Is My Room Too Small for Diffusors?

[ruanddu]

I have a 16x15ft room with 7foot ceilings that I use for music recording and rehearsal. I was wondering in your opinion if you feel diffusion is warranted in a room this size? If so, which type do you think is best? - wood block, random pattern, curved panel, angled panel, etc? Or, would I be better off just using absorption?

Thanks,
Josh

[Mr.Smith]

I don't know the answer to all your questions but the acoustic Gods tell us that diffusion works best at 6ft or more, from source/listening position.

good luck!

[gremmy]

No, the room is not too small. The conventional wisdom is to make sure your listening position is removed from the diffuser by a distance equal to or greater than 3 times the wavelength of the lowest frequency diffused. So, in other words, diffusers that operate lower require more distance.

However, I've also seen information suggesting that not all diffuser types require the same distance... This has been gnawing at me lately so I'll go see if I can dig it up.

[Mr.Smith]

gremmy,

can you explain that to me?

At 6 ft the wavelength would be about 188hz. Which seems rather low so even in a small room, I'm trying to see how it would make sense. I even asked Pilchner himself about diffusion and he told me no less than 6 ft. In the OP question, his room would be sufficient for diffusion. But does that mean you can can diffuse 376 hz and up if you sit 3 ft away?

Help me understand.

[gremmy]

Ah cool, I finally found what I was looking for, at this link: http://www.rpginc.com/news/library/T...ffCritList.pdf

I pulled some text out of the doc that basically asserts that different types of diffusers require different distances between diffuser and listener, in the following order, from most distance required to least:

1) Flat Surface (bare wall)
2) Curved Surface (poly)
3) 1D Phase grating (QRD)
4) 2D Phase Grating (2D QRD PRD SKYLINE)
5) 1D BAD Panel
6) 2D BAD Panel
7) absorber

Here is some of the relevant text:

The level of the scattered sound and the resulting
interference in the total field decreases in the following
order: flat surface, curved surface, 1D phase grating,
2D phase grating, 1D amplitude grating, 2D
amplitude grating, absorber. In light of these remarks,
it is important to consider the temporal, spatial and

spectral response of a sound diffusing surface.

The frequency response of the
total field more closely resembles the direct sound,
since diffusion has minimized the interference.
Importantly, the listener no longer picks up the regularity
of the nulls and maxima that were seen for the
flat surface in Figure 9, and so the spectral changes
introduced may be less noticeable. This is illustrated
in the bottom time and frequency response of Figure
9, for a 1D diffusor. 2D diffusors, which direct more
energy away from the listener, will further reduce the
level of scattered energy in the direction of the listener.
Recent research has now led to hybrid
reflective/absorptive surfaces, which consist of reflective
and absorptive areas. These diffusors provide
both absorption and diffusion and may allow the listener

to get even closer to the scattering surface.

[gremmy]

Originally Posted by Mr.Smith

gremmy,

can you explain that to me?

At 6 ft the wavelength would be about 188hz. Which seems rather low so even in a small room, I'm trying to see how it would make sense. I even asked Pilchner himself about diffusion and he told me no less than 6 ft. In the OP question, his room would be sufficient for diffusion. But does that mean you can can diffuse 376 hz and up if you sit 3 ft away?

Help me understand.

Yes, the wavelength of 188hz is about 6 feet, so you'd need to be sitting 18 feet away from a diffuser that goes that low. Remember, you need to sit 3 times the distance of the lowest frequency diffused. However, usually we do not diffuse frequencies that low because it's too much work to build something like that and most of us don't have rooms where it would be useful.

Conversely, a diffuser that diffuses 1000hz and up would require less than 4 feet of distance.

P.S.

That 6 foot figure you were quoted is a generality, and it's probably based on commonly available commercial diffusers (the most common of which tends to be an N=7, I believe) with a depth of less than 6 inches.

P.P.S.

I just realized that I initially forgot to include the "3 times" in my original post, which undoubtedly caused some confusion here. I went back and updated my original post for clairty.

[Mr.Smith]

Okay, now it makes more sense.
yeah thannks for that. In general (even in a small room) what is considered a good starting point for diffusion? 500 hz?
I ask because I am trying to see if it will work in my room so maybe if I build something less deep, I can retain some high end.

[gremmy]

Originally Posted by Mr.Smith

Okay, now it makes more sense.
yeah thannks for that. In general (even in a small room) what is considered a good starting point for diffusion? 500 hz?
I ask because I am trying to see if it will work in my room so maybe if I build something less deep, I can retain some high end.

500hz is a pretty reasonable target. I've got a great big N31 that goes down to 388 (or something like that), but after piecing together that beast I started opting for shallower, lower order panels that go down to about 600hz or so.

Have you downloaded QRDUDE yet? If not, playing around with that is a great way to see what your options are.

P.S.

According to the RPG doc I linked to above, what Ethan commonly says around these parts is quite true: even if you have to sit very, very close to a diffuser, it's better than sitting right next to bare wall.

[Mr.Smith]

But is it better than sitting near an absorber? especially in a small room?

Originally Posted by gremmy

500hz is a pretty reasonable target. I've got a great big N31 that goes down to 388 (or something like that), but after piecing together that beast I started opting for shallower, lower order panels that go down to about 600hz or so.

Have you downloaded QRDUDE yet? If not, playing around with that is a great way to see what your options are.

P.S.

According to the RPG doc I linked to above, what Ethan commonly says around these parts is quite true: even if you have to sit very, very close to a diffuser, it's better than sitting right next to bare wall.

[Glenn Kuras]

Originally Posted by Mr.Smith

But is it better than sitting near an absorber? especially in a small room?

No you can be much closer to a absorber vs a diffusor.

[boggy]

Originally Posted by gremmy

........

Conversely, a diffuser that diffuses 1000hz and up would require less than 4 feet of distance.

....

I agree, and also, for small rooms, it's really enough to have diffused frequencies from 1000Hz and upward...

I use "four wavelengths minimum distance from diffusor" principle, but 3x can probably work too. Some people can be very sensitive to nearfield diffusion.


regards

Boggy

[gremmy]

Originally Posted by Mr.Smith

But is it better than sitting near an absorber? especially in a small room?

What Glen says above is correct. You can sit much closer to an absorber than to a diffuser without experiencing any nearfield artifacts. However, like most things, the Devil is in the details. I think we have to quantify what we mean by "sitting near," as there is a difference between sitting 1 inch away and sitting 2 feet away, especially if the type of diffuser your dealing with is a BAD.

[Glenn Kuras]

Originally Posted by gremmy

What Glen says above is correct. You can sit much closer to an absorber than to a diffuser without experiencing any nearfield artifacts. However, like most things, the Devil is in the details. I think we have to quantify what we mean by "sitting near," as there is a difference between sitting 1 inch away and sitting 2 feet away, especially if the type of diffuser your dealing with is a BAD.

Agreed.
BTW the rule of thumb I have always gone by is for every inch of depth in a diffusor you want to be 1' away. Well that is for QRD type diffusion anyway. But even if you are say 3' away it still is going to work, but maybe not as well.