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Baffle Wall, Allison Effect, Baffle Step, and Half Space Radiation Discussion. Calling acoustic...

post #1 of 8
Thread Starter 
Common convention here is to build lots of absorption behind the mains to abosorb the omni radiation of the mains as best as possible. Many do this out of covenience or other restrictions. I'm not contstrained by those and want to explore how to get a ruler flat midrange without the Allison effect causing its normal disturbance for speakers ~20" off the front wall.

I will be building 2 way controlled directivity speakers soon and they will not have baffle step compensation incorperated into the crossovers.

Baffle walls. They effectively make the speaker radiate into half space and do solve the Allison effect causing time/phase issues above the calculated frequency the baffle wall reflects which is dictated by the baffle size and its thickness/density.

I've read some of John Murphys writeups based on Harry Olson's work. From their formula it seems a 4'X4' piece of 3/4" plywood "baffle" should reflect the frequencies above 95Hz, with 95Hz actually being the -3db point.

F3 = 380/W

W= Width of baffle in feet.

F3= -3db point

If I cut a 4x4 piece of 3/4" plywood, cut a hole for speaker, insert speaker, and attach to my Acoustically transparent screen wall, the speaker should radiate half space according to this calculation correct?

Is this a silly quest or should I just load up the front wall with absorption?

What other concerns are there?
post #2 of 8
Yep you are basically right that you do not need a continuous baffle wall to get the baffle step frequency low enough, though that seems the simplest to build to me unless you are trying to include bass trapping in the front wall.

For example I designed a baffle wall recently for a client which went across a 2ft deep alcove in the front wall of his room. I designed cut outs in the top of the baffle wall and in the space behind the baffle wall and the back of the alcove hung fiberglass panels spaced out from the wall by ~8". It worked really well, actually better than I expected. In day to day use you cant see the cut outs as the entire baffle wall is covered with multiple layers of acoustical foam.

If you speaker is not designed for flush mount then you will need to EQ out the boost that you have just introduced by reducing the baffle step frequency. But the boost is a minimum phase effect and so can easily be dealt with.
post #3 of 8
Thread Starter 
Hello Nyal,

Yes this is exactly what I was thinking. My first thought was to fill the screen wall with Acoustic insulation which I touched on inthis post.

Then further thought made me question the need at all for the treatment on the front wall and what benefits it offered over just going with speakers that are fully baffled. the 4x4 was just chosen as it was manageable and it would cover the majority of the bandwidth of the mains. My screen wall is already built and not as easily accessible as I would like. I'm thinking about mounting these 4x4 squares to the back of the wall and building a shelf in them to hold the mains.

The remainder of the screen wall would be filled with Roxul and spaced 24" off the wall to work as bass trapping.


Now as far as the baffles themselves:

1. Do they need to be covered with treatment?
2. Is 4" Roxul OK?
3. If I put treatment on the face of the baffles, either the 4x4 baffle will have to be behind the speaker baffle face to allow for treatment
or the treatment will have to extend beyond the plane of the speaker baffle. Which is preferred?

I plan on using Roxul Safe-N-Sound as I prefer it to fiberglass. I already have a lot from my last bit of Acoustic treatment. I had some pink fluffy in my theater for about 1yr and it stank. I could smell that stuff right up to the day I ripped it out. Maybe I used the wrong stuff IDK.
post #4 of 8
You will want some kind of absorption on the face of the baffle wall. Any acoustically transparent screen will reflect higher frequencies back to the speaker/baffle wall. You want to absorb those back reflections.
post #5 of 8
Thread Starter 
Thanks Dennis,

Should the speaker baffle be flush with the absorption or flush with the baffle wall?
post #6 of 8
Nick,

Where did you go to learn all this and what did you ultimately learn or decide ? Advice for me ?
post #7 of 8
Thread Starter 
Hey Mfusick,

I ended up doing 4x4 panels for each of the LCRs.... It seems to have worked well as i have solved the allison effect reflections off the front wall i was seeing in my ETC.
post #8 of 8
I have been reading biggrin.gif Why did you choose a panel instead of a wall ? Just for ease of construction purposes ?
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