Diffusion

[dantheman]

I'm going to embark on adding some diffusion to my bedroom system. I should be ordering some diffusors next week. My question is this: how does one measure the effectiveness of diffusion with common acoustic measuring equipment? Should I just do normal room responses and see if it's smoother? BORING. Or is there a good way to generate a polar plot before and after the diffusion of the wall? I know this sounds crazy, but it's something I'd like to do and in a sensible/presentable manner. I've seen the graphs in the Master Handbook of Acoustics, but I don't really know how they were generated. My thought is to make a rig to stand the diffusor on my speaker measuring rig, then place a mic about a meter from the diffusor. A fairly flat on axis/narrow directivity speaker would be placed about a meter away at a 45 degree angle from the diffusor. With a stack of a large absorber and some open cell foam between the speaker and the mic I'd then take a series of swept sine wave measurements while rotating the diffusor to different angles.

Any thoughts on the usefulness, oversights or foolishness of this exercise? All help appreciated. I'm really just stretching for ideas. I'd also need a control of some sort.

Thanks,

Dan

[terry j]

hi dan

not diy for these??

what type of diffusors will you be getting??

anyway, my only hint would be it is 'useless' to try and do it in the room. It MAY work if you can somehow isolate the room from your measurements....

I have tried it with some qrd's I built, and very quickly indeed came to the conclusion it was a waste of time to do it in room. We all know hown many multiple reflections occur in a small unit of time, they will swamp what you are trying to measure really.

The only way I thought I may be able to do it was outside, apart from the ground no reflections. Then what you envisage (or variations of the same) may lead to results.

[Ron E]

Cookies Required

http://www.rpginc.com/news/library/tyndall_paper.pdf

http://books.google.com/books?id=jYB...fusers&f=false

In freefield, I would suppose you could use a nondirective source, measure the direct and reflected sound together, and compare the diffuser to a flat wall.

In room, I would expect listening tests might be more useful, just put a piece of hardboard in front of the diffuser.

[dantheman]

Hey Terry. I'm going to start off with a couple of these GIK D1 diffusors:
GIK Acoustics: GIK D1 Diffusor
Look pretty good to me for starters. I will probably do some DIYing of diffusion when I get time, but that will be a while. I was thinking about doing some in room measurements and looking at the impulse response and various decay graphs to see changes. Might show something worth seeing.

Thanks for the links Ron! Those should give me what I need to know. I don't really have the ability to make to measurements at once, but a do have a sheet of hardboard that I could attach to the diffusor. We'll see what happens. This is still several weeks away, but it will be fun.

Thanks Gents!

Dan

[terry j]

the other suggestion is to ask this over at gearslutz, then you will certainly get into decay rates, initial time gaps etc etc!

What I would be looking for is the indication of specular reflections, and then you have to consider where you put the speaker (for the incident sound) as well as the mic.

Maybe using rew you can either do a band limited sweep )no point in starting at 20 hz for example) or else a custom pink noise, that could be a real time test and watch what happens to the response as you rotate the panel (or keep the panel still and move the mic).

You could take snapshots of the response along the way.

Hmm, had thought of making a clone of that gik panel, I have a method that could knock it up in under half an hour.

Of any length required.

But, intellectual property and all that so I decided to go PRD using published data (and calculators).

[dantheman]

Good thinking Terry! I'll go check out gearslutz and also try the test you suggest. It sounds like a good idea to me.

I'm hoping to measure a few types of diffusors and then quite possibly DIY or order a number of the best depending on what I feel I need. I still have much to learn about it--but a book to read.

The space I'm building will be used for practicing guitar/piano/sanshin, recording and mixing as well and I really want it to come out nice. I've built some massive absorbers for it already that turned out to be the most successful treatments I've made.

Thanks for the help Terry,

Dan

[norman bates]

I had luck using 12" cardboard sonotube.

Foam was too dead, you can use too much.

Norman