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« The Soundroom Project Part 6. The Floor and Ceiling | Main | The Soundroom Project Part 4. Subwoofers, Rear Channels, and Other Things »
Jun 28 2013

The Soundroom Project Part 5. Absorption Vs. Diffusion.

My schedule and the arrival of the sound treatment tiles finally coincided so I was able to put the finishing touches in the sound room yesterday.

Foreground: Auralex DTS Sound Diffusion Tiles (before painting) A bass trap is visible in the background to the right of the R900 and T301 speakers. I’m not a big proponent of over-treating a room until all the life is sucked out of it (but you still can’t have too many bass traps!), so I worked one reflection point at a time–listening to the results–until I was happy with the result.

I’m lucky to be working in a big room with only four chairs in it (and of course the stereo and home theater systems), and I had left notes on the walls (and ceiling) when I took my measurements, so I was able to get started installing the treatments straight away. I’d definitely recommend putting all of the furnishings in the room before treating it, as soft chairs and hard tables will have a drastic affect on the reflections in a room. It’s easy to get excited about treating your room as soon as possible but there is nothing more frustrating than getting an empty room to sound really good and then putting all the stuff back in it only to find that the room sounds horrible again. Save yourself the trouble and ignore your excitement to “finish” your room by furnishing it first!

I had previously used the mirror test to find my reflection points ( ) so all I really had to do was put the treatments on the wall and ceiling. With heavy drapes, an industrial wall-to-wall carpet and an area rug in the room, I didn’t need any more sound absorption, so I went against what you’ll read in most How-To’s by placing sound diffusion tiles at my first reflection point.

A Brief and Non-Technical Description Of Early vs. late Reflections

Think of being at a fireworks show. If you’re far away from the actual explosions, you tend to hear each individual explosion, even during the finale. This is because the sound waves arrive at your ears spread out enough for your brain to decipher them as individual sounds. If you insist on standing as close as possible to the launch / detonation area, the finale becomes a cacophony of noise that’s hard to distinguish as anything other than one constant noise wave (not to mention the actual pain involved!)

In a room, this is (sort of) the same principle. Sound leaves the speakers, with some waves going directly to your ears while others bounce around the room. Most of the waves that bounce around the room end up harmlessly somewhere other than your ears but the problem is the early reflections that arrive just behind the direct waves. Your brain simply cannot decipher them as separate sounds so it tries to listen to them as one singular sound–one singular, messy, cacophonous sound that isn’t pleasing to listen to.

By scattering the first reflections (therefore slowing down their arrival at the listener’s ears) I’m eliminating (or at the very least lessening ) the fight in the listener's brain between early reflections and the direct waves from the front speakers. Most rooms are treated at the first reflection point with sound absorbing foam, but this can lead to an overly dead room with no natural bounce or reverb. A room that is too dead is just as unpleasant a listening environment as a room with too many early reflections.

Reverb is good. It’s natural and we shouldn’t fight it–we should just learn to reasonably control it.

Because I’m stuck with a large window at the first reflection point on the listener’s left, I’ve opted to leave the drapes open and let the glass scatter as much reflected energy as possible. Because the windows face a busy street and parking lot, I placed some absorption tiles near them on both walls, but not in a place where the absorption would interact with any of the important reflections.

At the first reflection on the listener’s right I used a 3x3’ array of sound diffusion tiles. I also placed sound diffusion tiles on the "acoustic" ceiling at the first reflection point as well. The increase in clarity and articulation was noticeable, as was the fact that I needed less volume to accomplish the same feeling and reaction from the music or soundtrack–a sign of a properly tuned room.

In the rear of the room, on the wall opposite the main front speakers I also placed a 2x3’ array of diffusers (I used a mirror to mark on the wall where to place them).

Between the rear diffusers I placed a 4x4 section of absorption material, but after listening and smacking my little bits of plywood together a few times, I felt I'd reached the point of too much deadness in the room so I removed them. I always place my tiles and foam temporarily at first–once you put the glue on the wall you're stuck repairing the wall if you decide to change anything. Over the years I've constantly been reminded that when it comes to room treatment, less is usually right, but sometimes you have to cross the line to find out where the line is.

There are two heavy industrial wood doors right next to each around the area of the second reflection, but since this is after the listening position I didn’t worry about them. I did place an absorbing tile opposite the doors just to help quiet hallway noise a little.

*  *  *

The bass traps made the biggest difference–the bass in the room is now punchy and articulate with a very natural feel to it. The sound diffusers at the first reflection point (wall and ceiling) also made a big difference, especially in the range of the human voice. Any absorption I added was to simply quiet external noise, rather than dampen any reflections in the room. If reflections happen after the listening position I don’t worry about them too much: At that point reflections typically add to the liveness of a room rather than detract from the articulation of it.

I’ll talk specifically about the ceiling and floor next week.

Jack Sharkey for KEF America

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