Diffusion is a Wonderful Tool – by Jeff Hedback

Diffusion is a wonderful tool in small room acoustics. OK, nice enough, but what is diffusion, why is it wonderful and where would it be wonderful in my room? Below I will answer those questions and offer effective application options.

Mathematically-based/purpose-designed acoustical diffusors are young, only 30 years in development. The first commercially offered options didn’t appear until the 80’s. The origination of these “widgets” was driven by large room auditorium/theater acoustics. It wasn’t until the past 15 years or so that more specific units were developed for small room applications. This background is important only to note that there may still be much debate in types and applications of acoustical diffusors in studios and residential theaters…no worries, back to the first statement: diffusion is a wonderful tool in small room acoustics.

The QRD (Quadratic Residue Diffusor) type diffusor is a mathematically non-repeating design featuring varying sized “wells” where the well width determines the high-frequency performance and the well depth determines the low-frequency performance. The purpose of the QRD is: (1) evenly disperse the sound energy back into the space (retaining similar amount of energy) and (2) diffuse the phase content of the reflection so it does not smear (comb filter) with the oncoming energy (varying depth = varying time= varying phase response).

The Polycylindrical diffusor is based on a curved surface which randomly re-directs sound energy based on the radius of curve and angle of incident of sound energy. This type of diffusor again retains the acoustic energy but it does not affect the phase relationship of the energy.

In terms of frequency response, quality QRD’s are typically effective down to 400-500 Hz…which the D1 does. The effectiveness of a poly is determined by the depth, width and radius of the curve.

The cool thing about all quality diffusors is that below the diffusion cut-off point, they yield some amount of low frequency control (bass trapping).

So what does a Poly sound like verses a QRD:

A poly is a victim of peer pressure. By this, they will sound different depending on what the neighboring surfaces are. By itself, next to hard surfaces it can be unpleasant creating detectable pings. But, when properly located in clusters, in relation to boundaries and/or with absorption, they truly sound totally neutral in a most cool manner. You do immediately get a bigger feel psychoacoustically.

The QRD really holds its own sonic imprint: warm, non-directional yet sonically cohesive. A great acoustic guitar played into a single quality QRD just feet away will sound like an even greater version of same guitar.

Here is a good analogy for the “studio types”: a QRD is more like a mic pre where you select it for its character and ability to translate the frequency response in a desirable manner. Whereas, a Poly is more like a microphone where you select it for its frequency response and polar pattern. If you think about locating a QRD or Poly in these terms you can allow your imagination to run.

Uses:

Both QRD and Poly diffusors are one-dimensional, meaning that sound is dispersed perpendicular to the orientation of the linear features of a QRD or in the same plane as the radius of the Poly. Immediately you might see how the use of clusters of units where adjacent diffusors are rotated 90 degrees is a tremendous design thought.

Using absorption in combination with either diffusion type is how overall balance can often be obtained. In fact, the combo use of absorption can be essential with Poly’s.

Applications for Studios:

The “classic” application is the rear wall of Control Room. First, you want 8′ (as a general rule) minimum from mix chair to back wall to implement a QRD. The use of the GIK QRD diffusors covering 24 sq ft minimum can greatly enhance the sweet spot at mix position: you can move more freely from side to side/forward & back and hear consistent content in terms of mid/hi response.

Any large exposed sidewall and ceiling surfaces in the rear half of a control room are targets for diffusion. The QRD is a better option in general, but paired with absorption, the Poly could direct energy in very pleasing ways to the client seats.

Here is a control room rear wall Poly application I recently designed for ZenProAudio.com. The given factor is that the walls are 100% absorptive in the mid/hi bands. Thus the Poly’s became a wonderfully exciting option to retain life in the space while not creating any detrimental acoustical glaring, smearing or other mix position horrors. By rotating the adjacent units that are on rear wall and rear ceiling and placing specifically sized and space slats on rear sidewalls; Warren Dent has a mix space with great accuracy. As you might expect, the client position is a little unbalanced (being right against the rear wall) but still comfortable and more friendly than a totally dry listening space.

In a small booth with very effective broadband panels like the GIK 244 Bass Traps & Monster Bass Traps, you can place the QRD in a cluster on the ceiling (in rooms as low as 8′) and get a modern vibrant timbre without detrimental artifacts. The goal in “capture” or tracking phase is to find the most exciting representation of the performance…but it has to remain exciting with other tracks. Specular reflections off hard flat surfaces create artifacts that only get more problematic with other tracks and typical mix processing. Sure you can absorb such hot spots, but that doesn’t lead to excitement.

If you have 10′ ceilings or higher the Poly becomes a leading contender for a bigger feeling space. You would look at 10-20% coverage of a surface to have significant impact. Spacing Poly’s 6″ apart minimum is absolutely imperative in most applications. Keep in mind that you have to look at all surface interactions in this type application.

Check out the rear wall behind drums in JR Robinson’s drum room which I did treatment design. The QRDs are dispersing the hot sidewalls flanking his high frequency absorbing tapestry. This immediately improved the tonal definition of toms and cymbals (remember the QRD is smoothing out the mid bands very effectively) while leaving JR the bright room he desired…a little went a long way!

One big tip that can allow a little diffusion and a lot of hard “un-treated” wall to yield great results: stagger or offset diffusors between parallel walls. I recently did an acoustic make-over of a room (17’x18’x10′) that was nearly all absorptive on the walls and had some less effective one-dimensional diffusors on the ceiling. I re-appropriated his absorptive panels from walls to 20% coverage of ceiling (evenly spread), and other than very select absorption zones, used a quality QRD type diffusor to clean up hot spots on the vertical walls. We also added a bamboo floor and placed the lesser effective diffusors surrounding window. We created a very cohesive, bright , yet controlled tracking room just by locating diffusion on the walls in relation to each other and accounting for the manner that sound would propagate in the room.

TABOO TOPIC…diffusion above mix position. Yes it’s true and is now in print for others to read. I have successfully applied QRD style diffusors above mix position and increased the ability to determine spatial and depth decisions…lock in on attack/release compressor settings, combine reverb and panning elements. It is not a “one size fits all” application BUT it can work especially if the mix engineer has a greater sensitivity to vertical reflections.

Applications for Home Theaters:

Depends, depends, depends…speaker type, speaker location, proximity of seats to boundaries…but:

The rear third of most residential theaters is prime for diffusion. Most application use Bi-Polar surrounds and diffusing this energy truly does yield a more cinema like response: you hear the surround information, can track it during dramatic panning situations, but like a good commercial theater you never become aware of surround speaker locations.

If you have a ceiling 10′-14′ you can use diffusion on the ceiling to mimic the large room yet intimate feel of a commercial cinema.

As most THX style center channel speakers are designed to have very polite vertical polar response, you can use QRD style diffusors on the halfway point between speaker and seats to yield a more intimate theater response: tying the audio and video together wonderfully. This can be done in lower rooms and even rooms with vault profiles. Alternating diffusion and absorption panels on such vault angles can yield a more even response from seat to seat than just absorption panels.

If you have more than 6′ distance from boundary to nearest seat, then sidewall columns of Poly’s could work wonders! Not that “tighter” applications wouldn’t ever be possible, but you would have to be concerned about glaring and smearing with the direct sound.

OK, the above is long-winded, has some purely technical, some anecdotal and some true & successful diffusion applications…BUT I trust the article is helpful and I want to extend my greatest appreciation of GIK Acoustics for their off-the-charts value driven product line and true desire to help their customers enjoy great audio!

By Jeff Hedback