The Diffusor Thread

[Ethan Winer]

Folks,

Last weekend I drove down to Wes and Lisa Lachot's recording studio in North Carolina to learn first-hand about diffusors. It was 1200 miles round trip and took 11 hours each way, but it was well worth the effort because I finally got to hear diffusors in action. Which took less than ten seconds to understand and appreciate!

Knowing what diffusion is all about had become very important to me lately because people ask questions about it here all the time. But without having actually heard a diffusor all I could do is repeat what I've read. And we all know how Dangerous that is. Dr. Peter D'Antonio of RPG, who designs and makes diffusors, says they should be no closer than ten feet from your ears. Wes has said they can be a little closer and still work well. Kurt Foster insists he's seen high-end studios that presumably know what they're doing use them in very small rooms. So what's an inquiring mind to do? Drive for 22 hours practically non-stop, that's what! :D

Following is my report. At the very end are two paragraphs I just added to the forum FAQ, under the photo of ACID Acoustics's commercial diffusor. But that addition falls far short of describing my personal experience at Wes and Lisa's studio, Overdub Lane, which I want to share here.

The photos on their web site www.overdublane.com hardly do justice to how well-appointed and comfortable this three-room complex really is. They have a real Hammond B3 and Leslie, a fabulous Yamaha grand piano, not one but two 24-track 2" analog recorders, a full Pro Tools rig, more high-end mikes than I've ever seen in one place, plus a ton of other very cool current and vintage gear I won't bother to list. Frankly I was more impressed by the cartoons on the wall, the lava lamp, the Beatles memorabilia, and many other warm, personal touches. It's a given that the acoustics in all the rooms are first rate - this is Wes Lachot's own studio! I even brought a CD of my current projects to check there because I knew the control room would be dead-on accurate.

When I first stood in front of the diffusor wall at the rear of the control room I didn't notice anything unusual. These RPG diffusors are the "well" type comprised of many recessed channels, as opposed to the "skyline" type made of small squares you may have seen. Blame my short auditory memory, but I need an A/B test to be sure of what I'm hearing. So I asked Wes to bring out one of his gobos with a large glass divider, and he put that in front of half the diffusor wall. I stood just a few inches in front of the glass and said loudly "Testing 1,2,3" - what else do you do? Then I stepped a few feet to the side and did the same into the diffusor.

In that one instant the value of diffusors became undeniably obvious, and all I could say is Wow! Yes, these things really do work, and they really are worth having in a control room. Where a flat wall sounds boxy and hollow due to the numerous peaks and dips caused by comb filtering, the diffusor sounded incredibly open and natural. Talk about lifting a veil! In no way was this a subtle effect. Even Wes seemed a little surprised at the obvious difference since he said he's never actually A/B tested a diffusor. But wait, there's more.

Besides giving an open and natural sound in a control room, Wes explained to me how diffusors have a great use in studio live rooms too. At Overdub Lane one wall in the larger live room is covered with a long row of RPG well diffusors. As we all know, a big problem when recording multiple performers in one room is leakage. Reflections off a wall, even an angled wall, can get into the wrong microphone adding an undesirable and distant sound. But with diffusors the reflections are scattered far and wide, so whatever sound makes its way into the wrong microphone is greatly attenuated. Contrast that with a flat wall where all of the reflected sound is beamed coherently in one direction.

One other thing I noticed when talking directly into a diffusor at very close range is the wells have many narrow resonances. So depending on the depth of the well you're talking into, it sounds a little like a parametric EQ with a lot of boost and a high Q. This is where the "ten foot" rule comes from - as you get farther away from a diffusor all the individual resonant components have a chance to combine again. But even at a foot away the improvement of a diffusor wall over a flat wall was very obvious, and the effects of the multiple resonant chambers was minimal.

I still need to hear the skyline type of diffusor. Does anyone near me have a wall covered with those? I'm willing to drive! :D

--Ethan

Recently added to forum FAQ:

In truth, the angled and curved walls described above are reflectors, not diffusors. A true diffusor scatters sound Waves in all directions, rather than merely redirecting the waves while letting them remain coherent. This is an important distinction because a flat surface that is angled still fosters the boxy sounding response peaks and dips known as comb filtering. A real diffusor actually avoids direct reflections altogether, and thus has a much more open, transparent, and natural sound than a simple flat or curved surface. Besides sounding less colored than an angled or curved wall in a control room, diffusors serve another useful purpose in recording rooms: they can reduce leakage between instruments being recorded at the same time. Where an angled wall simply deflects a sound - possibly toward a microphone meant to pick up another instrument - a diffusor scatters the sound over a much wider range. So whatever arrives at the wrong microphone is greatly reduced in level because only a small part of the original sound arrived there. The rest was scattered to other parts of the room.

Unfortunately the better commercial diffusors are not cheap. So what are some alternatives for the rest of us? Aside from the skyline type diffusors, which are sometimes made of plastic and sold for not too much money, you can make a wall either totally dead or partially dead. For someone with a very small budget, making the rear wall of a control room totally dead may be the only solution. At least that gets rid of flutter echoes between the front and rear wall, though at the expense of sounding stuffy and unnatural. But it's better than the hollow boxy sound you get from a plain flat reflective surface. Another option is to make the rear wall partially reflective and partially absorbent. You can do this by making the wall totally dead, and then covering it with thin vertical strips of wood to reflect some of the sound back into the room. If you vary the spacing from strip to strip a little, you'll reduce the coherence of the reflections a little further improving the sound.

[macmod]

thanks for the report Ethan...
regards,
Macmod

[macmod]

/snip/
But even at a foot away the improvement of a diffusor wall over a flat wall was very obvious
/snip/


this observation appears to be in contrast to comments given to Q's about using diffusion on back walls in small studio's in this forum (like my own mini studio 3mx2mx2m..jpg soon to follow!)....

Now, where do we go from here regarding this particular topic Ethan, Wes and others?

very best of regards,
Macmod.

[Recording Engineer]

Yeah, I've wondered about diffusion for my control room back wall too. I know I need some bass trapping in the rear area at least; if not some diffusion on the rear wall as well.!

[knightfly]

My thoughts on this -

1. Ethan said "One other thing I noticed when talking directly into a diffusor" - I've yet to find anyone anywhere who thinks their voice sounds like it really does, without listening to the playback of a recording - so it seems likely that what Ethan heard when speaking into the wall or diffusor is NOT what would have been recorded, primarily due to the bone induction you hear when you speak.

2. Diffusion (even up pretty close) sounds really good compared to a flat reflected sound, until you realize that your center stereo image isn't there any more. That's the reason for the 10-foot rule - you want nothing coming back to your ears that's less than 20+ milliseconds later than the direct sound, so your ears/brain can localize any psychoacoustic panning in the mix.

The first point, I think, has been experienced by virtually ALL of us here - the second is (at this time) mainly conjecture on my part, based only on years of study. Before I get a chance to prove or disprove that part, I will have to nearly complete a new facility (working alone) that I've been planning but which won't even break ground for at least two years - I'm sure someone here will beat me to that one :=) Steve

[jazzman_in_pa]

Steve, I sorta grasp the theory too behind the 10 foot rule. And yet here's an example of a wall of (absorbing) diffusors practically right behind the mixing position (picture at bottom of page):

http://www.zainea.com/

BTW, anybody tried this guy's product yet?

Is it possible that such a product can diffuse so broadly that your brain is no longer confused by the close rear reflections, since they're so - well, diffuse?

Lee

[Recording Engineer]

I've seen that website too. They crossed my mind for one of my walls in the studio, but not the control room. I haven't decided on anything yet though.

[tubetime]

Thanks, Ethan, for going to such lengths to help de-mystify diffusion.

I've never heard or seen a real live diffusor, but I've researched this on the net, which is mostly opinion and to be taken with a grain of salt, and by reading Everest's 'Master handbook of accoustics'which is more reliable.

It seems there is agreement that diffusion is really important, especially in smaller rooms like mine (21 ft x 18 ft).
It also seems obvious that using a quadratic residue sequence (QRS) diffuser really works.

Using Everest's information, etc. it doesn't appear to be that difficult to construct a QRS diffusor. I'm thinking of building one to fit into an 8 foot square inset area on the back wall of my studio which is nearing completion. Using a formula I found at http://www.djlace.com/quad.html, I could build one flush into the inset in the wall which is six inches deep. Using a number sequence of 17 the deepest 'well' would be six inches deep and the width of each well would be 0.75 inches wide, with 1/8th inch seperators. This would provide diffusion down to 1130 hz, if my math is correct.

Using plywood or mdf, this shouldn't be too hard to construct, and they certainly look cool. I'll probably build a few modules and see how it goes.

Thanks for such a great forum.

[Kurt Foster]

Proper acoustic design / treatment employs all three approaches, absorption, diffusion and bass trapping. Ethan has been instrumental in educating us in the ways of bass traps and absorbers, getting me to kick the foam rubber habit. Now with the addition of diffusion, the recipe is complete.

There are cost effective alternitives to expensive QRD style diffusors. Anything on the rear wall that creates an uneven surface, will aid in difusion. Bookcases are great for this. Shelves with vinyl LP collections are also good.. anything that presents an uneven surface (the greater the abberations, the better) and some mass will aid in diffusing your room.

I had a very bad slap back issue in my room which is 17 feet long. Snare hits refected back with a short delay and were really messing with me. I placed a narrow table across the back wall, and put my record collection on that. I then put up CD and DAT racks on about 2/3s of the wall and the slap problem dissappeared. This is a cheapo solution and I am sure a QRD diffusor would be better, but in my case it makes enough of a difference that I can do better work...