As you might have read at www.sensibleaudio.dk my main speakers are placed very close to the side walls.
This is not at all an ideal placement, as this will cause a smearing of upper mid frequencies and treble. In the time domain you’ll see this by looking at the impulse response, thet there’ll be a delayed pulse shortly after the main pulse. In the frequency domain you’ll se a very ragged frequency response.
Sound wise, you’ll experience a more harsh sound, and reduced sound stage.
The best solution would of course be to move the speakers away from the walls. But this might not be possible, which is the case in my room, and therefore you’ll have to do something else.
There are basically two ways to go. Two very different ways, but both usable.
One is Diffusion, the other is absorption.
Though I’m not that fond of applying absorption, as it takes life out of the music, I actually tried to put of som Rockwool bats on the side walls where the reflections are coming from (use a mirror to determine this). The result was not good at all. It did take the reflection, and the sound became more relaxed, but the stereo image was also lost. I didn’t try to use something less masive that the 10 cm bats I first tried, so I might have found a better compromise!
Diffusion is in my mind a lot better. I doesn’t thake out energy og the sound. Instead of converting the energy of the sound wave into heat, the sond waves are skattered , and a smaller part of the will be reflected towards the listening position. The skattered waves will be reflected by other surfaces before it reaches the listener , and will actually help in creating a fealing of room.
There are a number of different diffusor types. One of the best and best known are the Quadratic-Residue Diffusors (se more here). Problem is they are a bit hard to make, especially if they are to be effecient ove a larefe frequency band, and the looks are also not their strongest side.
I have decided to go for something a bit different, where I actually reflect the sound wave in a controlled manner, avay from the listening position.
Take a look at the reflections here.First the reflections without treatment:
Reflections without diffusor Here is a draving of the reflections with the diffusor in place:Reflections from left side wall with diffusor
Reflections from left side wall with diffusor
Now, at least for mid to high frequencies the sound fom the side wall is reflevted away from the listening position and back towards the rear wall, and the poly diffusor placed there.
Building plan for the diffusor
The base is made out of 60 by 60 cm, 10 mm chipboard. Onto this is placed some ribs of 22 mm chip board.
Large venting holes are made under the ribs. This allows for a good venting of the back side whan you are allpying the epoxy.
I actually made some early prototypes, where no venting hles where made. This caused the epoxy to not cure on the back side, and they kept smelling for months … so venting of the backside is essential.
These holes will also allow for the inside to be stuffed with mineral woll, insulation fiber glass or polyester batting. This will dampen the the inside.
Mounting the fleece
After making the frame, it’s time to mount the fleece cloth. I just bought a very cheap fleece blanket (25 DKK ~ 5 USD). It is more flexible in one direction, than the other.
Lay it out carefully so that the flexibility is in the width direction of the frame, so you can stretch it dovn into the groves between the ribs. Use a stabler to fasten the cloth in the groves. Stretch well to streighten out all wrinkles.
Use a stapler to mount the fleece in the groves
When you have mounted the cloth in all groves it’s time to turn the frame over and mount the cloth on the back side.
Mounting the cloth to the back side
Again make sure to stretch enough. Use plenty of staples.
Cut all excessive cloth and finish off with some Gaffa tape.
Finish off on the back with gaffa tape
Epoxy / Polyester
Now it’s time for epoxy. I actually used polyester, as it’s a bit cheaper. For this purpose it dosent matter. I gave the cloth two layers (don’t be shy here), and used between 1,5 and 2 liters. Use plenty of hardener, to make it cure faster.
Two layers of polyester or epoxy is applied
Sanding and paint
The surface will end up quite rough, so it needs some sanding. You could probably use some polyester filler to even the surface, but I just sanded the surface, and ended up with something that looks a bid like a rough concrete surface. I used selant in the groves to hide the staples.
Surface sanded and groves filled with white selant
Next step is to apply some paint. First a layer of base coat, then two layers of normal wall paint in the color of your choosing.
A layer of base coat applied
I chose to give the diffusors the same color as the walls, to help it blend in. You could have choosen a darker color to make it stand out a bit more. Think that would also have looked great.
Hollows filled with polyester batting
Finally the difusers can be tried out. I put on some wooden laths so I could just put the difusers onto the walls, without mounting them. This way I could easily set them up and take them down to hear the difference with diffenent music.
Two screws holds the difuser in place on the walls. I think they turned out quite well. It was not that much work, they look quite good, and the sound improvement is quite big.
Final diffusor mounted on the wall just in front f the main speakers
I first listned to music, using the temporary stands to make it easy to put them up and down. From the very first note, thare was no doudt that it was a big improvement.
The sound stage was wider and deeper. The room simply feels larger. Tha sound seems much more detailed, and it’s easier to pick out different instruments or place the singer on the stage. Also the aggressiveness in the upper mid to treble is much less pronounced. The sound seems more carm, and you can play louder before it starts to get tireing.
Well, after the first listening sessions, with a feeling that this is just better, it was interesting to make some measurements.
First take a look at the impulse response for the left side (the result is the same for the right side). Without the diffusors, it’s easy to see the first reflection only 1 mS after the main pulse. It’s actually quite large, and it’s quite obvious that it will smear the sound.
With the diffusor in place, the reflection is simply gone. The beauty of this solution, compared to absorption, is that the energy and sound of the reflection is not gone. It is still there. It’s bounced back at the backwall and the big polydiffusor there, and the sound will arive later to add to the feeling of space.
Left Side Impulse without diffusor
Left Side Impulse with diffusor
Also the frequency response is improved, but with the used measurement software Arta, it’s maybe a bit harder to spot. Anyway the level in the upper mid to low treble is now lower, (thats on of the reasons it sounds less agressive), and the response is more smooth.
Left Side frequency response without diffusor
Left Side frequency response with diffusor
Was it worth it?
Yes without doubt. Maybe it’s not the most pretty solution, but I can hardly think of something more efficient, and without the downside of absorption.
How big is the improvement?
It’s in the same order as going from a good speaker to a very good speaker. It’s just so obvious that there is an improvement. This is nothing like fiddleing with two types of capacitors, different op-amps in your amp, cables or anything else. This is quite measurable, the sound is just so much better.
So the conclusion: Go do it! You will not regret it 🙂