Sonotube Diffusor

[bignick]

Hey guys -

Finishing up the treatment of a new tracking room and I'm having an echo issue in certain spots. My treatments so far (room is 20L'x17W'x10H'):

-(7) 4'x6'x4" bass traps/absorbers
-(8) 18"x5' absorbers
-Acoustic tile dropped ceiling
-4'x12' poly diffusor on front wall

Everything sounds good except for a few spots where there are parallel walls that aren't treated (ie, spacing between traps on adjacent sides). I was thinking - instead of going through the trouble of building a bunch of shadowbox poly's or adding more absorbers (which I don't want to do, the room is sounding good aside from the few parallel walls spots), would half a Sonotube work? Looking around the net, I found this site with got me thinking:

ACS Articles - The Sono Tube Fix!

I know half a circle isn't as optimal as a poly, but for $5 each + some burlap, this seems like a good idea to me. Anyone think otherwise?

-Nick

[avare]

Originally Posted by bignick

I know half a circle isn't as optimal as a poly, but for $5 each + some burlap, this seems like a good idea to me. Anyone think otherwise?

Yes, I don't think it is a good idea, I think it is fantastic! Use the largest you can fit aesthetically, and cut into thirds, not halves. If you thin about the geometry, the parts of the cylinder near the ends when cut in half are reflecting almost nothing. By diffusing the sound, the room will become somewhat deader. Finish the tubes with a paint etc, not burlap if possible.

Why do you think arcs of a circle are not good? With smaller arcs, like 120°, the difference between a circle and a hyperbola is minimal. The the theory also ignores real world diffusion at the relevant wavelengths. Even a hard surface like glass when struck by sound at a non normal angle will have diffraction.

Andre

[Ethan Winer]

I'm not a big fan of poly surfaces as diffusors, but I guess it's better than a bare wall. The problem with a row of polys is that for every convex surface which is good, there's a concave surface between each poly section and that's bad. If you use this I agree with Andre to have them as large as possible, and cut into thirds.

--Ethan

[Musiclab]

you can even buy wood veneer and glue it

[bignick]

Originally Posted by Ethan Winer

I'm not a big fan of poly surfaces as diffusors, but I guess it's better than a bare wall. The problem with a row of polys is that for every convex surface which is good, there's a concave surface between each poly section and that's bad. If you use this I agree with Andre to have them as large as possible, and cut into thirds.

--Ethan

Ethan -

Would the concave surface issue be reduced by making the tubes slightly different depths (ie, 1/3 of one tube, 1/4 of another etc) to make the resulting wells more irregular? Thanks for the replies, I'm gonna give this a shot - I really dig the wood veneer suggestion too.

-Nick

[avare]

Originally Posted by bignick

Would the concave surface issue be reduced by making the tubes slightly different depths (ie, 1/3 of one tube, 1/4 of another etc) to make the resulting wells more irregular?

I do not know what concave surfaces both of you are talking about. The wall between diffusors is flat, not a curved. As far as variation in depth goes, instead of changing the arc of various diffusers, use different size diameters if you are concerned about repeating patterns.

Andre

[avare]

I am trying to figure out what concave surfaces you two are talking about in this room. I made a sketchup drawing with what information is in the first post of how the room might be treated. It is looking into the room from the wall that has diffusors on it. The walls actually have less space for treatment as there is no window or doors. The cylindrical diiffusers are scaled as 120° sections from 24" circles.


Getting confused,
Andre

[bignick]

Originally Posted by avare

I am trying to figure out what concave surfaces you two are talking about in this room. I made a sketchup drawing with what information is in the first post of how the room might be treated. It is looking into the room from the wall that has diffusors on it. The walls actually have less space for treatment as there is no window or doors. The cylindrical diiffusers are scaled as 120° sections from 24" circles.


Getting confused,
Andre

Andre,

Unless I misunderstood, Ethan was referring to the point at which the circles meet when two are placed together.

Your sketch is pretty accurate (), except the backwall has two of the 4'x6' bass traps, adding space between the other traps and absorbers along the parallel walls. Some spots have more space, as I placed them closer together where the drums are placed, so having to use two Sonotubes would be probable - creating the concave surface where they meet.

[avare]

Originally Posted by bignick

Unless I misunderstood, Ethan was referring to the point at which the circles meet when two are placed together.

Now we (or at least I) are getting somewhere. My comments in this thread have been written in regards to your specific room. Not generalizations.

Your sketch is pretty accurate (), except the backwall has two of the 4'x6' bass traps, adding space between the other traps and absorbers along the parallel walls.

Not bad acoustic analysis, if do say say so. Seriously, and this is a complement to the detail, in the information that you did give. It permitted further accurate analysis.

Some spots have more space, as I placed them closer together where the drums are placed, so having to use two Sonotubes would be probable - creating the concave surface where they meet.

How wide are the spaces? A general guideline is that 25% of an area should be covered to prevent flutter echoes, assuming of that the contiguous ares are are not large relative to the wavelength of the frequencies involved. In my sketch, I used a criteria of 33% as a guideline. The diffusors have a width of 21". Using the 25% guideline, it would be good in the middle of an 84" surface. Using my guideline, a 63" surface. From what I can gather of your treatment description, the larger spaces are due to one less absorber. If this is all in one area, the length is about 5'6". A diffusor from 24" diameter would still meet the 33% guideline.

In researching this subject, I found out that Sonotubes are available in up to 60" in diameter. This would provide a 120° diffuser 54" wide. This would also be 1' in depth! If you contemplate such a large poly but are daunted by the depth, you can segment the arc in same manner that Fresnel lens is constructed. The attachment shows a scale drawing of such a diffusor with a maximum depth of 6". If you are concerned about close repetition, and still want full coverage, use bigger Sonotubes, which is pretty well what I wrote in my first post.

With the price of Sonotubes being what they are, poly's can be made very inexpensively. The finish would literally be the most expensive part. With the research your post started, it is obviously an overlooked tool in the DIY acoustic treatment toolkit. Thanks for the lead!

Andre

[Ethan Winer]

Originally Posted by avare

I am trying to figure out what concave surfaces you two are talking about in this room.

This is from a post I made elsewhere last year:

The biggest problem with curved surfaces hasn't been mentioned yet, so I'll do that now. In a small room, which is what most of my posts address, it's not practical to have a single huge piece of bent plywood. Let's say 6 feet high by 12 feet wide, bent so the center sticks out at least two feet from the wall. Even two feet is probably not enough to reduce comb filtering by very much, but let's say it is enough for the sake of this discussion.

So instead of building one huge curved piece you instead have to build a series of smaller "modular" sections. Here's a lame ASCI drawing looking down from the top:

UUUUU

Each outer "U" section is curved, so let's say it deflects adequately. But in between each curved section is a concave area that focuses the sound. This not only negates any benefit from the convex portions, it creates new focusing and resonant areas. Try it, as I've done, and you'll clearly hear the nasal quality imparted by each concave section's resonances.

I'll add that polys are fine for reducing flutter echo, but comb filtering is just as big a problem, if not more so, in small rooms. In my experience polys are not as good at reducing comb filtering as QRD diffusors.

--Ethan

[avare]

Thanks for the clarification Ethan.

As far as depth goes, I posted about segmenting it like a Fresnel lens.

As far as comb filtering goes, poly diffusers reduce the level of the reflection, reducing comb filtering.

As far as temporal diffusion goes, they do nothing.

The OP asked about flutter echo problems, which we have addressed in our posts. Poly-diffusers are another tool in acoustics treatment toolkit. They a tool, appropriate in some situations, not in others.

Andre

[avare]

Originally Posted by Ethan Winer

This is from a post I made elsewhere last year:

I have been trying to understand this for a couple of hours now, with no luck. Diffusers are not effective below the (take your pic: Davis, Shroeder #1, Schroeder #2, 3 modes/Hz) cutoff frequency of a room. Huge polys would not be used in a small room to begin with, if the room is designed by someone who knows what they are doing. For small rooms, such as we are discussing in this forum, this typically is around 200 Hz where modes become dominant in the room response. Working with test results from Schroeder diffusers, the effectiveness of diffusers is about an octave below the theoretical frequency.

For a poly diffuser the low end cut off is defined by length of the chord. Taking 300 Hertz as the design criteria gives a chord length of 3'. A 120° dispersion angle poly with a chord of 3' has a depth of approximately 5".

Your (and as you clearly wrote poor) ASCII diagram also shows the diffusers adjacent to one another. In a properly designed room, they would be spaced out, similar to what I show in post #7 in this thread. Ignoring that, attached is a drawing of 4 diffusors right against one another. Where are these concave surfaces?

Clarification is requested please!

Note: References to properly design are meant for people reading this thread who are not familiar with acoustics, not directed at Ethan in any way, shape or form.

Andre

[Ethan Winer]

Originally Posted by avare

Clarification is requested please!

Looking at the drawing you posted, with a poly diffusor that is rather shallow, there are multiple concave sections between each convex part. These concave sections focus sound, which of course is not desirable. Now, it's true that both the concave and convex parts are shallow, which reduces this effect. But the shallower the convex parts are, the less they "diffuse" and also the less they reduce comb filtering. So the more effective (curved) the polys are, the worse the artifacts and vice versa.

If you "talk into" a wall made this way you'll clearly hear the resonance inside the concave sections, and you'll also hear comb filtering against the convex parts if they're shallow. Even a stack of filled soda cans is sufficient to hear these effects, and is a good "ghetto" test that anyone can do.

If there's enough interest, maybe I should make a video comparing the sound of polys versus QRD diffusors, similar to the diffusor video on my company's Videos page. Let's have a show of hands.

--Ethan

[avare]

Thanks. I am starting to understand what you are trying to say.

Originally Posted by Ethan Winer

Looking at the drawing you posted, with a poly diffusor that is rather shallow,

I think part of the confusion is being shown by the above abbreviated quote. The sections in my drawing are designed to spread the waves through out a 120° arc. If a repeating poly is desired, any wider dispersion angle will result in waves reflecting from one poly, then the adjacent one and back into the room CREATING EXACTLY THE EFFECT YOU DESCRIBE.

This is a result of no acoustic design. I was going to write poor acoustic design, but that implies even a rudimentary knowledge of acoustics on the part of the designer of the space.

What it boils down to is that, like Schroeder diffuser design, you have to know what you are doing. If you don't, the results will not be optimum at best, or worse than nothing at worst. Like most things in acoustics, the last thing it is is intuitive.

Andre

[Jason Jones]

Originally Posted by Ethan Winer

Looking at the drawing you posted, with a poly diffusor that is rather shallow, there are multiple concave sections between each convex part. These concave sections focus sound, which of course is not desirable. Now, it's true that both the concave and convex parts are shallow, which reduces this effect. But the shallower the convex parts are, the less they "diffuse" and also the less they reduce comb filtering. So the more effective (curved) the polys are, the worse the artifacts and vice versa.

If you "talk into" a wall made this way you'll clearly hear the resonance inside the concave sections, and you'll also hear comb filtering against the convex parts if they're shallow. Even a stack of filled soda cans is sufficient to hear these effects, and is a good "ghetto" test that anyone can do.

If there's enough interest, maybe I should make a video comparing the sound of polys versus QRD diffusors, similar to the diffusor video on my company's Videos page. Let's have a show of hands.

--Ethan

The sections between the polys are NOT concave. The portions from both polys coming together are convex. It won't focus the sound like a real concavity. However.......you are right that multiple diffusers side by side will cause problems. This is because a long row of anything is non-random (very much so ).

A long non-random sequence of polys has a lower scattering coefficient than just one. A long non-random sequence of QRD's has the exact same problem....and they have no concavities (pseudo or otherwise).

Second, polys diffuse every bit as well as a QRD. In fact a true semicylinder measures better than every available commercial diffuser (purely on scattering).

However, you need to be further from a poly to use it effectively or it will cause comb filtering. The reason you can be closer to a QRD is because of the slight absorption of a QRD and probably because of the time based diffusion offered by a QRD.

So in small rooms polys are not a good solution: in medium rooms they can definitely work, but probably not for any strong early reflection: and in big rooms, knock yourself out with polys (metaphorically).

Jason

[Glenn Kuras]

However, you need to be further from a poly to use it effectively or it will cause comb filtering. The reason you can be closer to a QRD is because of the slight absorption of a QRD and probably because of the time based diffusion offered by a QRD.

Any good general rule of how far away the poly should be? I know with QRDs 1 foot for every 1" of depth.

Glenn

[Ethan Winer]

Originally Posted by Jason Jones

The sections between the polys are NOT concave. The portions from both polys coming together are convex. It won't focus the sound like a real concavity. However.......you are right that multiple diffusers side by side will cause problems.

Yes, and my calling those sections "concave" was shorthand for the basic "innie" shape. I understand it's not a true concave. But the focusing effect is similar, and even a simple soda can test as I described above will show that.

polys diffuse every bit as well as a QRD. In fact a true semicylinder measures better than every available commercial diffuser (purely on scattering).

It depends on what you measure.

In a large room flutter echo is the main problem, and polys work well enough there. But in small rooms comb filtering from nearby surfaces is a bigger issue, and polys are not as effective as QRD diffusors, nor do polys "sound as good" as QRDs. At least in my opinion.

you need to be further from a poly to use it effectively or it will cause comb filtering.

Yes, this is exactly my point. And in a domestic size home studio room you're close to all surfaces.

and probably because of the time based diffusion offered by a QRD.

No "probably" about it. QRD diffusors reflect on-axis sound at different stepped delay times, versus the continuous and more gradual difference of a curved surface.

So in small rooms polys are not a good solution: in medium rooms they can definitely work, but probably not for any strong early reflection: and in big rooms, knock yourself out with polys (metaphorically).

Good summary, and again this is my main point too.

--Ethan

[Glenn Kuras]

Jason asked me to post this graph for him. Damn he is so special, I OWN THE COMPANY BUT HAVE TO CARRY THE Q CARDS!!!!!!!!

[Jason Jones]

Originally Posted by Glenn Kuras

Jason asked me to post this graph for him. Damn he is so special, I OWN THE COMPANY BUT HAVE TO CARRY THE Q CARDS!!!!!!!!

Here is a graph from the D'Antonio and Cox book (page 252 for those of you following along at home). What we see here is the comparison of the diffusion coefficient of one RPG 7-well QRD diffuser in red and the diffusion coefficient of a long line of the same diffusers in black.

The long line, or array of QRD's performs very poorly. There are no concavities or pseudoconcavities to blame for this. It is simply because there is too much repetition - it is non-random. Note, that these effects do not cause any problems with the time based diffusion - that is still working, but the spreading in space is not. (I hope I am being clear.)

The same effect happens with poly's - heck, even with absorbers....but 0+0+0+0 doesn't cause much of a problem.

I am sure that soda cans sound terrible for this reason and others. Are you following me?

Jason

[Jason Jones]

Originally Posted by Glenn Kuras

Jason asked me to post this graph for him. Damn he is so special, I OWN THE COMPANY BUT HAVE TO CARRY THE Q CARDS!!!!!!!!

BTW, I prefer to be called "differently abled", not "special".

[avare]

Originally Posted by Glenn Kuras

Jason asked me to post this graph for him. Damn he is so special, I OWN THE COMPANY BUT HAVE TO CARRY THE Q CARDS!!!!!!!!

EDIT! I read the graph wrong and Jason politely corrected me two posts down.

Nice graph. What is it supposed to be showing? One line is for a single diffuser, and the other for a repetitive array, with the number of units unspecified. As Jason wrote earlier in this thread, any diffusers, including Schroeder types, will have a reduction in scattering if put in a repetitive pattern.

I don't understand why showing repeating cycles. The effects have been documented by Cox and D'Antonio. IIRC by Angus too.

Confused again,
Andre

[andrebrito]

For a poly diffuser the low end cut off is defined by length of the chord. Taking 300 Hertz as the design criteria gives a chord length of 3'. A 120° dispersion angle poly with a chord of 3' has a depth of approximately 5".

Andre, how do you can calculate the depth of the poly regarding its angle of dispersion ? Snell's law ? To be honest, that kind of curvature of the poly is quite small, I how say it acts almost as a flat wall.

Polys - > better spatial scattering, poor temporal diffusion...

QRDs - > worst spatial scattering, nice temporal diffusion

Both suck if placed in a repeated sequence, but they have been used both in large CR for the back wall so even they worst behavior is better than a flat wall

[Jason Jones]

Originally Posted by avare

Nice graph. What is it supposed to be showing? One line is for a single diffuser, and the other for a repetitive array, with the number of units unspecified. As Jason wrote earlier in this thread, any diffusers, including Schroeder types, will have a reduction in scattering if put in a repetitive pattern.

I don't understand why showing repeating cycles. The effects have been documented by Cox and D'Antonio. IIRC by Angus too.

Confused again,
Andre

It is not showing repeating cycles. It is:

• Test A: The diffusion coefficient vs. frequency for one 7 well QRD.
• Test B: The diffusion coefficient vs. frequency for a bunch of the same 7-well QRD's in a row.

The point is to illustrate to Ethan how significant this effect of repetition is. I agree that the effects are well known. I am trying to explain this to Ethan.

Jason

[avare]

Originally Posted by Jason Jones

The point is to illustrate to Ethan how significant this effect of repetition is. I agree that the effects are well known. I am trying to explain this to Ethan.

My apologies. I read the captions wrong. A VERY good graph!

Andre

[avare]

Andre, how do you can calculate the depth of the poly regarding its angle of dispersion ? Snell's law ?

Not Snell's law, but still ray tracing from optics, or specular reflection if you wish. Trying to keep it simple for other readers, the cylindrical surface is thought of as perfectly reflective. The angle of the incident ray equals the angle of the reflected ray. For a 180° diffusion pattern, which means 90° for the included angle, the maximum angle of the arc is one half: 45°. Then using the length of the chord as the variable, I just used geometry and trigonometry.

Reading the last two sentences, they still read like a diffuse field (acoustical humour). The attached drawing helps show it.

To be honest, that kind of curvature of the poly is quite small, I how say it acts almost as a flat wall.

Agreed. I learned of that design descriptor from Tremaine's Audio Cyclopedia, which I purchased when it was still in print. I was in high school at the time and I spent many hours going over the section and trying to understand the physics behind the values without having the physics or mathematical knowledge yet to understand it. Eventually it made perfect sense, once I had the math and physics learned. It seems to be forgotten to use ray tracing for spacial scattering design.

As this thread shows, polys can be quite practical for scattering.

Polys - > better spatial scattering, poor temporal diffusion...

QRDs - > worst spatial scattering, nice temporal diffusion

Total agreement.

Both suck if placed in a repeated sequence, but they have been used both in large CR for the back wall so even they worst behavior is better than a flat wall

I will be posting more about that shortly, hopefully. Glenn's post #16 in this thread started me thinking.

Andre

[andrebrito]

Snell's law is really the angle of the incident ray equals the angle of the reflected ray.

I forgot everything I learned regarding optics during my physics degree. The book I used at university for optics was "Fundamentals of Optics" from Francis Jenkins and Harvey White, lots of figures and lots of maths ! A nice yellow book.

[andrebrito]

I know Fresnel principle is used in acoustical diffusion prediction also... I never did it myself to be honest.

[avare]

Originally Posted by andrebrito

Snell's law is really the angle of the incident ray equals the angle of the reflected ray.

You are joking, right? I find it shocking that Snell's law has a different meaning in Europe to what it is in North America.

Snell's law in North America refers to the angle of refracted rays in a medium with a different velocity than what the ray is coming from. It is that the ratio of the sine of the incident angle to the sine of the refracted angle is equal to the ratio of speed of propagation in the two media. The teaching example typically used is light entering water.

The most famous application of Snell's law in commercial acoustics is the JBL 2301 acoustic lens/horn assembly. Some pictures of it are here.

Regardless, you were asking if I used "the angle of the incident ray equals the angle of the reflected ray", and the answer is yes.

Andre

[andrebrito]

Yes Snell's law is applied to refraction but I thought the angle of incidence equals the angle of reflection was one of its derivations. But it isn't, on the contrary, the generalization of the Law of Reflection is Snell's law, which is derived bellow using the same principle : Fermat's principle.

Sorry my mistake !

[Ethan Winer]

Originally Posted by Jason Jones

The point is to illustrate to Ethan how significant this effect of repetition is.

Maybe I'm color blind, so just to be clear, the upper line is one diffusor and the lower (smaller value) line is a row of repeating diffusors?

If this is the case, and the point is to avoid rows of diffusors, how does one treat an entire wall? In every big-name studio I've seen that has RPG diffusors in the rear of the control room, it was a repeating row. Or am I just dense and still missing something?

Also, my "pet theory" of diffusion in smaller rooms is their value is more from the staggered reflection times than any particular lateral spreading of the waves. But I'll be glad to be educated.

--Ethan