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Some quick thoughts on reflection…

Posted by David Aiken (A) on February 11, 2011 at 23:38:43

In Reply to: RE: QRD placement, grouping and orientation posted by Mark UK on February 10, 2011 at 15:23:57:

pun intended—actually thoughts on diffusion but still.

I don't think losing diffusion at the lower end is likely to be an issue. Decay time is longer at lower frequencies because they get absorbed more slowly, and diffusion in a room increases as decay time increases though modal activity isn't diffuse in one sense. Even losing some bass diffusion with a given layout, you're still going to have more diffuse mids and highs, a longer decay time at low frequencies and some modal activity at low frequencies. In fact you'll have that sort of picture whatever you do.

I don't think that whatever audible differences there are, it would be the difference in lower end diffusion that would lead you to choose one approach over the other. I think it's more likely to be what's happening in the mids on up and how that affects things, including how that affects imaging and soundstage, regardless of whether you prefer those precise or not.

Just be aware that you need space between the listening location and the diffusors. Sit too close and you'll hear some problems because the diffusion won't have integrated over the whole diffusion bandwidth and audible anomalies can occur. If I remember correctly, RPG suggest a minimum of 10' between diffusors and listening position somewhere on their web site. That distance varies a bit based on diffusor type and bandwidth but I suspect it's a good guide for QRD type diffusors like you're considering building.

If you can't provide that sort of distance you may well be better off considering absorption rather than diffusion. If you go the absorption route, consider using a number of 2' wide panels, place 2 at the rear wall first reflection points, and experiment with different placements for the others. I would expect you to get best results with some space left between panels and the alternation of absorption with reflection from bare wall will tend to promote a bit of diffusion anyway but nowhere near as much as you would get from using diffusors.

Once again, however, how much absorption you would use with this sort of approach and how you space the absorbers out is going to be a bit of a personal matter in my view.



David Aiken

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