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Back Wave Solution: Is this what I think and is it effective?

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  • Back Wave Solution: Is this what I think and is it effective?

    Came across this drawing some time back and saved it out of curiosity. There is a stack of blocks on the back wall with a progressive rotation as they were stacked. I assume this is being used to mitigate the back waves from the upper mid/woofer. Has anyone tried this? Was there a measurable difference if so?

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  • #2
    not sure about this, but try transmission line for back wave absorption. Bill tried it with his dual exhaust build.

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    • #3
      The goal of the blocks is 3 fold. 1st is to introduce non-parallel sides these should help cancel standing waves. 2nd add mass to the wall to help eliminate vibrations. 3rd get rid of shop scraps too small to make a proper brace.
      Does it work...yes...is it the best way ever...

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      • #4
        I believe that irregular surfaces will aid in breaking up sound inside the speaker. The amount shown in the picture is not enough to do much of anything. You can absorb the back wave, or try to diffuse it a bit, or both. Just keep in mind the frequencies, and wavelengths involved when you are planning. Take a look at an acoustics book for some technical information. Simple materials can be used effectively if you want to get creative. For example pegboard. If you were to space a sheet across the back wall of a speaker with a 1/2" of airspace behind it, it would tend to absorb, and diffuse sound. Probably at mostly higher frequencies though. The number, and size of holes makes a difference too. Imagine if the box was filled with practice type golf balls. (The light weight plastic ones with holes.). Sound waves would have a hard time going from one side of the box to the other. (At least the higher frequencies.) A long time ago, I lined the walls of a speaker with foam coffee cups. The cups had some randomly punched holes in them. The big end of the cup was glued to the wall. I had an idea one time that I never tried. That was to oversize the box, and then fill it with three or four inch diameter styrofoam balls. A big plastic, or wood salad bowl would probably work pretty effectively.

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        • #5
          Strikes me as acoustic diffusion for mid to high frequencies, but I like Ken's idea that it's a shop scrap reduction
          Electronics engineer, woofer enthusiast, and musician.
          Wogg Music

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          • #6
            Originally posted by Kevin K. View Post
            Came across this drawing some time back and saved it out of curiosity. There is a stack of blocks on the back wall with a progressive rotation as they were stacked. I assume this is being used to mitigate the back waves from the upper mid/woofer. Has anyone tried this? Was there a measurable difference if so?
            In theory it is a good idea, it can't hurt. But more than likely it will only work with very short wavelengths, higher frequencies. I'd prefer to use open cell egg crate foam instead, because it would have better absorption. Diffusion will get you very little inside an enclosure.

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