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On-Wall MTM Design: Advice Greatly Appreciated

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  • #31
    Originally posted by mobius View Post
    I'm going to again suggest that in the special case of an on-wall speaker (or a slightly proud sitting in-wall speaker as the case may be), you need to include both the baffle diffraction effects and the rear wall boundary gain effects in your sims
    There is no rear wall gain. Sims model in half-space, and that's what you have with a flush mounted or nearly flush mounted speaker. You will muck up the works if you employ BSC, because there's no baffle step either. This is a good thing, as that's two hoops you don't have to jump through to get an accurately modeled result.

    www.billfitzmaurice.com
    www.billfitzmaurice.info/forum

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    • #32
      Yes, you could choose to just use the driver's infinite baffle response for the sims but the moment the box ceases to be flush with the wall, then you introduce changes to the frequency response due to the diffraction that occurs at the baffle edges and due to the interference that results from the reflection of sound waves off the back wall in particular the suck-out that gets created. In effect, there is indeed no rear wall gain and no baffle step loss because they cancel each other out but the sims model more than just these transitions between half-space and full-space, they give you the effects of the edge diffraction and the rear wall interference. When just relying on sims as opposed to measurements, I would want to include those effects in this situation (though minor they may be) to try to improve the accuracy in the sims.

      For the OP, maybe the easiest thing to do since it sounds like you are able to mount the speakers into the wall, is to choose a cabinet depth that will allow flush mounting to the wall, so basically just an in-wall speaker instead of an on-wall speaker. That way this discussion is mute and you can ignore any potential baffle and rear boundary effects. If you can that is. The in-walls could still be hidden with any kind of acoustically transparent art work if you or your significant other want to.

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      • #33
        Upon a little more consideration, I think I need to amend my suggestion for this particular situation of a wide baffle speaker only slightly raised off the wall. Thanks to Bill's post, I started thinking a little more about the transition from half-space to full-space and I've realized that the accuracy of the rear wall boundary simulation probably depends on the relationship between the baffle width and the distance to the rear wall which the software doesn't account for.

        More specifically, we know that on any given flat baffle, the higher frequencies propagate forward into half-space and the lower frequencies propagate both forward and backwards into full-space and that the transition frequency between the 2 depends on the baffle width. Or to put it another way, we can say that in order for the HF's to start going in the backwards direction, the wavelength has to be greater than the width of the baffle. So in the case of a 15" baffle, the corresponding frequency is about 900Hz. So in other words, all the frequencies above about 900Hz are only going in the forward direction and are therefore not going to create any reflections off the back wall.

        Now if we look at the case of the rear wall boundary gain again when the baffle is only 3" from the rear wall, we can see that the null that is created (and any ripple beyond that) sits at about 1600Hz (see below). But when the baffle is 15" wide, only the frequencies below 900Hz are going backwards. Therefore that rear wall null at 1600Hz and any other ripple can't actually be created because those frequencies are not moving backwards and are not reflected off the wall and therefore cannot create the kind of out of phase interference necessary to produce the null and ripple in the 1st place.

        At least that's what my logic tells me today.

        So in this case, I don't think adding the rear wall gain to the baffle diffraction effects is going to produce a simulation that isn't flawed. I do still think though that if the cabinet is sitting forward from the wall, the baffle edge diffraction effects but without the 6dB of baffle step loss still need to be added to the infinite baffle measurement for more accuracy. That'll take some manipulation of the baffle sim in something like Response Modeler perhaps but I'm not sure if there isn't just a bit of guess work involved at this point. Adding large edge round-overs or chamfers to the cabinet will diminish those effects of course.

        Or again, flush mount it if you can. That is if you can't take the in-wall/on-wall measurements.

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        • #34
          Regarding the potential for an Allison Effect sourced response dip, go back and read Post #7.
          www.billfitzmaurice.com
          www.billfitzmaurice.info/forum

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          • #35
            Point taken.

            I don't know for other people, but for myself I often don't completely get an idea until I've worked it out for myself.

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            • billfitzmaurice
              billfitzmaurice commented
              Editing a comment
              I can see your point as well, but no matter how many times one tries to reinvent the wheel it always ends up round.
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