QRD Calculator and Diffusion
One of the great, overlooked aspects of acoustics. The first thing everybody does when they build a new studio room is slap foam everywhere. Usually this goes very badly, because they use the wrong foam.
Eventually they wise up and use the right foam (like rigid fiberglass or acoustic batts), but it’s still only part of the equation. Absorbers can only absorb so much, and you still end up with sound bouncing around – and those straight-line reflections can cause all sorts of trouble with phase, comb filtering, resonances, etc. That’s where diffusion comes in – a diffuser
breaks up those reflections and steers them off in different directions.
The reason it’s so often overlooked in studio construction is that it’s difficult to do properly. And it’s expensive to do properly. And diffusers are often bulky. And…well, you can see the issue. A lot of home studios use the “bookcase trick” – that is, they stick large pieces of oddly-shaped furniture on the rear wall and expect that to scatter and diffuse
sound. That…that sort of works. But it’s not easy to control, and lord only knows just what frequencies are and aren’t being scattered.
There are many commercial diffusers – RPG, Auralex, GIKAcoustics etc make some of the best-known ones – but they aren’t cheap. The 1-foot-square auralex T-fusor panel retail for about $60 retail each, and a 2x4ft RPG can run upwards of $250.
Plus, these aren’t the kinds of things you need just one of. Considering you can make a 2×4 broadband absorber panel for about $10 each, that’s the fruit people usually go for.
The quadratic residue diffuser, or “Schroeder diffuser” (named for Manfred Schroeder, of Univ. Gottingen, who developed the formulae used below) is one of the more common methods of handling diffusion. Using a panel with “wells”
of specifc varying depths, it can “steer” wavefronts in various directions, acting to diffuse sound. The frequency of the diffusion varries based on the number of wells, and the well width and depth. Well width defines the highest frequency diffused, well depth defines the lowest. The downside is that they tend to scatter only in one dimension. To scatter in multiple directions, the diffuser needs the be built to do so, and something like a “skyline diffuser” needs to be employed. And the math for that is painful (well, to me, at least).
What’s handy about QRD’s is that they are the kind of thing someone who’s somewhat decent with a set of tools could
conceivably build. You’d need a backboard of some kind, some rigid strips of material, and thin strips to be the “fins” between the wells. And, of course, a good tape measure. Wood construction is common, but as that weight can add up quickly, they can also be built from rigid polystyrene or thermoplastic, if you’ve got access to that sort of thing. Most
calculators I’ve found start from the QRD’s “design frequency” – the target frequency it’s designed to diffuse – and then provides calculations for width and depth from there. Problem is, for someone building one at home, who’s going to go to the local Home Depot and buy a 2.632″-wide strip of moulding lumber? Who’s good enough with the mitre saw to get that depth cut to the second decimal place? So I’ve taken the liberty of flipping the equations around so you can put in well widths and a maximum depth and then see what shows up.
|Number of Wells|