If sound hits a diffusor....like an RPG one... something that diffuses well, if those sound waves are scattered, wouldn't comb filtering be prevented no matter how close the source?
I understand that the sound wave might be so powerful that it bounces through the diffusor, reflects off the flat wall and still bounces straight back causing frequency nulls or peaks. But what about the human voice or an acoustic guitar which have weaker sound waves compared to a kick drum or even cymbals. So then even though the mic might be close to the diffusor, the sound should have diffused somewhat..
If the capsule of the microphone is 1 inch. How much space do you really need to avoid the same sound waves coming back into the microphone without being sent in different directions??
What if you have an instrument 2 feet off the floor (a drum for example). you have 7 feet above it to the ceiling. On the ceiling you have a diffusor and you have a cardiod mic (back facing the diffusor by 1 foot), would this still not benefit from the diffusion?
Even if the distance was required for the mic.... wouldn't the source being the correct distance away (even though the mic is close to the surface) still benefit since sound that you would hear in the room should be more natural and have that ambience?
Sorry if i'm not making much sense...i'm a bit confused about this topic... any help?
Can anyone explain why diffusors don't work at close range?
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TomM
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Ethan Winer
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Re: Can anyone explain why diffusors don't work at close ran
Tom,
> wouldn't comb filtering be prevented no matter how close the source? <
I'm not a diffusion expert, but I have observed this: All the small cavities in a QRD diffusor resonate, so when you're close it can sound like a bunch of boosted EQ peaks set with a high Q. As you get farther away the individual resonances average together. Or something like that.
--Ethan
> wouldn't comb filtering be prevented no matter how close the source? <
I'm not a diffusion expert, but I have observed this: All the small cavities in a QRD diffusor resonate, so when you're close it can sound like a bunch of boosted EQ peaks set with a high Q. As you get farther away the individual resonances average together. Or something like that.
--Ethan
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drfrankencopter
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The problem is that for diffusion to work, you have to perceive all those individual reflections as a group of diffuse echoes. When you get too close you actually hear the discrete echoes themselves as opposed to a grouping of diffuse sound.
The closer you get to a diffuser, the less discrete echoes you end up hearing, partly a result of the inverse square law, and party a result of the inability of the diffuser to scatter sound at a wide enough angle. If you're say 10 feet from a diffuser, your ear will hear returns from a wide field of vew (i.e. it 'sees' much of the wall/diffuser and gets lots of echoes, thus the reverb field is diffuse). When you're closer to the wall, your ear sees less of the wall, and gets far fewer discrete echoes in the reverb field and it starts to sound odd.
Like Ethan, I've heard this effect myself, and it does indeed sound a little like combfiltering...hard to describe it though.
Cheers,
Kris
The closer you get to a diffuser, the less discrete echoes you end up hearing, partly a result of the inverse square law, and party a result of the inability of the diffuser to scatter sound at a wide enough angle. If you're say 10 feet from a diffuser, your ear will hear returns from a wide field of vew (i.e. it 'sees' much of the wall/diffuser and gets lots of echoes, thus the reverb field is diffuse). When you're closer to the wall, your ear sees less of the wall, and gets far fewer discrete echoes in the reverb field and it starts to sound odd.
Like Ethan, I've heard this effect myself, and it does indeed sound a little like combfiltering...hard to describe it though.
Cheers,
Kris
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TomM
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AVare
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First one easy. More than three wavelenghts of the lowesst frequency diffused. Yes the lowest frequency is another can of worms!TomM wrote:What is the ideal distance from a diffusor? and does it vary per model/make?
What distance should the instrument be, what distance should the microphone be..... or if mixing ...what should be the distance between your head and the diffusors??
For the second paragraph, it is subjective. What ever sounds good.
Andre
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dcharrison
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Diffusion at close range
There is a simple explanation of why some diffusors DO work at close range and why some DON'T.
Diffraction grating diffusors like those based on quadratic residues or other math esoterics are designed to break down the sound into specific frequency bands and to delay those frequency bands individually at random intervals. The reflections are, by design, phase incoherent. (This is basically the same thing as crossover distortion.) This may work for some when it comes to listening room diffusion, but you would never want to hear it in your microphone.
Polycylindrical diffusors are designed to produce time aligned reflections across a broad spectrum and thus are phase coherent and can be used at close range. In fact, there is a whole cult of producers, engineers and musicians who have made an art of this. The technique is called the Quick Sound Field ™ and was commercialized by Acoustic Sciences Corp. http://www.asc-studio-acoustics.com/
You probably have seen the ASC Tube Traps in hi-fi showrooms around the country, but they have also earned a home in some of the most prestigious recording studios on the planet. Pete Townsend, Bruce Swedien, and Ed McMahon swear by them and Jennifer Lopez actually gave Art Noxon and Acoustic Sciences Corp. album credit!
The Quick Sound Field relies on a psychoacoustic phenomenon called the Haas effect. According to the research conducted by Mr. Haas, sound reflections that reach the listener within the first 30 milliseconds after the direct sound are perceived to be part of the initial sound.
By arranging a series of reflective cylindrical diffusion columns in an arc around the sound source and separating them by a foot or so, multiple reflections reach the mic within the first 30 milliseconds, but are quickly attenuated as they leak out through the gaps. If you had only one reflection like this, you would get comb filtering and the sound would be thin and undesirable. If you get 40 or 50 reflections in this first 30 seconds, they are summed by the brain into one really fat sound.
The backsides of the QSF Tube Traps are absorptive so that any reflections coming back from the room are severely dampened. I recently recorded an acoustic guitar in this manner and was blown away by the effect. Mic placement is less critical so the guitarist can move around without drastically changing the sound, I was never even tempted to use an equalizer and when I laid in reverb, the reflections in the verb came back articulate and clean because they were acting on the sound of the guitar and not the echoes in the room.
Had this been done with Quadratic Diffusors, I suspect that the results would have been unusable.
Diffraction grating diffusors like those based on quadratic residues or other math esoterics are designed to break down the sound into specific frequency bands and to delay those frequency bands individually at random intervals. The reflections are, by design, phase incoherent. (This is basically the same thing as crossover distortion.) This may work for some when it comes to listening room diffusion, but you would never want to hear it in your microphone.
Polycylindrical diffusors are designed to produce time aligned reflections across a broad spectrum and thus are phase coherent and can be used at close range. In fact, there is a whole cult of producers, engineers and musicians who have made an art of this. The technique is called the Quick Sound Field ™ and was commercialized by Acoustic Sciences Corp. http://www.asc-studio-acoustics.com/
You probably have seen the ASC Tube Traps in hi-fi showrooms around the country, but they have also earned a home in some of the most prestigious recording studios on the planet. Pete Townsend, Bruce Swedien, and Ed McMahon swear by them and Jennifer Lopez actually gave Art Noxon and Acoustic Sciences Corp. album credit!
The Quick Sound Field relies on a psychoacoustic phenomenon called the Haas effect. According to the research conducted by Mr. Haas, sound reflections that reach the listener within the first 30 milliseconds after the direct sound are perceived to be part of the initial sound.
By arranging a series of reflective cylindrical diffusion columns in an arc around the sound source and separating them by a foot or so, multiple reflections reach the mic within the first 30 milliseconds, but are quickly attenuated as they leak out through the gaps. If you had only one reflection like this, you would get comb filtering and the sound would be thin and undesirable. If you get 40 or 50 reflections in this first 30 seconds, they are summed by the brain into one really fat sound.
The backsides of the QSF Tube Traps are absorptive so that any reflections coming back from the room are severely dampened. I recently recorded an acoustic guitar in this manner and was blown away by the effect. Mic placement is less critical so the guitarist can move around without drastically changing the sound, I was never even tempted to use an equalizer and when I laid in reverb, the reflections in the verb came back articulate and clean because they were acting on the sound of the guitar and not the echoes in the room.
Had this been done with Quadratic Diffusors, I suspect that the results would have been unusable.
David C Harrison
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lovecow
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TomM,
If you're really interested in the science of diffusion, you should check out the new book by Cox and D'Antonio, Acoustic Absorbers and Diffusers: Theory Application and Design. Pricey, but worth it, IMO, and quite readable despite a little "heavy" math in some sections. You have to really want to know, though, to put out that kind of dough.
In that book, the entire concept of near-field and far-field effects from 1D and 2D diffusers (their spelling - I prefer "diffusors") is explained in detail, which is what you're asking about. The above explanations are all completely valid. I have recently been doing a bunch of diffusion measurements and the types of effects that are described in the book are completely measureable. You really do want to be in the far-field of diffusors (the 3x wavelength thing André gave above) to get the full diffusive benefit. I often call this "getting your money's worth" from the devices.
Also of note, considering David's post above, is the information given on semicylindrical diffusers. Comb-filtering effects are quite measureable from arrays of semicylinders, even though the dispersion of sound (spatially) can be quite uniform. This may or may not be a desirable by-product of such devices. Usually not, me thinks...
A comment from the book might be worth noting here:
If you're really interested in the science of diffusion, you should check out the new book by Cox and D'Antonio, Acoustic Absorbers and Diffusers: Theory Application and Design. Pricey, but worth it, IMO, and quite readable despite a little "heavy" math in some sections. You have to really want to know, though, to put out that kind of dough.
In that book, the entire concept of near-field and far-field effects from 1D and 2D diffusers (their spelling - I prefer "diffusors") is explained in detail, which is what you're asking about. The above explanations are all completely valid. I have recently been doing a bunch of diffusion measurements and the types of effects that are described in the book are completely measureable. You really do want to be in the far-field of diffusors (the 3x wavelength thing André gave above) to get the full diffusive benefit. I often call this "getting your money's worth" from the devices.
Also of note, considering David's post above, is the information given on semicylindrical diffusers. Comb-filtering effects are quite measureable from arrays of semicylinders, even though the dispersion of sound (spatially) can be quite uniform. This may or may not be a desirable by-product of such devices. Usually not, me thinks...
A comment from the book might be worth noting here:
FWIW!Cox & D'Antonio, "Acoustic Absorbers and Diffusers," Chapter 10 wrote:Certainly, semicylinders are an enigma, they appear to be a near perfect diffuser from dispersion graphs, but they do not sound like a perfect diffuser.
---lovecow---
It is easy enough to be friendly to one's friends. But to befriend the one who regards himself as your enemy is the quintessence of true religion. - Mahatma Gandhi
It is easy enough to be friendly to one's friends. But to befriend the one who regards himself as your enemy is the quintessence of true religion. - Mahatma Gandhi
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Ethan Winer
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Jeff,
> Comb-filtering effects are quite measureable from arrays of semicylinders <
Thanks for posting that, because it confirms what I always assumed. My few experiments with small semis were not very favorable, but I've heard people say large semis can be excellent. Someone on another forum once said large semis are even better than QRDs, which made no sense to me. Your point about comb filtering versus dispersion explains the issue exactly.
--Ethan
> Comb-filtering effects are quite measureable from arrays of semicylinders <
Thanks for posting that, because it confirms what I always assumed. My few experiments with small semis were not very favorable, but I've heard people say large semis can be excellent. Someone on another forum once said large semis are even better than QRDs, which made no sense to me. Your point about comb filtering versus dispersion explains the issue exactly.
--Ethan
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alejandro
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Diffusers, near field, distortion, cilinders and more...
I'll try to be as short as possible.
There are several phenomenons going on here.
1st, youre talking about QRDs, probably a low length sequence. The lower the sequence the more susceptible to lobing the diffuser array is... And the Schroeder diffuser works only for some discrete frequencies (there is no continuous diffusion spectrum)... And the diffusion it brings (just a simple sequence) is defined as same energy in the diffraction lobes, ... And for non diffusion frequencies you'll get specular reflection (and comb filtering). I repeat: Just for low length sequence numerical diffusers.
2nd, youre hearing some resonance near this diffuser. This is wrigth!, the cells resonate and this is distortion (but not comb filtering)!. Some designs minimize this by eliminating the cell dividers. But certainly you'll hear this if you're VERY close to the diffuser.
3rd, youre talking about cilinders too. this is a unit that diffuse in space only. An array of several cilindres shows lobing too.
4th, youre forgetting the "time spreading" of the energy. Cylinders have no time spreading, so the wavefront is unique. This is why I call them "non optimal diffusers".
5th, today there are MUCH better developments than the "simple schroeder sequence", with minimized resonances and lobing and extended bandwidth. For example, I developed a new kind of diffuser mixing the advantages of numerical and cylinder diffusers.
6th, You'll have to design with diffusers primarily for sound energy decorrelation in the "early time field" (improving ASW) and / or in the "late time field" (improving LEV).
7th It has been shown that diffusers that work ok in the far field they do the same in the near field, but please, use improved designs of the initial number theory sequences, not just the early designs if you're going to use them very professionaly.
Hope I could help.
Kind regards.
Alejandro Bidondo
There are several phenomenons going on here.
1st, youre talking about QRDs, probably a low length sequence. The lower the sequence the more susceptible to lobing the diffuser array is... And the Schroeder diffuser works only for some discrete frequencies (there is no continuous diffusion spectrum)... And the diffusion it brings (just a simple sequence) is defined as same energy in the diffraction lobes, ... And for non diffusion frequencies you'll get specular reflection (and comb filtering). I repeat: Just for low length sequence numerical diffusers.
2nd, youre hearing some resonance near this diffuser. This is wrigth!, the cells resonate and this is distortion (but not comb filtering)!. Some designs minimize this by eliminating the cell dividers. But certainly you'll hear this if you're VERY close to the diffuser.
3rd, youre talking about cilinders too. this is a unit that diffuse in space only. An array of several cilindres shows lobing too.
4th, youre forgetting the "time spreading" of the energy. Cylinders have no time spreading, so the wavefront is unique. This is why I call them "non optimal diffusers".
5th, today there are MUCH better developments than the "simple schroeder sequence", with minimized resonances and lobing and extended bandwidth. For example, I developed a new kind of diffuser mixing the advantages of numerical and cylinder diffusers.
6th, You'll have to design with diffusers primarily for sound energy decorrelation in the "early time field" (improving ASW) and / or in the "late time field" (improving LEV).
7th It has been shown that diffusers that work ok in the far field they do the same in the near field, but please, use improved designs of the initial number theory sequences, not just the early designs if you're going to use them very professionaly.
Hope I could help.
Kind regards.
Alejandro Bidondo
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dcharrison
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dcharrison
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...also, does Massenburg wear protective eyewear in there?
IMHO, there is a fatal flaw there. Surely the diffusion field will be drastically altered as the musicians cram chewing gum, cigarette butts and broken guitar picks into the cracks and crevices.
He should have covered it with chickenwire.
IMHO, there is a fatal flaw there. Surely the diffusion field will be drastically altered as the musicians cram chewing gum, cigarette butts and broken guitar picks into the cracks and crevices.
He should have covered it with chickenwire.
David C Harrison
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Pennywizz6
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AVare
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No. Diffusionis provide an "air" or ambience. Correlated signals provide an echo.dcharrison wrote:Would you agree that diffusion schemes based on frequency specific delays inherently lack phase coherency and are thus unsuitable for applications where the diffuse reflection is intended to be part of the recorded signal?
Andre
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alejandro
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about:
"Would you agree that diffusion schemes based on frequency specific delays inherently lack phase coherency (THIS IS THE DECORRELATION PROCESS GENESIS) and are thus unsuitable (THEY ARE VERY SUITABLE) for applications where the diffuse reflection is intended to be part of the recorded signal? (THE STEREO RECORDED SIGNAL GAINS QUALITY IF IT HAS A HIGH DEGREE OF BINAURAL DECORRELATION)"
The only way of adding binaural decorrelation is by means of lateral diffusers in a stereo miking. If you are doing a mono miking the diffusers will complete the natural decay of the instrument's sound so it will be enriched.
Conclusion: capturing Binaural Decorrelation is equivalent to more sound quality (ever). This one of the variables of ASW and LEV (if they are positioned correctly in time).
Kind regards,
Alejandro Bidondo
"Would you agree that diffusion schemes based on frequency specific delays inherently lack phase coherency (THIS IS THE DECORRELATION PROCESS GENESIS) and are thus unsuitable (THEY ARE VERY SUITABLE) for applications where the diffuse reflection is intended to be part of the recorded signal? (THE STEREO RECORDED SIGNAL GAINS QUALITY IF IT HAS A HIGH DEGREE OF BINAURAL DECORRELATION)"
The only way of adding binaural decorrelation is by means of lateral diffusers in a stereo miking. If you are doing a mono miking the diffusers will complete the natural decay of the instrument's sound so it will be enriched.
Conclusion: capturing Binaural Decorrelation is equivalent to more sound quality (ever). This one of the variables of ASW and LEV (if they are positioned correctly in time).
Kind regards,
Alejandro Bidondo