QRT Diffusers - what effect at close range?

[Riddler]

Hello,
I've read some really quite conflicting reports on the effect of QRT diffusors at close range. Some people say that they cause a comb filtering effect, so one needs to use them in a big enough studio to keep a distance away from them, and others argue that this is rubbish. I have seen diffusers used in small booths at commercial studios where it would be impossible to get any distance away from them.

I realise that asking this question here may just shove the issue into further confusion but hopefully someone might be able to provide some more useful information than I've seen so far!

Thanks,
Tim.

[Glenn Kuras]

first up QRDs are awesome, but 2 things.

1)They have to be deep to pick up more of the low mids
2)You have to be a good 5 to 6 feet from them to get the true effect.

Here is a link on how diffusion works. It may clear up a few things for you.
GIK Acoustics

I also emailed our diffusion expert, Jason, to see if you could chime in to help you out a little also.

Glenn

[Jason Jones]

The issue with QRD diffusers is that they work by reflecting several sound waves back in such a way that they will interfere with each other in a controlled manner. The result of this controlled interference is a reflection that is:

1) Equal in frequency to the sound being reflected.
2) Equal in intensity in all directions.
3) Broken up into several "smaller" waves, one after the other.



It takes some distance for this "controlled interference" to take place and produce the final desired result. The word on the street is that this is accomplished pretty much completely at 3 times the wavelength.

So for a diffuser that is designed to work down to about 565 hz, you would want to be 6 feet away.

Jason

[Riddler]

Thanks for the response, I'd read about the 3x wavelength rule before but also read some conflicting views.

So just to clarify, if you are capturing frequencies down to 500Hz you should be at least 2m away from a diffuser and if you are capturing frequencies down to 1000Hz you should be at least 1m away.

Does this mean that you can use a diffuser with wells of a relatively shallow depth and short width for taming higher frequencies in a smaller room? Would this help to lower RT60 without over deadening the room?

Also, what is the exact effect inside 3x the wavelength (before the sound wave has completely developed)?

Thanks for your help!
Tim. thumbsup

[Ethan Winer]

Originally Posted by Riddler

I've read some really quite conflicting reports on the effect of QRT diffusors at close range. Some people say that they cause a comb filtering effect, so one needs to use them in a big enough studio to keep a distance away from them, and others argue that this is rubbish. I have seen diffusers used in small booths at commercial studios where it would be impossible to get any distance away from them.

Tim, the conventional wisdom is as Jason explained, but I have my own theories.

I agree with the rules for distance versus well depth, and the best way to learn more is to get a real QRD and experiment yourself. I've done that with my standard test of "talking into" the front face, and I noticed a bunch of resonant reflections from each of the wells. These reflections sound sort of like a wah-wah pedal left in one position, or a parametric EQ set to boost the midrange with a high Q. When you get farther away the resonances tend to combine and you don't notice each resonance individually so much. However, a QRD diffusor that's "too close" still sounds a hell of a lot better than a bare reflecting wall.

Would this help to lower RT60 without over deadening the room?

I think you're missing something important here. The whole point of absorption is to lower the RT60 when needed, and the whole point of a diffusor is to avoid flutter echo and reduce comb filtering when you don't want to lower the RT60. Lowering RT60 and deadening the room are one and the same. Absorption and diffusion both avoid flutter echo and comb filtering, and which you choose depends directly on whether you also want to lower the RT60.

--Ethan

[Riddler]

Hi Ethan, thanks for posting! I've been reading your website for the last couple of days and it's been really helpful. thumbsup

Lowering RT60 and deadening the room are one and the same. Absorption and diffusion both avoid flutter echo and comb filtering, and which you choose depends directly on whether you also want to lower the RT60

Sorry - I do sound stupid now I'm looking at it! You better expressed what I actually want to achieve which is a well controlled yet reasonably live space.

What I really need to know is where does flutter echo occur; at all frequencies or is the frequency related to the distance between parallel surfaces? If a diffuser works down to 1000Hz (therefore only lose 1m of space to this "wah" effect) and is used in conjunction with panel traps will this reduce flutter but keep the room reasonably live?

I'd like to experiment with some QRD diffusers but they are a little expensive to buy just for testing purposes!

Thanks,
Tim.

[Jason Jones]

Originally Posted by Riddler

What I really need to know is where does flutter echo occur; at all frequencies or is the frequency related to the distance between parallel surfaces? If a diffuser works down to 1000Hz (therefore only lose 1m of space to this "wah" effect) and is used in conjunction with panel traps will this reduce flutter but keep the room reasonably live?

I'd like to experiment with some QRD diffusers but they are a little expensive to buy just for testing purposes!

Thanks,
Tim.

That can be a reasonable compromise in some situations. I would definitely avoid a 1Khz and up diffuser at any early reflection points, though. What are the specifics of the room you are treating?

Jason

[Ethan Winer]

Tim,

Originally Posted by Riddler

You better expressed what I actually want to achieve

Hey, that's my job!

What I really need to know is where does flutter echo occur; at all frequencies or is the frequency related to the distance between parallel surfaces?

Flutter echo is definitely related to the wall-wall spacing. In fact, it's exactly the same as a room mode, but acts mainly at the mode's multiples (harmonics) since those "multiples" frequencies are what get excited when you clap your hands or whack a snare drum etc.

If a diffuser works down to 1000Hz (therefore only lose 1m of space to this "wah" effect) and is used in conjunction with panel traps will this reduce flutter but keep the room reasonably live?

Yes, that's the whole point. A diffusor avoids flutter echo and comb filtering without having to resort to absorption which also makes a room more dead sounding. However, I agree with Jason that diffusion is not appropriate at reflection points in most rooms. Anywhere else is a potential candidate though.

--Ethan

[Riddler]

I would definitely avoid a 1Khz and up diffuser at any early reflection points, though.

Is there a specific way to work out early reflection points? Does it not depend on the location of the sound source(s)?

I know my room doesn't have the best ratio (1.00:1.68:2.38) dimensions 8.2' x 13.8' x 19.5' but I understand that it could be worse.

I've worked out the axial modes as below.

Length Width Height
Frequency Frequency Frequency
28.90 40.86 68.80
57.81 81.71 137.60
86.72 122.57 206.40
115.63 163.42 275.20
144.54 204.27 344.00
173.45 245.13 412.80
202.35 285.99 481.60
231.26 326.84 550.40
260.17 367.70 619.20

If the accuracy of these results can be trusted (they were calculated using an online tool ) then it looks as though all modes are evenly spaced. Are tangental and oblique modes a consideration or are they too weak to worry about? Is it true that modal problems above 300Hz are rare?

Up until recently I have been using acoustic flats made from pegboard with rockwool on the inside. It was surprising how well they worked, and is particularly apparent now they have been removed and the room is bare. I've decided that since this room is purely for recording sounds, it's time to install something a bit more permanent.

I hope you can appreciate why I don't want to lose 6ft around the edge of the studio while I wait for 3x 500Hz to develop! (If I can possibly avoid it that is!)

Thanks again for the help from you guys, it really is appreciated,
Tim.

[Jason Jones]

The location of the early reflection points can be found by putting a mirror along the side wall, back wall or ceiling. Where you can see the sound source in the mirror from your seating position - that is the early reflection.

As to room modes, I would strongly recommend tearing down the entire building and starting from scratch! Really don't worry about them. You do need to treat the corners with bass trapping. Short of cranes and wrecking balls that is the usual best course.

What are you using the room for? You have enough room to use some good diffusers on the back walls and back side-walls if you want...unless you are trying to make it into a 30 seat theater or something.

Jason

[jayfrigo]

It's certainly not rubbish. It is dependent on frequency as has been observed already, and there may still be some beneficial effect if you are somewhat closer than 3 wavelengths, but you clearly don't want to be extremely close. There are other methods and designs to deal with problems up close. These may include different diffusor designs if you don't want to absorb mids and highs, coupled with low-mid to bass absorption, or even reflection using changes in geometry to aim reflections away from your listening position and into areas of trapping.

There are no people more expert than Cox and D'Antonio when it comes to diffusion. They have the most experience with design, implimentation, testing, and even new math and methodolgy for evaluation. They can show you measurement plots of the differences among different diffusor types and at different distances. It's proven scientifically both theoretically and through observation and objective measurement.

Still, this does not mean that you would never use any kind of QRD in any application where you have limited space. You simply must use the right design for your task and space. And there are reasons beyond the 3 wavelength rule as to why it is preferable to have 10 to 11 feet behind you, whether or not you have diffusion on the back wall. The ITD gap (Initial Time Delay) is one such reason.

BTW, note it is QRD, not QRT.

Originally Posted by Riddler

Hello,
I've read some really quite conflicting reports on the effect of QRT diffusors at close range. Some people say that they cause a comb filtering effect, so one needs to use them in a big enough studio to keep a distance away from them, and others argue that this is rubbish. I have seen diffusers used in small booths at commercial studios where it would be impossible to get any distance away from them.

I realise that asking this question here may just shove the issue into further confusion but hopefully someone might be able to provide some more useful information than I've seen so far!

Thanks,
Tim.

[Jason Jones]

Originally Posted by jayfrigo

And there are reasons beyond the 3 wavelength rule as to why it is preferable to have 10 to 11 feet behind you, whether or not you have diffusion on the back wall. The ITD gap (Initial Time Delay) is one such reason.

I am unaware of any empirical work that indicates that the time delay by itself is relevant. The combination of delay, intensity and location all have been shown empirically to make a difference.

I may be missing something but, all I have seen regarding time as an independent variable is based upon extrapolations from research that is not quite matched to our situations. I have seen nothing verifying that it is relevant to typical audio situations. Have you seen such?

Jason

[Ethan Winer]

Originally Posted by Riddler

Is there a specific way to work out early reflection points? Does it not depend on the location of the sound source(s)?

Yes, look here:

RealTraps - Creating a Reflection-Free Zone

And here:

RealTraps - How To Set Up a Room

If the accuracy of these results can be trusted (they were calculated using an online tool )

And here:

RealTraps - ModeCalc

Are tangental and oblique modes a consideration or are they too weak to worry about? Is it true that modal problems above 300Hz are rare?

Yes and Yes. Further, it doesn't really matter what mode frequencies you calculate or measure because the solution is always the same regardless - as much bass trapping as you can possible manage.

--Ethan

[Glenn Kuras]

Jay,

It was great to meet you at AES. You are one hell of a smart guy and LOVE THE STUDIO on your web site. The best word I can think of is "WOW".

[jayfrigo]

Originally Posted by Jason Jones

I am unaware of any empirical work that indicates that the time delay by itself is relevant. The combination of delay, intensity and location all have been shown empirically to make a difference.

If there's one thing common to acoustics and my posts on the subject, it is that nothing can be taken "by itself." It all works together. But that doesn't mean you ignore the individual elements; simply that you integrate all into the system.

As for any evidence for the ITD gap, I'd imagine I've just seen the same stuff as you. There was Leo Beranek's work decades ago, and the later point about needing to increase the gap when your control room is smaller than your recording room (pretty common), and delay of early reflections as it pertains to RFZ etc...

The simplified point here is that you don't want to be right on top of a boundary, whether it has diffusion applied or not.

[jayfrigo]

Originally Posted by myfipie

Jay,

It was great to meet you at AES. You are one hell of a smart guy and LOVE THE STUDIO on your web site. The best word I can think of is "WOW".

The pleasure was mine. I'm glad we had time to chat at the "gearslutz lounge." Thanks for the compliment. Next time I'm in ATL (family and in-law visiting permitting) I'll look you up.

[Glenn Kuras]

Originally Posted by jayfrigo

The pleasure was mine. I'm glad we had time to chat at the "gearslutz lounge." Thanks for the compliment. Next time I'm in ATL (family and in-law visiting permitting) I'll look you up.

I am going to hold you to that!!!

Glenn

[Jason Jones]

Originally Posted by jayfrigo

If there's one thing common to acoustics and my posts on the subject, it is that nothing can be taken "by itself." It all works together. But that doesn't mean you ignore the individual elements; simply that you integrate all into the system.

As for any evidence for the ITD gap, I'd imagine I've just seen the same stuff as you. There was Leo Beranek's work decades ago, and the later point about needing to increase the gap when your control room is smaller than your recording room (pretty common), and delay of early reflections as it pertains to RFZ etc...

The simplified point here is that you don't want to be right on top of a boundary, whether it has diffusion applied or not.

Definitely room acoustics is the result of many factors "taken together". But, I mean something slightly different. I have read some of the older work where they thought that a small ITD gap in a control room would mask the larger ITD gap from the recording studio.

However, in all of the empirical research I have seen (Hass, Meyer and Schodder, Lochner and Berger, Olive and Toole), the effect is related to the time plus intensity as a unit. So, just time by itself would literally be meaningless. An imperfect analogy would be if I said that this morning I drank 7 coffee. We would need to add ounces or cups (or maybe even pots in my case ).

I hope my explanation makes some sense. Just wondering if there was something out there I had missed.

Jason

[jayfrigo]

Originally Posted by Jason Jones

Definitely room acoustics is the result of many factors "taken together". But, I mean something slightly different. I have read some of the older work where they thought that a small ITD gap in a control room would mask the larger ITD gap from the recording studio.

However, in all of the empirical research I have seen (Hass, Meyer and Schodder, Lochner and Berger, Olive and Toole), the effect is related to the time plus intensity as a unit. So, just time by itself would literally be meaningless. An imperfect analogy would be if I said that this morning I drank 7 coffee. We would need to add ounces or cups (or maybe even pots in my case ).

I hope my explanation makes some sense. Just wondering if there was something out there I had missed.

Jason

Sorry I disappeared for a bit; didn't want you to think I was ignoring you...

Intensity is undeniably linked to time at the hip (and frequency can be a significant variable too according to Schodder, Olive & Toole etc.). To take your analogy, in this thread I'm just trying to say I had some coffee, and not even mentioning the 7, therefore never getting all the way to cups or ounces. Just keeping it practical insofar as the thread is concerned; since we're talking about very close side walls, or even rear walls, in a standard rectangular and untreated room, the reflections are in all probability going to be of sufficient intensity to cause interference to a large enough degree to care about.

I'm not really trying to rehash the history of the Hidley/Westlake room and later LEDE certified rooms (not the newer, looser use of the term LEDE for any RFZ room with a diffusor in back, but original Chips Davis LEDE rooms) which both have been superceded since their shortcomings became known, which would take too long to describe here. Their use of ITD gap certainly is one thing that wasn't quite right, though they both did take important steps in the right direction, especially considering what came before. Westlake rooms also had some reflective sides toward the front that caused trouble the way they were done at the time, and don't even get me started on compression ceilings!

The intended point was only that the 3 wavelength guideline for diffusors is not the sole reason to get a little distance from the room boundaries; but I haven't seen anything new on ITD that would make me discount the contributions of Hass, Schodder, or Olive and Toole, or to make me isolate time from intensity. Boers also has some interesting things to say on the subject. Then again, I certainly can't read every paper that comes out, so if you see something interesting, let me know!