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[L_Tofastrud]

Francis asked me to write a little about my experience with room with a lot of diffusion.

I did a couple of rooms in Norway where all surfaces were covered with diffusers (there were still a 30cm tall area below and above the diffusers that was just drywall).  We built the diffusers from 50mm thick Styrofoam and the max depth was 400mm.  They were 1.2 meters wide (It was a 23 sequence QRD).  I decided to use the wide sequence because of the wide bandwidth of the unit.  I designed them to cover most of the frequency range of the vocals – it was fairly effective from 150 Hz up to 5 kHz – the calculated range is 270 to 3400 Hz.

The rooms are used for voice work but also some instruments.

The thought behind it was:
- Diffusers remove quite a bit of energy from the direct reflection so there is some absorption in the way they work.
- One of the issues in smaller rooms are often the _lack_ of reflections rather than too many.  This can cause very obvious coloration and the extremely dense “reverberant” field in these rooms was free from any ridges in the waterfall measurements that could indicate unwanted coloration.

The corners of the rooms had pretty wide and deep porous bass trapping.  At the deepest point they were more than a meter deep.  The exposed area of porous traps was only a fraction of the exposed area of diffusers.

We concluded it was necessary to have diffusers in the ceiling too as the first reflection from the ceiling was very obvious.  We used the same diffusers in the whole ceiling.

The feedback I got on these rooms was very positive.  They said if was a very natural sound and it was also very easy to add additional reverb.  I think some of the voice talent found the room unusual and this might be an issue -one of part of it was the visual appearance of the diffusers.  My perception of the sound in these rooms was that I asked myself "where are the walls?"  There were energy in the room but it was impossible to tell how big/small the room was and the dense low-level “reverb” of the room was apparent by talking or clapping but the rooms were definitely not “live”.

The measurements in the room looked very dense, even and “theoretically ideal”.  It really looked like the ETC and waterfall measurements of the “Quick Sound Field” (TM ASC) (page 469 in the 4th edition of the “master handbook of acoustics”).  I believe the description of the sound in these rooms would be a good way to describe the sound in these diffuser rooms too, but I don’t have enough experience with this treatment that I will recommend it unless the engineers/owner understands that it is a little bit of an experiment.  Especially with loud instruments there was a little bit of “honkyness”.  I think this was related to the Styrofoam (it not being heavy/dead enough) and/or the need for diffusion even higher up in frequency.

For those willing to experiment with this I’d say:
- cover the diffusers with fabric as they can be visually distracting and unusual
- the diffusers need to cover a wide frequency range
- make sure you absorb effectively from 200 Hz or so and down as far as needed.
- use a heavier and deader material than Styrofoam.
- consider adding diffusion up to even higher frequencies. (Higher sequence diffuser and narrower wells)

Questions?

Regards
Lars Tofastrud

[J.F.Oros]

Hi Lars,

This is Flaviu from Romania. Remember me ? We changed some (accidental) mails a few years ago about acoustics and studio design.

I'm really glad that you are back at FM Design and that you and Fran are here to share your valuable knoledge with us.

I must admit I had at some point the crazy idea of applying for a job there too , but I came down to earth and now I run the first studio acoustics design firm in my contry    (hey, are you interested in an Eastern-Europe FM design office    ? )

Anyway, back on topic (I'm known to be a verry off-topic person on some other forums   ). I have a question :

How big were the rooms and at what distance from the walls were the sound sources ? In LEDE control rooms you usually have to keep at least 3m from QRD's (or other 1D diffuser) to your ears, to let the diffusion develop...

[Guy Staley]

J.F.Oros wrote on Thu, 30 November 2006 14:35

How big were the rooms and at what distance from the walls were the sound sources ? In LEDE control rooms you usually have to keep at least 3m from QRD's (or other 1D diffuser) to your ears, to let the diffusion develop...

Yes, same inquiry here. What were the dimensions of this room?

Fascinating!

[L_Tofastrud]

Flaviu,

Yes I remember you!  It's been a while!  Good to hear that you are still in the acoustics business too.

The rooms were fairly small.  I think the smallest dimension there was just a barely 5 meters.  The longest dimension was probably in the 6-7 meter range..  Ceilings were at about 3 meters.

I agree with your comment about havign some distance to the diffusors.  I still found that the distance to the diffusors were acceptable since so much of the wall surfaces were covered.

I would like to do more experiments with rooms like this and the distance to the diffusor is one of the things that it could be interesting to know more about.

Regards
Lars T

[J.F.Oros]

Well, judging after the dimensions here in my country people build their recording (and mixing too) rooms, those you mentioned are not small rooms  

But they are of course somewhere at the limit where you would use diffusion as main treatment, especially QRD's.

Just qurious, didn't they showed specific frequency absorbtion, because of the wells spaces acting as "tuned" resonators ? Did you use wells dividers for them ?

[Sin x/x]

What would be the upper limit for the size of a diffuse room?

And why can't you use diffusers for bigger rooms?

[Die BREMSSPUR]

Sin x/x wrote on Wed, 06 December 2006 00:41

What would be the upper limit for the size of a diffuse room? And why can't you use diffusers for bigger rooms?

I wasn't aware that rock and roll had rules...

[L_Tofastrud]

Sin x/x wrote on Tue, 05 December 2006 16:41

What would be the upper limit for the size of a diffuse room? And why can't you use diffusers for bigger rooms?

There is really no upper limit but as the room gets bigger it will at some point where there is actually a diffuse field without diffusers.

In one of the studios I mentioned we also treated one of the long walls in the large room with the same type diffusers.  There were also 4 other diffusers in this room but not much treatment apart from that (some low-frequency treatment and that was pretty much it).

LT

[L_Tofastrud]

J.F.Oros wrote on Fri, 01 December 2006 15:22

Just qurious, didn't they showed specific frequency absorbtion, because of the wells spaces acting as "tuned" resonators ? Did you use wells dividers for them ?

I didn't really see any specific "tuned" absorption but they definitely absorb quite a bit - maybe I should say it appears they absorb a lot of sound - at least in theory it is really just the energy/time relationship that is shifted, not the energy level as such.

I did not use wells on these.  We did build one prototype with dividers but it was just too much work I'm sure dividers will have a positive influence on the efficiency of the diffusers but it also complicates the assembly.

LT

[jfrigo]

L_Tofastrud wrote on Fri, 08 December 2006 09:54

Sin x/x wrote on Tue, 05 December 2006 16:41 What would be the upper limit for the size of a diffuse room? And why can't you use diffusers for bigger rooms? There is really no upper limit but as the room gets bigger it will at some point where there is actually a diffuse field without diffusers.

Diffusion is important even in large spaces, like concert halls. Classics, like Symphony hall, Grosser Musikvereinssaal, and Concertgebouw, have much ornamentation, statuary, niches, and architectural details that provide diffusion.

Many modern halls have a more sparse aesthetic, but several employ extensive use of QRDs to increase diffusion and improve acoustics for music. Tokyo's Opera City Concert Hall has many QRDs in its pyramid shaped top.The volume of this hall is 540,000 cu. ft. (15,300 cu. meters). This certainly supports the idea that diffusors can be useful in larger rooms. I doubt we're going to be building recording studios with live rooms of that size!

[jfrigo]

L_Tofastrud wrote on Fri, 08 December 2006 10:19

I'm sure dividers will have a positive influence on the efficiency of the diffusers but it also complicates the assembly.

For optimum and predictable performance, the well dividers are important. However, There certainly still will be plenty of positive effect from diffusors withouth divders, and there are some diffusors designed without dividers (like the 2D Skylines from RPG) that certainly are capable within their design goals.

It does complicate construction, and in the end it needs to be decided whether the application is critical enough to warrant the added complexity, time, and expense. For the back of a control room, I wouldn't skip the well dividers (and the smaller area makes it less of an encumbrance), but in a larger live room with lots of area covered, it's often not nearly so critical.