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QRD Diffusiors: Minimum Distance To Seating Position

Brandon Drury —  March 24, 2014 — 24 Comments
Seating Position

Quite a few people here at RecordingReview have expressed an interest in buiding QRD Diffusors. I’m not exactly sure of the draw of the QRD serpeant, but I’ve been bitten by that snake, too. Here’s a few guesses:

  • Diffusors can be just technical enough to challenge a guy who wished he designed stealth bombers.
  • The actual construction chops required aren’t prohibitive.

It’s no wonder why DIY Diffusor projects are so popular.

Dumb Diffusor Thought Experiment

It won’t be long from now that we’ll be planning a new house and a new home studio along with it. Just for my own amusement, I pondered what it would take to get a 2D QRD diffusor effective down to 50Hz. I fired up QRDude and selected, again just for fun, a 43-well QRD diffusor with a design frequency of 50Hz. Go big or go home.

QRDude screenshot

I expected the thing to be 12′ deep. That didn’t surprise me. The number that truly elated me was the Minimum Distance To Seating Position figure: 812.59 inches

Yes, that’s 67.71583333333333 feet.

For this diffusor to do much of ANYTHING, you need to be 67′ from the damn thing.

Granted, the definition of “anything” is always up in the air. The guys drawing the line between an effective and ineffective diffusor have the same problem as the guys trying to figure out when upgrading from an $100 mic to a Neumann U87 becomes “effective”.

Regardless, that 67′ number is enlightening. I’d be more than thrilled if I had the studio space to that was 40% that dimension. Of course, this was just a thought exercise like swimming to the moon.

Even when adjusting QRDude for a much less-impractical 300Hz design frequency, we still get a minimum distance from seating position at 135..43″. (11′). When I built my diffusors, I knew I was going to be on the low side of that, but went for it anyway. I regret it.

After recording in 100% dead spaces, highly diffused spaces, very live spaces with no diffusion, and just about everything in between, I can’t help but think that the major reason most unseasoned home recorders build diffusors has almost nothing to do with a sound. I can’t see why it’s worth fretting about whether we bust a bullet into a hundred ineffective bb’s (diffuse it) or just catch it (absorb it).

I know they use words like “oppressive” to describe a room heavily covered in Roxul. I think that’s an irresponsible use of the language and an example of extreme sensationalism when used to describe a near-100% dead room. (Actually get 100% dead requires enormous efforts.) I think “surprising” may be a better term.

I know people like to say that a diffusor will “liven” up a room. I’m still trying to figure out what they hell they are talking about. That’s not what diffusors do. Even if you really did want to “liven” up a room, according to Philipp Newel, author of Recording Studio Design, you need to cover 25% of that room in a “livening” material.

I think people believe that adding a ton of diffusors to a crappy space will suddenly make that space sound outstanding. In most cases, I’d bet people want their little bedroom to “open up” like the live rooms at Ocean Way. That’s definitely NOT what diffusors do, either.

Diffusors won’t “liven up” a room, either, unless the thing is REALLY freakin’ dead and you’ve got a few truckloads of diffusors (for a small-ish room). If you are used to hard reflections, you are going to wonder what happened when you bust those up.

Conclusion

  • If you are using QRDude to put together some DIY diffusors, take that “minimum distance to seatig position” seriously. Its there for a reason. While the definition of “effective” may be debated, remember you are playing with a fire.
  • Keep in mind this is idea of diffusion sits well, philosophically, to us humans, but the real-world, tangible benefits of the extreme investment in time to make these things is limited to a small percentage of people actually partaking.
  • Don’t make DIY diffusors just because they look pretty.

Brandon

Brandon Drury

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Brandon Drury quit counting at 1,200 recorded songs in his busy home recording studio. He is the creator of RecordingReview.com and is the author of the Killer Home Recording series.
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24 responses to QRD Diffusiors: Minimum Distance To Seating Position

  1. Yeah, you have to be careful with what you use different tech to accomplish.

    If you had raised the diffusion requirements to 1kHz and relied on absorption products or Helmholtz resonators to deal with problems from 300Hz – 1kHz and trap the lower frequencies below 300Hz I think you might find that diffusers work quite well to keep a room live while combatting comb filtering and other unwanted issues in the mid to high mid regions.

  2. Great article Brandon – I wonder sometimes if we don’t get caught up in the psychosomatics of these “improvements” and don’t really spend quality time doing real science or real unbiased research. Everybody wants a new toy & we have to justify the cost by attributing terms to the “improvement” which have little-to-no real meaning. Enjoy your articles. Congrats on your new house and upcoming soundroom.

  3. Great article Brandon – I wonder sometimes if we don’t get caught up in the psychosomatics of these “improvements” and don’t really spend quality time doing real science or real unbiased research. Everybody wants a new toy & we have to justify the cost by attributing terms to the “improvement” which have little-to-no real meaning. Enjoy your articles. Congrats on your new house and upcoming soundroom.

    Yeah, I’m probably the world’s worst about this. I took a LONG time to build my diffusors. I saw no improvement in sonics that would allow me to charge bands more money. That’s what I wanted! Oh well. In retrospect, that time sure could have been put to use actually recording. I missed out.

  4. If you had raised the diffusion requirements to 1kHz and relied on absorption products or Helmholtz resonators to deal with problems from 300Hz – 1kHz and trap the lower frequencies below 300Hz I think you might find that diffusers work quite well to keep a room live while combatting comb filtering and other unwanted issues in the mid to high mid regions.

    You are correct that raising the frequency allows for the Minimum Distance To Seating Position to go down radically. The numbers I showed were extreme, but that was just for fun.

    The real question is whether treating problems at > 1Khz is best done by diffusors. My personal opinion is the diffusors are very high cost and don’t do anything particularly special. I think the idea of diffusion is sexy, but I feel 99% of us would be better served by using good ol’ absorption.

  5. I’m a bit confused with your article.

    First, your diffuser scatters effectively at half that distance at that frequency. It isn’t truly diffuse, but scattering is a lot better than a direct reflection.

    Second, your article mentions the minimum seating distance from a diffuser, but not why it is bad. You just mention that it is “playing with fire”. Why do you regret not being quite 11′ from your 300 Hz diffuser? What do you hear?

  6. First, your diffuser scatters effectively at half that distance at that frequency. It isn’t truly diffuse, but scattering is a lot better than a direct reflection.

    If we are trying to keep that reflection from coming back, scattering may be better than nothing. However, if we don’t want that reflection to come back, I’d rather just absorb it and be done. SMILEY

    Second, your article mentions the minimum seating distance from a diffuser, but not why it is bad.

    Good point. I should go back and correct that. More or less, if we are too close to a diffusor, we won’t be able to hear it working. For anyone in a tiny or even “just small” room, the proximity to the diffusor is going to be big issue.

    Why do you regret not being quite 11′ from your 300 Hz diffuser? What do you hear?

    I had a wall of 2D skyline QRD diffusors I had built that was about 32 sq ft. I took ‘em down and toss 6 bags of unopened Roxul there instead with fabric covering the whole mess. The fabric tamed the top end and the huge bags of Roxul significantly improved the low end. So to answer your question, I really can’t say. I’m not sure the diffusors did when being too close to them.

    The overall theme here is that the times when diffusion is the right choice for acoustic treatment are fairly rare at least in home recording land. Stick with absorption, kill the bad stuff, and go back to mixing. SMILIE

    Brandon

  7. RoyJohn Wheelock March 25, 2014 at 1:27 pm

    just you say your studio is located in Missouri, where abouts in Missouri?

  8. This is one of your best quotes I’ve read in the past 2 years !

    “The overall theme here is that the times when diffusion is the right choice for acoustic treatment are fairly rare at least in home recording land. Stick with absorption, kill the bad stuff, and go back to mixing. SMILIE”

  9. Harland Giesbrecht March 25, 2014 at 9:21 pm

    What you want in a home studio for diffusion is a bookshelf(s), ideally a large one with no back to it. Not only does it diffuse, it has some ability to absorb and it has some effective mass. Plus it’s interesting. And it has an aesthetic.

  10. I always love it when someone writes something about diffusion, DIY and getting the biggest bang from your acoustic treatment budget.

    I am completely biased, I make diffusers for $. I spent quite a bit of time studying diffusion and scattering, testing methodologies, doing real-world listening testing and trying to wade through all of the confusing material that is available out there.

    Rectilinear QRDs, especially with wide wells, and especially in the vertical position are particularly prone to yucky sounding artifacts if you get too close. Ethan Winer has a great video showing QRD artifacts. (Around 5:30–> https://www.youtube.com/watch?v=4nzmBhkR4JQ)

    Using smaller wells, like in BBC or Skyline style diffusers, or angling the rear of the wells helps a lot. I have done listening tests that are even sounding with excellent imaging even at 3 feet away from a foot deep alternative diffusers. Running the diffusers horizontal make the artifacts less noticeable too, because we tend to move our heads side to side and not up and down.

    Your right about cost/benefit. For most people that are hobbyists or recording musicians, diffusion isn’t cost effective. They should stick to a nice pair of headphones for critical listening and use monitors for panning and imaging. I would caution over doing it with absorption.

    However, people that really care about getting the most accurate sound quality for critical listening aren’t going to be satisfied with a room over-treated with absorption. A dead room doesn’t sound like rooms in which people listen. Engineers working with dead rooms tend to exaggerate high frequencies, add too much reverb and slap delays-essentially they have a translation problem.

    Non-flat high surface area rear walls of studios are great. Diffusers are a good way of getting a non-flat high surface area rear wall. You get some sense of space and a more natural sound to the room. Use wide-bandwidth absorbers in the corners and in early reflection points. Try out a set of diffusers behind your mix position and listen to spatial cues and imaging. If it doesn’t make a difference that you like, you can return them or, if you DIYed it, burn them for kindling.

    There are very few things better than a metal garbage can fire in the backyard! It would be an awesome photo op.

  11. thanks for saving me the time and money..

  12. Hey Brandon and all you guys.

    No shit about that, Brandon!
    And what about G. Massenburg’s studio, those at Blackbird you’ve visited once? Remerber that room, full of sticks – instead of “Full of Mirrors” like Hendrix once said?
    Didi you think that that room is a mistake? Because it doesn’t respect this law.

    Hey ho, PL.

  13. Sorry the type errorrrrrrrrsz…

  14. “Don’t make DIY diffusors just because they look pretty.”

    Too late. At least they impress people when the come to record. As do my $6 “Studio Monitors” that I picked up from goodwill and mounted to the wall above my desk :P

  15. I have been at this a while, you know the sound engineering thing, and I have to agree with Brandon to a great degree. If your the owner of a large commercial room, diffusion may make sense. But for the small budget home studio using near fields, absorption goes a long ways. I built absorption panels and one at a time placed them on the walls. I next listened to the changes and then added more until the room was tamed to a great degree. The room never became dead or “oppressive”, simply manageable with less direct reflections. As well a cloud above the mix position can be helpful. My room still has bass management issues that I may or may not address. Why? Because I’ve learned the room, I know it’s deficiencies and have adjusted my mixing to deal with them. In a perfect world I wouldn’t have to, but then I’ve never had anyone call my mixes low end deficient and there’s been a hell of a lot of great recordings made in a hell of a lot worse conditions than a studio like mine. The bottom line is, tame it up as best you can and get on to making music.

  16. Doug Hazelrigg March 28, 2014 at 4:19 pm

    Personally, I think the word “oppressive” is pretty accurate concerning my reaction to a small room with too much absorption. In any case, isn’t room treatment about choices and personal preference? I just prefer to have a back wall that has a good deal of diffusion. I would agree 100% that the investment in time and money may not be proportional to the benefit, unless you’re really finicky about the benefit.

  17. Personally, I think the word “oppressive” is pretty accurate concerning my reaction to a small room with too much absorption. In any case, isn’t room treatment about choices and personal preference?

    I think a bunch of kids are going to get confused. They’ll leave a really bad sounding room live just because they are afraid of this “oppressive” thing. When a room sucks (a laymen will know based on what the monitors are telling them), you’ve got to get that bad stuff out. Diffusion is the icing on a cake that is rotten. In my view, diffusion won’t get far in fixing a bad room and I’d imagine that’s what 99% of the home recording world is fighting through.

    The reason I brought up the minimum distance issue to a diffusor is fairly straight forward. If your room ain’t big enough to get the diffusion results you want WITH the minimum distance required, the room simply isn’t big enough for diffusion. That’s it. If the room is big enough for diffusion, a person is still going to want a shit load of absorption if any mixing will be done in the room.

  18. What you want in a home studio for diffusion is a bookshelf(s), ideally a large one with no back to it. Not only does it diffuse, it has some ability to absorb and it has some effective mass. Plus it’s interesting. And it has an aesthetic.

    I’ve never seen any evidence that a book shelf can actually diffuse sound at least when using the formal definition.

  19. Harland Giesbrecht March 28, 2014 at 6:19 pm

    From Wiki:
    ” Compared to a reflective surface, which will cause most of the energy to be reflected off at an angle equal to the angle of incidence, a diffusor will cause the sound energy to be radiated in many directions, hence leading to a more diffusive acoustic space.”

    Compared to a wall, a bookcase, especially one as I described, will do this to a great degree. It did in my room, anyway. ymmv

  20. I have seen photos of QRD diffusors behind acoustic instruments as they are being recorded. Does anyone have any experience (or even just opinions) about using diffusors in a tracking room? We built two diffusors several years ago, but moved them out of the control room into the tracking room on the advice of the John Sayers forum. Now they just stand there looking pretty :) I have tried moving them around while recording, but I wasn’t really sure what I was doing.
    (Our tracking room is 28x20x14)

  21. Does anyone have any experience (or even just opinions) about using diffusors in a tracking room?

    Yes, the results are the same. If you are too close to them they don’t do anything. All concepts in this article pertain to tracking rooms and control rooms.

    You’ll find that the visual appear of diffusion is radically more pronounced than any sonic appeal. I’m scratching my head looking for times when a reflection is “bad”, but absorption would be “bad”, too. In either case, the result is a mostly-neutralized reflection.

    Brandon

  22. Fair enough, so what I am getting is that for a typical small room project studio just spread around the absorption until it sounds good and then deal with the lows with trapping and get on with the mixing. Makes perfect sense, most existing structure rooms aren’t anywhere near ideal acoustically so as you say diffusion is like icing on top of a rotten cake.

    But how about in a properly designed purpose built control room?
    Say a room with an outer isolation shell of Louden 1:1.40:1.90 dimensions that is 11′ x 15.4′ x 20.9′ and an inner reflection control shell with splayed walls and bass trapping shaped to create an LEDE environment with a reflection free zone at the mixing position.

    Do you think a Quadratic diffuser would make sense in that case or just save the cash and toss up some strategic absorption on that back wall?

  23. Harland Giesbrecht April 25, 2014 at 10:22 am

    You’ll never really know until you put one in there and hear it. If you are purpose designed for LEDE then it makes more sense to have a diffuser on the LE than absorption.

  24. But how about in a properly designed purpose built control room?

    If the room was designed without diffusors, I certainly wouldn’t want to mess with what the original designer had in mind.

    I think people drift towards diffusion because they are looking “life” in their room as opposed to a “dead” room. This is a problem with semantics, I think. A room without significant reflections is not “dead” (lifeless/bad sounding). It’s just not ambient. If we want to increase the ambiance in a room (not something I’d gravitate towards too much in a control room), diffusion isn’t really the way to do that, either, necessarily. I’m probably repeating myself, but in Phillip Newell’s Recording Studio Design http://www.recordingreview.com/blog/recording-equipment-reviews/recording-studio-design-review/ he mentions that if you want to liven up a room, the new hard material you add needs to cover at least 25% of the room to be effective.

    The trickiest part is diffusion doesn’t necessarily increase the ambiance in a room, either. In my experience, diffusion reduces the effective ambiance of a room. It’s my rudimentary understanding that busting a direct reflection up into hundreds or thousands of pieces makes them more subtle at least in terms of human perception. This is probably not too away from adding a bunch of stuff to the room (much of it not so absorbing) and the room still reduces in ambiance.

    I’m trying to think of the times when we need to bust up a hard reflection into a bunch of little pieces, but don’t want to absorb. I’m at a loss particularly absorbing that one particular reflection is going to pay dividends in absorbing the bass mess in most rooms.

    Brandon

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