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boundry cancellations inside the cab

Discussion in 'Amps and Cabs [BG]' started by will33, Oct 21, 2010.


  1. will33

    will33

    May 22, 2006
    austin,tx
    Here's the conundrum. Looking at doing a pair of cabs to match the footprint of a specific amp, Peavey Classic400, big tube amp. Needing the footprint limits me to building however high I need to reach a workable internal volume.

    What if the height ends up equaling the width? In this case, that's 23". Looking at the boundry cancellation chart, http://www.padrick.net/LiveSound/CancellationMode.htm 23" is 1/4 wavelength at 7.66 ft. That ends up between 35 and 40hz. Total non-issue if that gets cancelled out. 23" is a full wavelength between 145 and 150hz, do not want to lose output there.

    Are these cancellations really narrow? Does having 2 dimensions the same measurement make the cancellation deeper but no less narrow?

    The depth is 15", that's 1/4 wavelength at 5 feet or between 55 and 60hz. Don't want to lose output there. It's also a full wavelength between 225 and 230hz.....really don't want to lose anything there but I have other cabs with similair depth and they don't seem to be missing anything.

    Are these cancellations, although they may add up to a lot of db, in such a narrow bandwidth where I'm over-thinking this? Will it really screw things up if 2 of the 3 dimensions are equal?


    Edit: Should note that I've read things saying it's a bad idea to build a cube, ie: 3 equal dimensions. Also read that the "ideal" ratio should be .6 depth to 1 width to 1.6 height although I haven't reverse engineered these to figure out why or what you lose by not doing it that way.

    Another Edit: Rule of thumb is never put a sub 6 feet from the wall. That's a full wavelength cancellation between 45 and 50hz. Using the 1/4 wavelength method would put you at 18" or between 185 and 190hz. That should be close to or above where you should lowpass a sub at so I'm not sure if I'm calculating the cancellations correctly using the 1/4 wavelength or full wavelength method or maybe going the wrong way, multiply instead of divide?

    Thanks
     
  2. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Cancellations inside the cab are mainly limited to cone to rear panel reflections. If that distance is the typical 10 to 12 inches of a bass cab the null will be in the vicinity of 300Hz. That can be damped out with the usual damping materials, and is one reason why damping isn't optional, it's mandatory. Said null can be as much as 24dB deep.
    The modes created by side to side and top to bottom reflections are resonant, so they add response peaks. Damping also takes care of them unless they are long wavelength, so avoiding internal dimensions longer than 3 feet is prudent. Also avoid squares and cubes, as they can result in peaks and nulls so severe that damping won't suppress them.
    Leo Fender did the same and left us with the curse of side by side drivers. IMO speakers aren't supposed to look good at the expense of sounding good.
     
  3. will33

    will33

    May 22, 2006
    austin,tx
    Yeah, I am putting the cart if not before the horse, at least beside it. The amps footprint is 24x17. Not uncommon external cab dimensions. It also weighs nearly 100lbs. so I don't want it teetering. 1/2" ply and a 1" baffle recess makes the inside 23x15. Thanks for the info, sounds like it'll be ok damping it good which was going to happen anyway.
     
  4. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    That puts the cone distance close enough to the back to keep the response bump at a high enough frequency to be easily damped. BTW, squares and cubes are common with hi-fi subs, but that's OK as the modes are at wavelengths above the subwoofer pass band. It's with full rangers that it's problematic, and that's the source of the Golden Ratio you referred to above, which aids in smoothing response with larger boxes that have modes at wavelengths too long to be easily damped.
     
  5. will33

    will33

    May 22, 2006
    austin,tx
    I've noticed a lot of subs like that that are cubey but yeah, if it happens above the xover freq., it doesn't matter. These would be, at least to start with, full-rangers with the option of adding a mid later if I can decide just what the hell I want in there. Been experimenting with pulling the mid out of a 3-way PA cab, bi-amping it, etc. I really like the openness and dispersion of the sound but as far as xover points and spl matches/mis-matches as it relates to the tonality of my style of bass playing.......if you put what my ear likes on paper it would likely have some checkmarks on it in bright red pen.
     

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