's fun to know you did it yourself). I looked into what is important when chosing the right material and what thickness I would need. Of course, the thicker the better, but there's only so much space between the speakers and the wall and since this is not a dedicated listening room but a living room I do need to think about aesthectics too. For low frequency absorption and thin panels (less than ~100mm) a dense material with high flow resistivity is prefered. I had decided to go with a 70mm panel since it would still look OK and it's easy to find panels and wood for the frames in 70mm thickness.

Flow resistivity is not something that a lot of manufacturers list in the spec sheet for their products. I had a look at different brands of rockwool insulation and couldn't find anything helpful. After some further googling I found this thread with some helpful data on flow resistivity for insulation products here in Scandinavia. Regular fluffy rockwool insulation seems to have values of around 10 kPa*s/m2 while the denser ground insulation slabs seemed to have values of around 40 kPa*s/m2 and up.

There's a great porous absorber calculator here at Acousticmodelling which turned out to be super helpful. I compared two absorbers, both 70mm thick and with 20mm to the back wall. 20mm is the thickness of the baseboard in my living room.

Model of two different absorber panels with different flow resistivity.

The green line is with the denser material and the blue is with the fluffier material. If my goal was to treat reflections anywhere in front of the loudspeakers I would have gone with the fluffier material since it is way better from around 250Hz and up. However, now I'm trying to treat reflections behind the speakers and for that I need good absorption in lower frequencies because those are the ones that wrap around the speaker and my main problem area seems to be between 100-200hz and in that range the green line (dense material) wins.

I ended up buying a pack of three Rockwool 70mm thick 600x1200mm ground insulation slabs. Then for the frames I bought two 4.2m long 28x70mm bits of wood and cut four 1.2m pieces and four 0.656m pieces. I hadn't decided on what fabric to wrap them in yet and since I wasn't too confident about my skills in wrapping frames with fabric I bought the cheapest stuff I could find at Ikea. It was a non-bleached cotton fabric and I started with buying just enough for wrapping one frame once.

One frame being wrapped with fabric.

Result of first attempt at wrapping a frame. Not pretty but the best I could do.

Wrapping the frame wasn't as difficult as I thought it would be. No problems at all getted the fabric stretched evenly. However, the corners had me confused and I also realised that I should probably get the back side stapled BEFORE wrapping the sides over. I ended up going with this triangular deal for the corners. I had a feeling that there is some other nicer way to do it but at the time I couldn't figure it out. Turns out that when I did the very last corner on the second round on the second absorber I finally figured it out. But then it was too late so I did the triangle anyways. Learn by doing! By this stage I had realised that the dirt cheap fabric from Ikea matches the colour of the wallpaper in my living room really well so I went back to the store to buy another three pieces of fabric so I could wrap the first panel with one more layer and then do two layers on the other panel as well.

Left panel with one layer of fabric, right panel with two layers.

Both panels finished.

Both panels finished.

The panels blend in nicely with the wall and don't attract too much attention. I had first planned to go with a coloured or more textured fabric but I'm glad I did it like this. As I mentioned previously my intention was to place these all the way against the back wall with only a 20mm gap for the baseboard down by the floor. Turns out this doesn't work because behind the right speaker there's a power outlet that stick out quite a bit. I ended up having to move the TV-bench and the speakers forward a bit to make enough room. Now the panels ended up 70mm from the wall and this changes the efficiency of them a bit.

Blue line is with 70mm air gap, green with 20mm air gap.

Not a big difference but it does give slightly better absorption below 200Hz. Good for me, because that's exactly what I needed. However, if we compare this new result with the fluffier material from before we get a different picture.

Blue line is the dense panel with 70mm air gap, green is fluffy with 70mm air gap.

Now my dense panel gets beaten by the fluffy panel from around 150Hz and the fluffy panel is way better from 150Hz up to 2-3kHz. This shows that you really need to measure and model BEFORE buying your material. Depending on your situation you might need something different than what you first thought.

Also, something that I didn't consider beforehand but I realised later is that a fluffy panel may sag a little from gravity over time. I'm not sure if it will, but the denser ground insulation panels are incredibly rigid and shouldn't deform over time. That could be something worth thinking about too. Weight is also something to consider if you want to mount the panel on the wall. Ground insulation slabs are HEAVY! My pack of three slabs weighed in at over 20kg. A fluffier material would way much less and be much more wall-friendly. I will end this post with some measurements with/without absorber panels and in the very end I will make a parts list and write the total cost for everything.

Left channel measured at ~1m on-axis with and without absorber panel. Very small difference in frequency response. 1/12 smoothing

Left channel RT60 with and without absorber panel. The biggest difference is between 150-450Hz, as
can be expected since the sound starts to wrap around the speaker at ~500Hz according to the GIK Acoustics article. 

Frequency response at sweetspot with both channels. Red is with absorber, blue is without.

RT60 at sweetspot with both channels. Not as big of a difference here but there is at least a slight difference.

Parts list:

• 1 pack of 3 70mm 600x1200mm Rockwool ground insulation slabs: 519SEK (~€50/$58)
• 2 pieces of 4.2m 28x70mm wood: 66SEK (~€6.50/$7.40)
• 4 pieces of 1.5m "Bomull" fabric from Ikea: 115SEK (~€11.30/$13)
• 1 pack of staples: 59SEK (~€5.80/$6.60)
• 16 wood screws: 0SEK (had those already)

Total cost: 756SEK (~€74/$85) and then I have one slab left so I could build a third absorber for only ~90SEK (~€8.80/$10). So price per unit turned out to be 378SEK (~€37/$42.50) when building two or 282SEK (~€28/$31.50) if building a third one.

Tools required:

• A saw to make accurate 90-degree cuts (or 45-degree cuts if you feel ambitious).
• A drill to pre-drill the holes for the screws. Makes assembly a lot easier and with less risk of cracking the wood.
• Screwdriver
• Staple gun
• Scissors for cutting the fabric.
• Earplugs (recommended because staple guns can be quite loud).

Time required: It depends on how good tools you have, how good you are at using these tools, and how skilled you are in general. I had a terrible saw and did all the drilling and assembly in the small kitchen in my apartment and wrapped the frames on the largest open space available in my apartment. I probably spent 2-3hrs on each panel. If you are more skilled than me (which isn't very unlikely) and have a better workspace you could probably finish a panel in 30-45 minutes.