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

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?

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.

Thanks Dennis,

Should the speaker baffle be flush with the absorption or flush with the baffle wall?