Acoustical Building Block #2 – Diffusion

Ok, so now you have the basics on absorption; now it’s time for the next tool – diffusion.

When you add liquids in a bowl for a kitchen recipe, you usually stir it to get a smooth, consistent mixture. Likewise, in a listening environment, the sound should be well mixed and consistent, for two reasons. First, you want everyone there to hear the same thing, no matter where they’re sitting. Second, and more importantly, you don’t want your sound colored and distorted by rogue reflections.

When sound leaves a speaker, it more or less radiates out in all directions within the room. Less so for the higher frequencies, but since sonic colorations in a room occur predominantly below 500 Hz. or so, this is our area of interest.

Radiating out, it first reaches the listener with the direct sound; sound also strikes all the room’s surfaces and reflects towards the listener. When these early reflections reach the listener, they’re delayed slightly from the direct sound, due to the longer distance they must travel. You get two effects.

First, the early reflections cause the apparent location of the sound to shift, since your ear is detecting sound coming from multiple sources. This creates a “smearing” of the sound and distorts localization. Everything moves around in subtle ways perceived more as an indistinct “bad sound”, than a specifically identifiable problem.

Second, take two identical waveforms and mix them together. Delay one of them by a few milliseconds, and look at the resulting frequency response. You’ll see points where there are notches in the response, occurring at regular intervals. Looks kind of like a comb, right? That’s why this is called a comb filter effect. This is sonic coloration personified.

There’s another issue. When you have two parallel surfaces in a room and generate a sound between them, you get this distinctive rapid, fluttery echo. This is best heard with an impulse, like snapping your fingers, and most often experienced in stairwells or hallways where hard walls face each other. More coloration!

So diffusion is the acoustical tool that serves as our big kitchen spoon. It mixes everything so that what you hear in one position is essentially the same in other positions. And it breaks up the early, hard, specular reflections, the “angle of incidence equals angle of reflection” stuff you learned in high school physics.

In my opinion, diffusion has the greatest affect on the actual sound quality in your room. When sound falls on a well designed diffusor, you don’t get hard, specular reflections. Instead, the reflected energy is broken into many smaller reflections, sent in many different directions. Unlike absorption, diffusion doesn’t remove energy, it spreads it around. So there’s also no turning up your level to accommodate losses, as with absorption.

By placing diffusive elements at the points where the early audio reflects back to the mix position (which I call “critical reflection points”), the reflected energy is better distributed throughout the room. This breaks up the delayed sound that causes the comb filter effects, the smeared sound, the distorted localization, and the flutter echoes. And there’s an easy way to find these critical reflection points.

Stand in the primary mixing position and have someone move a mirror on each side wall until you can see the speaker – that’s your critical reflection point. Repeat for all front speakers. By moving to the various listening positions and moving the mirror to see the speakers again, you will get a series of points. Install diffusion where it covers all these points. Repeat this for the other side wall, the rear wall, and the ceiling (harder to do, but very important).

Now the big question is just what gives the best diffusion? A common misconception is that geometric protrusions provide the best diffusion. They’re OK, but not the best. If you do choose this route, your best bet is actually rectangular solids, rather than the odd shapes that are out there. You can also use polycylindrical, phase grid, primitive root, and perforated surface diffusors.

But for the best performance, the most bang for your buck, use Quadratic Residue Diffusors. These are those strange looking devices that have rows of large slots of differing depths, like RPG sells. You can find them at a number of online resellers, but if you’re willing to do some math work, and you’re handy in the wood shop, you can design and build your own.

So now you have a grasp of diffusion. There’s a lot more to know, so if you REALLY want to delve into the subject matter, get F. Alton Everest’s Master Handbook of Acoustics. You will learn a great deal, and find out that the mysteries of acoustics aren’t the “black magic” secrets they may have seemed.