Absorption vs. Diffusion

[LCD]

Can someone break these down to the simplest level for me... I guess give me the "Acoustics for Dummies" version... I know what products are for sound absorption but what would I get for diffusion and why?

Thanks

-LCD-

[dementedchord]

readers digest version....

sound bounces off hard surfaces.... and if your walls are parallel (most reasonably are) it bounces back and forth at a freq = to the distance... these are called room modes.... the first thing we tried to do was deaden the thing.... so you would see guys putting carpet on the walls and ceiling... this is part of what gave the 70's sound... but as it evolved we found it best to controll the reflections by making them "spray" out across the room hence the idea of difussors..and then controll the bass primarily by absorbing what we could down there.... make sense???

[LCD]

Originally Posted by dementedchord

readers digest version....

sound bounces off hard surfaces.... and if your walls are parallel (most reasonably are) it bounces back and forth at a freq = to the distance... these are called room modes.... the first thing we tried to do was deaden the thing.... so you would see guys putting carpet on the walls and ceiling... this is part of what gave the 70's sound... but as it evolved we found it best to controll the reflections by making them "spray" out across the room hence the idea of difussors..and then controll the bass primarily by absorbing what we could down there.... make sense???

Yes it does! Thanks! Now my question is what type of treatment should I get for diffusion?

[Wayne]

Originally Posted by LCD

Yes it does! Thanks! Now my question is what type of treatment should I get for diffusion?

Before 'what kind' might be 'if and where'. It might not be applicable for a small room for example.

Here's a few go-to's.
http://homerecording.com/bbs/forumdisplay.php?f=20
http://johnlsayers.com/phpBB2/viewfo...1dded4acf8ac31

[Nutmeg II.]

To put it as simple as I could do:
empty room = echos
with diffusors = reverb (the echos get less noticeable)
with absorbtion = dead dry

Keep in mind diffusion does not work to well in small rooms!
A combination of absorbtion and diffusion in combination with a good room shape is the key to a good room sound.

[Ethan Winer]

> Can someone break these down to the simplest level for me <

There are several common acoustic problems, and diffusors can help some of them. Flutter echo and comb filtering at mid and high frequencies can be avoided by either diffusion or absorption. But bass problems are helped only by absorption.

Diffusion is used instead of absorption when you want to control flutter echo and comb filtering, but don't want to make the room any more dead sounding. In a smaller room you usually need to make it more dead sounding, because small room ambience is bad ambience. So absorption is more suitable for small rooms.

Most rooms are too live generally anyway, so absorption makes more sense in most cases. Diffusion in a few key places can be a nice touch in larger rooms though.

--Ethan

[LCD]

OK, I finally understand! Thanks to u all!

[fonman]

Originally Posted by Ethan Winer

> Can someone break these down to the simplest level for me <

There are several common acoustic problems, and diffusors can help some of them. Flutter echo and comb filtering at mid and high frequencies can be avoided by either diffusion or absorption. But bass problems are helped only by absorption.

Diffusion is used instead of absorption when you want to control flutter echo and comb filtering, but don't want to make the room any more dead sounding. In a smaller room you usually need to make it more dead sounding, because small room ambience is bad ambience. So absorption is more suitable for small rooms.

Most rooms are too live generally anyway, so absorption makes more sense in most cases. Diffusion in a few key places can be a nice touch in larger rooms though.

--Ethan

Ethan,

Do you neccessarily need the frames around panel absorbers ?

I currently have a bunch of panels made like: (1) 1" 705FSK (foil out), backed by (3) 1" 703's. All four are together making a 4" panel wrapped tightly in a homemade burlap "pillowcase".

When I put them in the corners as bass traps, I put the foil side away from the wall, and when I use them as absorbers, I put the foil side to the wall.

I just noticed a lot of people doing frames. Is there a reason other than aesthetics, and increased mounting ability ?

Have I done it completely wrong ?

P.S. Your video was great. thumbsup

[Ethan Winer]

> Do you neccessarily need the frames around panel absorbers ? <

If by "panel absorber" you mean a slab of rigid fiberglass, then No, a frame is not needed except to help mounting.

> When I put them in the corners as bass traps, I put the foil side away from the wall, and when I use them as absorbers, I put the foil side to the wall. <

That's correct.

> P.S. Your video was great. thumbsup <

Thanks!

(But which video? I've done several.)

--Ethan

[fonman]

If by "panel absorber" you mean a slab of rigid fiberglass, then No, a frame is not needed except to help mounting.

Is the 4" slab I described not a panel absorber ?

(But which video? I've done several.)

--Ethan

I only saw one video, a while back of you playing a bunch of cellos. I will have to check your site out again to view the other videos.
Thanks for your help, by the way.

[DeeDrive]

The videos on the real traps site are pretty fun to watch. Very informational. I love hearing the immeadiate change in acoustics of the basement when they're showing how to hang bass traps.

[Ethan Winer]

> Is the 4" slab I described not a panel absorber ? <

Yes, sort of. But "panel absorber" can mean other types of traps too, such as this wood panel trap I wrote about for Electronic Musician magazine some years ago:

www.ethanwiner.com/basstrap.html

> I only saw one video, a while back of you playing a bunch of cellos. <

That's my only music performance video, though I'm working now on another that's even more ambitious. As DeeDrive said, I've also made educational videos for my company's web site. They're definitely "home made" looking (because they ARE homemade! ) but hopefully the content comes through well anyway.

--Ethan

[fonman]

I've also made educational videos for my company's web site.

I watched several last night. Very nice. Thanks for the help.

[gjuodenas]

I would like to chime in as a manufacturer of Modified Schroeder Acoustical Diffusor panels.

It is a myth that diffusors do not work in small rooms. I have put our professionally designed diffusors in many small rooms (are there any large rooms left?). The proof is in the pudding as they say. The clients have been overwhelmingly positive, their jaws usually drop as they hear the change in the acoustical environment the diffusors create as they go up one by one. They can hear the not so subtle change in the sound of their own voice. Their room sounds large and there is air around the sound. Acoustics is a theoretical science and the final proof comes upon completion of an installation. You have to use acoustics theory as a starting point but you also have to recognize the effect of applied science.Check out some examples at http://www.srlaudio.com/acoustics.htm

I agree you have to first deal with the bass using corner absorbers. It is also a good iidea to have a "sprinkling" of absorption panels in key places. After a studio is up and functioning with the diffusion modules, I make it a point to ask visiting engineers what they think about the acoustics. The usual reply is they had not noticed them, this is the greatest compliment because they are not hearing the room, it is not a distraction, they are not compensating.

[Glenn Kuras]

Originally Posted by gjuodenas

I would like to chime in as a manufacturer of Modified Schroeder Acoustical Diffusor panels.

It is a myth that diffusors do not work in small rooms. I have put our professionally designed diffusors in many small rooms (are there any large rooms left?). The proof is in the pudding as they say. The clients have been overwhelmingly positive, their jaws usually drop as they hear the change in the acoustical environment the diffusors create as they go up one by one. They can hear the not so subtle change in the sound of their own voice. Their room sounds large and there is air around the sound. Acoustics is a theoretical science and the final proof comes upon completion of an installation. You have to use acoustics theory as a starting point but you also have to recognize the effect of applied science.Check out some examples at http://www.srlaudio.com/acoustics.htm

I agree you have to first deal with the bass using corner absorbers. It is also a good iidea to have a "sprinkling" of absorption panels in key places. After a studio is up and functioning with the diffusion modules, I make it a point to ask visiting engineers what they think about the acoustics. The usual reply is they had not noticed them, this is the greatest compliment because they are not hearing the room, it is not a distraction, they are not compensating.

I agree but at the same time disagree. Diffusion is awsome, but from all the rooms I have done/tested and all the pros I have talked to, you really should not have diffusion that is less then 5 to 6 feet from you. Trust me we are working on a new diffuser, so myself being against diffusion could very shorty be shooting myself in the foot!!

Glenn

[gjuodenas]

I agree with Glenn that 5 to 6 ft away is probably the theoretical minimum but that being said I have done some interesting experimenting. If you place a diffusion module say 3 ft in front of you and speak into it or play a speaker into it you do not hear any effect that you can directly identify. What you do get is a feeling of spaciousness.

In my experimenting there are a lot of subjective effects diffusion modules create that I beleive are not yet explained by the science of diffusors. I never would have believed it before I started experimenting. In one experiment I took a 5ft X 8ft diffusion module and placed it against a wall in a 20ft X 20 ft band practice room with parallel walls and virtually no acoustic treatment on the walls. I then took a high quality15"coax speaker and stood about 10ft away from the panel and aimed it around the room. When i would point it at the diffusion panel my jaw dropped. It sounded like I introduced a high end speaker that was very flat and had a lot of air (a good thing) around the music being played. This made me a true believer in lots of diffusion. After having applied a lot of diffusion panels in recording studios, I am still a true believer.

I get clients calling me back asking me to make gobos from the diffusion panels to place around acoustic instruments and vocalists. If you want a truely magical effect in a small room, put some diffusor modules around your acoustic instruments and you will swear they were recorded in a large hardwood room.

By the way I too have talked to a lot of pros about diffusion but I have found that most have not had the opportunity to listen to them applied in studios and have formed their opinions from what they have read or heard about.

[asagaai]

Hi guys

I am not an expert-been doing my own tacking room-6.5 metres by 4.6 metres, ceiling 8.5 foot high. Initially treated every second wall, and covered entire ceiling with 50 mm foam. Great drum room-but too dead generally.

So I have been building QRF Diffusors myself, in 600 x 600 squares, out of pine wood. Bluddy heavy-need to put beams up in trusses to hold. I have got 7 up, another 4 to go up tomorrow. Now, although the ceiling is only 8.5 foot, I have found there is more clarity and air.

Singers previously winged about the room-saying they struggled to sing, now they are finding it more pleasant.

So, although not a high ceiling, I am getting real good results. Planning on putting up 17 squares on ceiling, and then some on the main separation wall from control room.

I recommend it, but even DOI aint cheap-wood, time, glue, fixing beams and threads, stains.

Cheers

Gavin Jensen
Newcastle/OZ

[gjuodenas]

Gavin,
What do you mean 600 X 600 squares? I am curious what prime seed number you are using, in other words how many sections is each panel? One thing you have to be aware of when using Quadratic Residue Diffusors. If the prime seed is on the lower side say 7 thru 13, putting a lot of these in a row defeats your purpose because it actually begins to cause lobing. When using these smaller prime seeds in mulitples, you have to according to a certain pattern introduce a negative or reverse profile. That is why I recommend large high prime seed panels, when using them the above is not a concern.

[asagaai]

Ah man-dont give me a heart attack.

I have square sheets of ply-500mm by 500 mm. Each piece of wood is 42 mm by 42 mm, 12 pieces of wood fit across the board.

Lengths are 4 lengths as follows:

1. 0

2. 4.75 cm (38 pieces)

3. 9.5 cm (38 pieces)

4. 14.25 cm (40 pieces)

5. 19.01 cm (15 pieces)

According to the diffusor calculator, this gives me a low frequency of 905 hz, and a top end of 4095.23 Hz.

And unlike your crappy foam/hollow plastic diffusors, at least mine made out of solid wood will diffuse bass.

It has definately given brightness to the room, like more present crisper cleaner sound, and as I said, I am planning to put some on the flat control room wall facing into the tracking room.

Cheers

Gavin
Newcastle/OZ.

[asagaai]

Hey Gujodinas I just realized you have some diffusors on your thread, I did not mean to say your diffusors are crap-actually they look pretty cool, and if you were here in oz I would probably just buy that 8 ft by 4ft panel for the real of my control room. But here we cannot buy decent diffusors for reasonable $$ so I have resigned byself to building those QRD diffusors.

From what Heard foam and plastic duffusors do not do a decent job of diffusion below about 800 HZ.

Cheers

Gavin
Newcastle/OZ

[gjuodenas]

I am really sorry for being so dense but I still do not understand what you have done. Is it possible to post a photo so that we can all see it?

I do not understand what the 4 lengths are. Are they the width of the pieces or the depths inset from surface? I also do not understand what you are doing with the very large plywood panels or how the 12 pieces are mounted. Sorry for the lack of understanding.

I couldn't agree more with your feelings on the hollow plastic diffusors. I guess if they are very stiff, they might work.

[Ethan Winer]

George,

> It is a myth that diffusors do not work in small rooms. <

This can be true in some rooms. The issue is not how close or far you are as an absolute distance, but as a function of the diffusor's lowest effective working frequency. A deep diffusor diffuses to a lower frequency than a shallow type, so the deep models are usually best placed farther away. However, in very small rooms I still feel that absorption is generally a better choice.

> I have found that most have not had the opportunity to listen to them applied in studios and have formed their opinions from what they have read or heard about. <

I agree with that too. When I first became aware of diffusors I didn't know anyone near me who owned "good" ones - cheap stamped plastic does not count - so I drove 9 hours each way to visit a pro studio designer friend all the way down in North Carolina so I could hear them in person. This friend has RPG's deepest "well" diffusors lining the entire back wall in his medium size control room. I had him bring a large gobo with a glass upper portion into his control room, and we wheeled it right in front of the rear wall leaving some of the diffusors exposed. I first "talked into" the glass, then into the diffusors, and in those five seconds the benefit of good diffusion became perfectly clear. I urge anyone who has not experienced good diffusion, and wants to be educated, to do the same.

--Ethan

[asagaai]

Hi gjuodenas,

I feel dense as well, took photo but cannot compress it enough to attach-maybe I should run the F##r through my buzz.

Now picture square pieces of plywood, 504mm by 504mm. I have drawn grid patterns, essentially 144 squares of 42mm x42 mm, so that the entire square piece of pkywood has a grid pattern 144 squares of 42 mm x 42 mm.

Into these 144 squares that have been plotted, I glue varying lenghts of pinewood, in a QRD diffusor pattern.

The pieces of pine wood (42mm x42mm) vary in length, from 4.75cm, to 9.5 cm, to 14.25cm, with the longest pieces being 19.01.

Once all 144 pieces of pinewood of varying lengths are glued into the allocated plywood square -in teh QRF pattern (actually less, because there are squares where it is left blank,) I then attach the plywood against the ceiling, with all the pieces of pine 42mm x42 mm in varying lenghts pointing down-so it really feels walking underneath like a vampires worst nightmare.

If you do a search you will note that RPG does them under the name skyline diffusor. These are QRF diffusors, RPG does them with high density foam, I am doing them with solid pine.

It would be too expensive to build out of solid pine for commercial resale, because it is very time consuming. If you guys still want me to post the photo of my studio ceiling with my handmade QRF diffusors, I will do it tomorrow, is too late here in Oz.

GJ
Newcastle/OZ

Hop that makes sense.

[gjuodenas]

In my personal subjective opinion, I feel that the type of QRD Schroeder type diffusors that work best in the studio environment are those that are designed to work in a frequency renge from 500 to 750 in the bottom end and 3k to 4.5k in the top end. All of the technical papers I have read state that even though the design parameters state that the low end is .75 of the design frequency the diffusors are actually effective to at least an octave below the .75 of the design frequency. Let the bass traps do the lower bottom work. Most of these type of diffusors are designed around a target frequency and they are effect from .75 times the design frequency to 3.5 times the design frequency if the ideal values as laid out by Schroeder are chosen in the design. Large deep diffusors are hard to accomodate in small to medium rooms because of their size. The folks at RPG have certainly lead the way in advancing the theory of diffusors, they are very highly respected academics. However, that being said their products are very expensive and usually only found in high end rooms.

My apologies to the original poster LCD who asked for simplicity but acoustics is not a simple topic, there have been many books written about the topic. If you are going to do it yourself, read a few of these books.

[bpape]

Exactly Ethan.

Let's take for example the 900 Hz low end for a diffusor. Most of the male vocal range is well below that. To get something that will be effective over the full male vocal range will take something that is close to a foot deep. Just that fact in itself makes them questionable in a small room where space is at a premium.

In shorter distance situations like that, sometimes it's better to use dispersion instead of true diffusion. Something like a polycylindrical shape will scatter the sound evenly and is much more tolerant of sitting a little closer. While it doesn't do true random scattering, it does disperse things across one dimension very well and very evenly.

Bryan

[gjuodenas]

Gavin,

I just realized what the confusion was. I was trying to visualize QRD (quadratic residue diffusor) type schroeder diffusors while you are talking about skyline type diffusors. Sorry!

You are to be truly commended for building this type of diffusor, very labor intensive.
I am sure they will work great and as you sound like you are willing to do, experiment by using them in unconventional ways such as close in on acoustic instruments.

Yes, I would like to see a photo posted just so I can admire it.

Best of luck
George Juodenas

[asagaai]

Hi George, here are 2 shots-resolution is a little low, but you will get the idea.

Just put another 4 up today, now have 11 squares up, another 6 to go. Oo am just sooo sick of this building shit-and believe me when I start giving blood unintentionally I make sure everyone knows.

The sound is getting real good-I even dig the sound of my own voice-which is not cool because my voice stinks. But-I played shitty little stereo-cannot believe the sound -it sounds huge-diffusors seem to hi fi things.

Best way to describe, sounds like outside clean- and tends to increase volume in crystal clear way. Oh boy cannot wait to get the drum kit and snare under.

Cheers

Gavin Jensen
Newcastle/OZ

[gjuodenas]

Gavin,
Your studio is great! You deserve a lot of kudos (fame and renown resulting from an act or achievement; by extension is often used as a praising remark).

You are hearing the same things I have encoutered when installing diffusors, more "air" around the sound, more clarity to the sound, you notice your own voice sounds better, everything sounds better (more pleasing), it is like you hi-fi every sound. It is amazing what a single large diffusion panel on the back wall of a control room or listening room can do. When I started experimenting with diffusors, the results were very exciting. My first thought was I could kick myself for not doing it years earlier, instead I kept deadening rooms. This reslulted in the past in depending on the directly radiated sound from the speakers with no reflection.

Sounds you hear everyday exist in an acoustical environment, that is what we are used to. Multi-tracking and close miking techniques have taken away the ambience. Some attempt is made to re-introduce it in the mix. Listening rooms are then "deadened" to eliminate reflections. What a revelation it is when you create a diffuse sound field in the listening room or around acoustic instruments when recording, it is nothing short of pleasant.

[Scott R. Foster]

LCD:

The answer depends on the size and reverb time of your room[s].

To really get your arms around the concept/question you need to grasp the concept of the Schroeder frequency.

There is a cut off frequency below which modal behavior dominates and only absorption can be effective as an acoustic treatment. Above this cut off, diffusion becomes a practical solution. This range varys with room size [larger rooms can use diffusion effectively on larger waves].

To calculate this frequency you divide the room RT60 by the volume in cubic feet, take the sqaure root of the result, and multiply by 11250.

Using a full featured room mode calculator will calculate this cutoff for you for a given target RT60, and help you choose which type of treatments to use.

Here is a link to a spiffy web based room calculator you can use for this purpose.

http://www.bobgolds.com/Mode/RoomModes.htm

Here is a downloadable excel based room mode tool that will calculate the schroeder frequency for you - and just plain rocks [click on "Room Modes and Reverberation Time Calculator" to download - I think you have to log in - registration is free and easy]:

http://forum.studiotips.com/viewtopic.php?t=1414

As to why use diffusion... here is an informative explanation:

http://www.acoustics.salford.ac.uk/a...sign/index.htm

And this book contans a good but easy to understand overview of the topic:

http://www.amazon.com/exec/obidos/AS...486553-8829465

As to material or cost considerations, obviously the laws of physics don't give a hoot what you pay for the substrates in questions. A "cheap" plastic diffuser properly configured will work just as good as an expensive wooden one.

A very inexpensive and effective design is to simply bend a sheet of thin plywood into an elipsoidal curve [FKA polycylindricals, or "ploys"]. You can makes these very eaisly or buy them with nice built -in hanging fixtures, etcetera.

Scroll down to read about polys:

http://forum.studiotips.com/viewtopic.php?t=48

or look at these babies!

http://www.rpgdiffusors.com/products...dial/index.htm

[LCD]

^^^^ WoW! Thanks for the input and links... I have a good amount of reading to do