November 2013

Of Broken Mirrors And Building Your Own Sound Diffuser Device
Article By Jeff Poth

  Been a while, eh readers?  New role at my day job, and other good stuff keeping me from writing… but not from building, oh lord no!  It's about time I put pen back to paper.  One of the projects I'm working on is a very special set of cabinets for large format mid-basses (12", possibly 15") or subs.

Notice the holes?  The way the cabinet is arranged, I'm trying to make an incredibly strong enclosure with a highly diffuse internal profile which will not support any modal behavior.  A lot of scroll saw hours went into these puppies (and they're not done).  Those who have seen "Sonotube" subwoofers understand that the round structure is inherently very strong, similar to arches or eggshells.  Laminated plywood used "against" the lamination is also amazingly strong, and can be used for structural members in building construction (in place of steel).  The addition of the bracing scheme guarantees that no deflection of the cylinder walls will be possible- just too darned strong!

So, with dozens of layers of these shapes, with over a hundred little cutouts, what's to do with all this scrap material?  Why, diffusers of course!  My more loyal readers should be aware of how fond I am of recycling and repurposing.  In this case we're talking about a lot of semi-random shapes of high quality Baltic birch plywood, which would be a darned shame to waste.

So, I began laying them out and thinking about how to best press them into service.  A simple stacked assembly with randomized layers seemed like it'd work just fine.

Uh... Diffusor?

Okay, okay, I'll back up a few steps. We should all be familiar with the concept of acoustic reflections.  The surfaces of walls and other items reflect sound similar to mirrors and images, though they change behavior with frequency (walls are often "transparent" in the bass and increasingly reflective with increasing frequencies). It is very common to see acoustic treatments on the walls of dedicated listening spaces, or even on the ceilings or walls of restaurants that are trying to keep ambient noise under control. They are typically absorbers comprised of fiberglass or acoustic foam, soaking up the acoustic energy (sound) thus preventing reflections off a surface and reducing SPL within the space. It is common for half the sound in a given space to come from reflected sound, as opposed to direct.

A diffuser is an acoustic scattering device. One popular commercial model is the RPG "Skyline" diffuser, which utilizes blocks of differing heights.  The key feature is a varied profile which doesn't have a specific resonant character. By scattering the reflection, it becomes less coherent, and is thus easier for our hearing mechanism to disassociate from the preferred, direct sound. Diffusion can be accomplished with household goods, though they're often resonant structures which introduce their own problems. Something like an open china hutch full of wineglasses would be very diffuse at some frequencies, but also highly resonant at some other frequencies and thus unsuitable, as it would introduce more problems than it solves.

Many or most room treatments are focused upon adding absorption, and simply absorbing the reflections, but this can create an overly "dead" sounding space. The extreme case of this is what's called an anechoic chamber- a space where (effectively) all sound is absorbed. It sounds very unnatural to listen to music or voices within a space like this. Acoustic absorbers also tend to vary widely in effectiveness with differing frequencies. They generally are far less effective in the bass than the highs, and can thus wind up changing the balance of reflected sound to a very bass-heavy curve.  With half the sound coming from that reflected energy, it can dramatically change a speaker to be overly "dull" sounding. Humans are used to environments with at least some reflected sound; even outdoors there's still the ground, trees, nearby walls, and other boundaries in the general proximity to provide some reflected sound. Consider that a normal listening room usually has some overstuffed furniture, sometimes thick rugs, and is generally a soft, absorbent space. Adding more absorption can make the room over-damped and unnatural sounding.

A highly reflective sound is equally problematic; consider a funhouse hall of mirrors- a space that sounds the way it looks: bright, confusing, and reflective.  If you put a blanket over the mirrors, they're going to stop reflecting, both sound (to some extent) and light. You'll no longer be confused by all the reflected images, but if you put blankets over all the mirrors, the room will become relatively dark since you're giving up all that extra light energy that's been reflected by the mirrors; this is the equivalent of the over-damped acoustic space.

A proper listening space should (usually) be a balance, with absorption used in some spots and reflections tolerated or managed in other ways, in other spots. A diffuser is a method of scattering sound, by creating a randomized surface that remains reflective, rather than absorbent. In "Enter The Dragon", Bruce Lee has to fight in a mirrored room, and the reflections make it difficult to discern the real foe from the reflections. Similarly, coherent, mirror-like reflections can interfere with our hearing mechanism's ability to localize sounds within a stereo image. He overcomes this by breaking the mirrors- the reflections are now incoherent, though they're still there, and no longer interfere with his ability to find his enemy.

Similarly, an acoustic diffuser retains the reflected acoustic energy while reducing the effect of the reflections on image specificity and soundstage cues. Reflections can add to a sense of space and ambience in a home stereo system, and all but specialty (<0.01%) of recordings are mixed assuming some level of reflections within the listening space. Studios do not use all absorption but rather a mix of methods to create the desired acoustic. Some use a technique where one end of the room is reflective and the other is damped, "Live End Dead End".

Acoustic design is complex... but acoustic tweaking goodies are easy.

While getting a room "just so" will depend upon the speaker and careful planning and testing and adjustment, making some of the tools that allow you to tweak your acoustic space is not nearly so difficult. There are many designs around the web for acoustic absorbers, including bass traps designed specifically to address low frequencies, Helmholtz resonators tuned to frequencies of interest, absorbers for a variety of locations and styles, and also diffusers. This is just one way to build diffusers -- not inherently better than most other diffuser designs, but also quite easily accomplished with cutouts and wood that might otherwise be destined for the dustbin.  I find the look attractive, but others may prefer the more squared or finished look of some other designs.

This design relies upon a simple stack and glue and clamp process -- one builds up layers of cutouts by gluing and clamping them together, one layer at a time, until it reaches the desired size. Bigger will tend to be effective to lower frequencies, but is also... bigger.

I sanded off the ragged edges as I went, it would have been more efficient to pile them up by the belt sander and do it all at once. The cutouts are 0.75" Baltic birch ply, glued with Titebond II wood glue, and clamped together during glue-up.  This was a tedious process, but there's only so many clamps one fella can use  The nice part is that there's lots of glue area, and it won't be load-bearing, so you don't need to stress TOO much about getting the clamping force right. After the layers are done, a couple rattle-cans of Kilz sealer/primer (Zinsser BIN is similar and also excellent) and some sanding, and we are starting to get close.

I chose a metallic blue paint, with the idea of mimicking water. After a few rattle-cans (I should get a proper sprayer... someday) they were looking pretty good. The surface finish quality didn't need to be as good as a smooth panel, since they were highly randomized to begin with and the painting does a good masking job.

After looking at the blue for a bit, I wasn't quite happy with the look of water so decided to add some white to look like foam on the ocean, and installed them on the wall.  A little mounting hardware and some anchors and screws and they hung nicely.

I have a flat section of wall fairly close to the left speaker, and while I use horns, which reduce the influence of this wall, it's still problematic. The diffusers were a good solution for this spot, reducing the "image steer" towards the left that I was experiencing. Acoustic pyramid foam in this spot had looked much worse, and had somewhat of a disconnected sound when I used it, probably because the wall section and absorption were only on one side. The diffusers had a more coherent and stable soundstage and image than with a plain wall or acoustic foam.

Easy and cheap and very effective! Your results will vary based upon speaker, room, and personal preference, but having diffusers in your acoustic toolkit is very handy.