[vanderstephen]

Hi. I am DIY'ing some acoustic treatment in my listening room. I am in the process of making absorption panels for behind the speakers using 3.5" thick rock wool (Roxul Safe and Sound). For aesthetics, I do not want  something that thick on the side walls and I can't find rock wool locally that's any thinner. I can get 2" thick Auralex Wedge foam from a local guitar center (that's considered diffusion, not absorption, right?). Would that be just as effective to use at a first reflection point as rock wool? Would the egg crate and wedge foam  be effective for use on the wall behind the listening position?
Thanks!

[Jeffrey Hedback]

Hello Vanderstephen,

I understand your questions and it would be nice if this would work.  However, the thickness (and material properties) relates to the effectiveness of the absorber.  Most simply, the thicker it is the lower in frequency it will absorb effectively.

The Auralex wedge is an absorber (not diffusor).  Inch per inch, the Auralex foam is ~50% as effective as the rock wool you've found. 

So the answer is that no, 2" Auralex wedge foam would not be as effective as 3.5" rock wool at those locations in terms of absorption.

Now, the other topic altogether is in your thread topic: diffusion vs absorption.  That's a deeper question.  Please feel free to post a diagram of your room, maybe a pic or two and details on your speakers.  I could then offer a better answer on this question.

[DBeistel]

You could peel off layers of the 3.5" Rouxl until you achieve the 2" thickness.

[vanderstephen]

Thank you for the replies so far. I will try to post a photo later this week. Hopefully we'll determine that I can get by with just a few squares of the Auralex wedges per sidewall.
I tried peeling the thicker Roxul, but it comes off in chunks- it was worth a shot.

[DBeistel]

Can you use a knife and slice it to the desired thickness

[Bjorn]

I would advice you to use absorption for early reflection points in the front of the room (sidewalls, ceiling and carpet on the floor). Basstrapping behind speakers and in corners. Use diffusion on rear sidewalls or backwall, but make sure you have the distance that's required.

[Skye]

Use diffusion on rear sidewalls or backwall, but make sure you have the distance that's required.

What is the required distance?

[youngho]

It depends on the type of diffuser (i.e. true diffusion vs diffraction or scattering), but assuming something like the ones from RPG, look here and click on page 46: http://books.google.com.au/books?id=jYBR-aL2vJMC&lpg=PP1&dq=Acoustic%20Absorbers%20and%20Diffusers&pg=PA46#v=onepage&q=%22spatial%20response%22&f=false

[Bjorn]

Diffusion is appropriate when the listening distance is at least 3 wavelengths away from the diffusor of the diffused frequencies. So a 6 inch deep diffusor (500 hz) should be 7 feet from the listener.

[McTwins]

Hi
If you use Wing diffusor from SMT there is no need for any 3 wavelenght away from the diffusor. The Wing is perfectly suitible for small rooms. You can sit around 20 centimeters from it without any degradation from Wing's diffusions performance.

[Skye]

If I use the Cardas method of positioning, my listening seat will be six feet away from the back wall. Using your formula, one third of 72 inches is 24 inches, and a 24 inch wave is 565 Hz. In this case, I should build or buy a diffuser effective, if possible, down to 565 Hz?

[Bjorn]

If I use the Cardas method of positioning, my listening seat will be six feet away from the back wall. Using your formula, one third of 72 inches is 24 inches, and a 24 inch wave is 565 Hz. In this case, I should build or buy a diffuser effective, if possible, down to 565 Hz?

That's correct. But remember that 3 wavelengths is the minimum recommended distance. You might want to stick to diffusors that are effective down to about 700-600 Hz.
If you end up buying something, stay with products that show measurements. Like for instance RPG:
http://www.rpginc.com/
And you want 1D diffusors that only diffuse in the most important horizontal plane and don't waste energy in unwanted directions. You will need several for a good effect.

[Skye]

That's correct. But remember that 3 wavelengths is the minimum recommended distance. You might want to stick to diffusors that are effective down to about 700-600 Hz.
If you end up buying something, stay with products that show measurements. Like for instance RPG:
http://www.rpginc.com/
And you want 1D diffusors that only diffuse in the most important horizontal plane and don't waste energy in unwanted directions. You will need several for a good effect.

1) Are there consequences for buying diffusers effective to lower frequencies? Or they are just more expensive?

2) What are the prime frequencies needing diffusion? Obviously, as great a range of frequencies as possible, but what is the frequency range that the ear will most appreciate being diffuse?

3) Why only 1D? Why is reflection towards the ceiling or floor wasting energy?

4) When you say I need several for good effect, how many do you recommend and where in a room would you place them?

[Bjorn]

1) Are there consequences for buying diffusers effective to lower frequencies? Or they are just more expensive?

2) What are the prime frequencies needing diffusion? Obviously, as great a range of frequencies as possible, but what is the frequency range that the ear will most appreciate being diffuse?

3) Why only 1D? Why is reflection towards the ceiling or floor wasting energy?

4) When you say I need several for good effect, how many do you recommend and where in a room would you place them?

I'm just going to briefly answer them.

1) If you sit too close to a diffusor you will start to hear discrete reflections. They need distance for the sound to blend correctly back to the listener. But I think you will be ok with RPG diffusors like QRD and Diffractal. You could also lengthen the distance by having a basstrap in the middle and diffusors on the side of it. Not sure if you need to though.

2) Ideally diffusion should start about above the schroeder frequency (where the modes start in the room) and diffuse up to around 5khz. Diffusion above that is debatable, but it does have the advantage of removing flutter-echo and you avoid potential high gain reflections above the diffusion range. So Diffractals are nice if you can afford them.

3) Our ears are on the side and we are more sensitive to sound arriving laterally. We also want to have a boost of energy arriving from behind as long as it's arriving after a certain time. It not only brings liveliness and is preferable, but it's also an important psycoacoustic cue related to accuracy. 1D diffusors will remain more energy since they aren't sending diffusion vertically.

4) A minimum of 5 would be great. Placement depends on room layout, distance to listening position, etc. One could either cover most of the backwall in ear height and some on rear sidewalls or redirect the energy on backwall with a splaying wall/gobo and have every diffusor on rear sidewalls. One could also make a large circle of diffusion if the room is very wide, which is seen in the picture below. RPG are the diffusors.

 

[Skye]

3) Our ears are on the side and we are more sensitive to sound arriving laterally. We also want to have a boost of energy arriving from behind as long as it's arriving after a certain time. It not only brings liveliness and is preferable, but it's also an important psycoacoustic cue related to accuracy. 1D diffusors will remain more energy since they aren't sending diffusion vertically.

So potentially 1D diffusers on the walls and 2D diffusers on the ceiling above or behind the listener?

[Bjorn]

So potentially 1D diffusers on the walls and 2D diffusers on the ceiling above or behind the listener?

No need to use diffusors on the ceiling at all unless it's a sloping ceiling sending specular reflections back to the listener.

[kiwi_1282001]

So the recommendation is absorption in front of the listener and diffusion behind?

[Bjorn]

So the recommendation is absorption in front of the listener and diffusion behind?

Yes, but very generally said. More spesific would be to say that one treats early reflections with either absorption or redirection and later arriving reflections from behind with diffusion if the distance permits it. One needs to take note that absorption should only be used at those spots that create early reflections in sweetspot. We don't want to use absorption (and especially not thin panels) unecessarily which can easily leave the room dead and dry. The goal is to create an RFZ (reflection free zone) and that can also be done by redirecting the reflections to the rear of the room where they will eventually be diffused. In most cases for instances, provided the rear of the room is properly treated, there's seldom a need to absorp high frequencied behind the speakers. Most speakers don't backfire high frequencies. Bass absorption is crucial though behind them. To find out exactly what do to, measuring and utilizing ETC is the key.

[McTwins]

If I use the Cardas method of positioning, my listening seat will be six feet away from the back wall. Using your formula, one third of 72 inches is 24 inches, and a 24 inch wave is 565 Hz. In this case, I should build or buy a diffuser effective, if possible, down to 565 Hz?

Cardas positioning won't help you anything, he makes great cables, that's about it.

Wing diffusor goes down to 200Hz.

[kiwi_1282001]

Hi Bjorn,

Is the OP setting up a mixing desk / studio or a 2 channel stereo listening room?

If the later I'd generally be suggesting diffusion on the front wall (with traps in corners) and absorption directly behind the listener.  Personally, I've had good results using diffusion at the 1st side wall reflection points as well - but that may be a matter of personal taste.