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How to determine internal volume of a trapezoidal.?

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  • How to determine internal volume of a trapezoidal.?

    I have come across a pair of Peavey clones made by Sound Town and appear to be exact copies of the PV215 which is a 2 way PA.
    What I would like to know is the internal volume figuring they are using 3/4 inch MDF.
    The external dimensions are (H x W x D):Front:48.25" x 18.38" x 14.25" Rear:48.25" x 11.75" x 14.25"
    The cabinets appear to be of descent quality, but the internal drivers are crap.

  • #2
    Volume is equivalent to a rect. prism that's 48.25h x 14.25d (IF that's a measurement of the depth front-to-back, NOT the length of the measured (angled) side) x a Width that's the AVErage of the front & rear widths - so... 18.38+11.75=30.13/2 = 15.06".

    If the cabs are 3/4" walls, then your internal vol. should be close to 46.75 x 12.75 x 13.6 = about 4.7cf.

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    • #3
      Originally posted by Chris Roemer View Post
      Volume is equivalent to a rect. prism that's 48.25h x 14.25d (IF that's a measurement of the depth front-to-back, NOT the length of the measured (angled) side) x a Width that's the AVErage of the front & rear widths - so... 18.38+11.75=30.13/2 = 15.06".

      If the cabs are 3/4" walls, then your internal vol. should be close to 46.75 x 12.75 x 13.6 = about 4.7cf.
      Thank you Chris for the info, and if I wanted to use two Dayton Audio PA380-8's in them with the lowest F3 and maximum Xmax/power handling, what could be expected..?
      I am definitely interested in removing the stock drivers and crossover and upgrading it and improving the default F3 of 65hz to possibly 45 to 48hz. The port length would have to be adjusted.

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      • #4
        Assuming the horn structure used up about 0.3cf (leaving the volume around 4.4cf (for the pair), I'd try THREE 4"id ports that were each 6" long.
        WinISD ( @ linearteam.org - sc ) says that would tune the box to 51Hz w/an F3 also in the lower 50s.
        Xmax would limit power to just under 700w RMS down to the low 40s (virtually the entire music range) yielding 124dB in the low 50s.

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        • #5
          Okay just in case you don't know these cabinets do have dual 15's, also another thing I wanted to ask if there are any benefits with going with a sealed box.?
          Benefits such as increased power and Xmax and possibly lower F3 and what about adding Polyfill to make it think its in a bigger box..?
          I noticed that the Dayton Audio DC380-8 15" Classic Woofer is optimized for a 2 cu. ft. sealed with an F3 of 53Hz.

          Last edited by RedK9; 11-27-2020, 07:13 PM.

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          • #6
            2 diff. drivers is apples to oranges.
            But given the same driver, a closed/sealed box will most typically be roughly half the size, and you'll lose between 2/3-3/4ths to a full octave of bottom end. If designed properly, a vented box could also handle more power (around Fb - F3).

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            • #7
              Originally posted by RedK9 View Post
              are any benefits with going with a sealed box.?
              Benefits such as increased power and Xmax and possibly lower F3
              The opposite is what you'll get.
              what about adding Polyfill to make it think its in a bigger box..?
              Drivers aren't that easily fooled. Stuffing lowers Qa, which can lower a midbass peak, which a larger box also does. But it doesn't give the lower F3 and higher sensitivity of a larger box.
              I noticed that the Dayton Audio DC380-8 15" Classic Woofer is optimized for a 2 cu. ft. sealed with an F3 of 53Hz.
              It's optimized for a 5.5 cu ft ported box tuned to 19Hz. You could use it in a 3 cu ft sealed box, but there are far better choices for that option. Besides, it's not a pro-sound driver and is totally unsuited for that application.
              www.billfitzmaurice.com
              www.billfitzmaurice.info/forum

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              • #8
                Yes, sorry I should have said that I usually buy pro audio speakers for home stereo use. Mainly because of their high efficiency/sensitivity..
                In another forum post I mentioned that I had just purchased the JBL 4648 double 15 bass cabs which will be used as my mains in my living room.
                However for this post, these would be used in my studio at home.

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                • #9
                  Formula for Volume of a Trapezoidal Prism. If the prism length is L,trapezoid base width B, trapezoid top width A, and trapezoid height H, then the volume of the prism is given by the four-variable formula: V(L, B, A, H) = LH(A + B)/2. In other words, multiply together the length, height, and average of A and B.

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