need clarification regarding dual bandpass design

Chase lapsedChase lapsed Posts: 17
edited March 2010 in Custom Fabrication
this one here
http://www.polkaudio.com/caraudio/subbox/subbox_plan.php?id=47&mesurements=standard

The guide is not overly user friendly and polk does not give the length of each chamber, they give the volume, which is no good to me.

The box is supposed to be 35 1/2 inches in width 17 inches in length and 16 inches in height.
Accounting for the width of the 3/4 inch MDF, I would like to know if someone can figure out the length of each chamber, so I can start on the box blueprints.

I also am aware that there are 3 vents, 4 inches in diameter and 9 inches in length. however from the reference image given by polk the chambers appear to be staggered. Is this correct procedure ?

I would appreciate any help because this would be my first custom box build.
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Post edited by Chase lapsed on

Comments

  • Chase lapsedChase lapsed Posts: 17
    edited September 2009
    bump, did some algebra on the proper width of each chamber and they don't add up to the total box width. ie 62.68 cm width versus 90cm in width.
    chamber one volume (40.64*43.18) x = 32000 which X equals 18.235 along with chamber 3. Chamber two, (40.64*43.18) x = 46010 which X equals 26.2189 cm .

    plus no one wants to reply to this thread.
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  • FaceFace Posts: 14,714
    edited September 2009
    This section isn't as busy as the home audio area, give it till after the Holiday.
    "He who fights with monsters should look to it that he himself does not become a monster. And when you gaze long into an abyss the abyss also gazes into you." Friedrich Nietzsche
  • kcoc321kcoc321 Posts: 1,779
    edited September 2009
    while you do need to account for the thickness of the MDF in your calc,
    I checked the math on the sheet provided..
    no matter how you calc it, the specs linked are not accurate:
    either the areas are wrong or the wisth of the box is...

    chamber cross section
    height 40.64cm - 2*1.9cm = 36.84cm
    depth 43.18cm - 2*1.9cm = 39.38cm
    cross section area 1450.7592sq cm

    Result of your conversion:
    32 liters is equal to 32,000.00 cubic centimeters

    chamber #1 & 3:
    area spec'd: 32,000.00cc / section = 22.0574cm
    chamber #2:
    area spec'd: 46,010.00cc / section = 31.7144 cm

    length verification
    chamber #1 22.0574cm
    wall 1.9cm
    chamber #2 31.7144cm
    wall 1.9cm
    chamber #3 22.0574cm
    TOTAL: 79.6293cm
    vs: 89.94cm as shown on spec sheet......NO GOOD

    you might want to check with whomever provided you the specs for the box.
  • Chase lapsedChase lapsed Posts: 17
    edited September 2009
    kcoc321 wrote: »
    while you do need to account for the thickness of the MDF in your calc,
    I checked the math on the sheet provided..
    no matter how you calc it, the specs linked are not accurate:
    either the areas are wrong or the wisth of the box is...

    chamber cross section
    height 40.64cm - 2*1.9cm = 36.84cm
    depth 43.18cm - 2*1.9cm = 39.38cm
    cross section area 1450.7592sq cm

    Result of your conversion:
    32 liters is equal to 32,000.00 cubic centimeters

    chamber #1 & 3:
    area spec'd: 32,000.00cc / section = 22.0574cm
    chamber #2:
    area spec'd: 46,010.00cc / section = 31.7144 cm

    length verification
    chamber #1 22.0574cm
    wall 1.9cm
    chamber #2 31.7144cm
    wall 1.9cm
    chamber #3 22.0574cm
    TOTAL: 79.6293cm
    vs: 89.94cm as shown on spec sheet......NO GOOD

    you might want to check with whomever provided you the specs for the box.

    Thanks for the reply . I think that the 79.6293cm is correct, because the box total volume equals to 110L.

    "Overall External Dimensions
    Width 35-41/100" (89.94cm)"
    Good one, Polk employees.
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  • Chase lapsedChase lapsed Posts: 17
    edited September 2009
    I would appreciate if someone with some building experience can double check my cut-out sheets for my box.
    81.7x46.6cm 2 sheets (top/bottom)
    46.6x43.6cm 2 sheets (ends)
    81.7x43.6cm 2 sheets (sides)
    40x43cm 2 sheets (chamber seperators)

    and my drawings
    resizedscan0012776x930.jpg
    resizedscan0013778x927.jpg
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  • Chase lapsedChase lapsed Posts: 17
    edited September 2009
    Freaking bump
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  • Chase lapsedChase lapsed Posts: 17
    edited October 2009
    kcoc321 wrote: »
    while you do need to account for the thickness of the MDF in your calc,
    I checked the math on the sheet provided..
    no matter how you calc it, the specs linked are not accurate:
    either the areas are wrong or the wisth of the box is...

    chamber cross section
    height 40.64cm - 2*1.9cm = 36.84cm
    depth 43.18cm - 2*1.9cm = 39.38cm
    cross section area 1450.7592sq cm

    Result of your conversion:
    32 liters is equal to 32,000.00 cubic centimeters

    chamber #1 & 3:
    area spec'd: 32,000.00cc / section = 22.0574cm
    chamber #2:
    area spec'd: 46,010.00cc / section = 31.7144 cm

    length verification
    chamber #1 22.0574cm
    wall 1.9cm
    chamber #2 31.7144cm
    wall 1.9cm
    chamber #3 22.0574cm
    TOTAL: 79.6293cm
    vs: 89.94cm as shown on spec sheet......NO GOOD

    you might want to check with whomever provided you the specs for the box.

    I noticed something to add to the calculation. We have to add the exterior walls of 1.9cm on each end as well. I also did some work in excel to verify everything.
    Dual Bandpass Box Details for two MM2124	chamber 1	chamber 2	chamber 3
    height (cm)	36.84	36.84	36.84
    length (cm)	22.0574	31.7144	22.0574
    depth (width) (cm)	39.38	39.38	39.38
    chamber volumes (set as Vb) (cm3)	31999.97598	46009.95757	31999.97598
    			
    total interior length (cm)	75.8292		
    total exterior length (cm)	83.4292		
    total interior height (cm)	36.84		
    total exterior height (cm)	40.64		
    total interior depth (cm)	39.38		
    total exterior depth (cm)	43.18		
    			
    total interior volume (cm3)	110009.9095		
    total exterior volume (cm3)	146404.4969		
    			
    total external area (cm2)	17495.74149		
    total external area (m2)	174.9574149		
    			
    			
    total external area in sq. inches	2711.845354		
    total external area in sq. feet	225.9871129		
    total external length in inches	32.84614173		
    total external height in inches	16		
    total exterior depth in inches	17		
    
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  • Chase lapsedChase lapsed Posts: 17
    edited January 2010
    small update. I have cut the 4 inch diameter PVC ports now. When installing these, should I cover their outsides with dynamat to give them some weight or just nothing ? Or will the dracon stuffing inside the box be just fine.
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  • JstasJstas Posts: 13,709
    edited January 2010
    Adding mass to ports runs the risk of changing their tuning. Unless you know how the adding of mass will affect the resonances of the tubes and/or tube materials, I wouldn't do it. If you are worried about them vibrating against each other, unless they are very close together, the won't really vibrate.

    I have seen some designs where ports are run through internal bracing but with 4 inch ports only 9 inches long, there is plenty of material and heft that they shouldn't vibrate at all. You could always use some foam rings glued with like 3M General Trim Adhesive on to ports as spacers. They won't change properties as much as dynamat will.

    As for polyfil, I wouldn't put any in the vented chamber at all. You could use it in the sealed chambers if you like but I wouldn't stuff the outside chambers either. maybe half a pound of polyfill for each sealed chamber at the most.

    One thing though. If you are using different box dimensions, are you sure your ports are sized properly for the frequency that your bandpass is tuned at? If not, you could end up disappointed in performance.
    You're just jealous 'cause the voices don't talk to you!
  • Chase lapsedChase lapsed Posts: 17
    edited January 2010
    Jstas wrote: »
    Adding mass to ports runs the risk of changing their tuning. Unless you know how the adding of mass will affect the resonances of the tubes and/or tube materials, I wouldn't do it. If you are worried about them vibrating against each other, unless they are very close together, the won't really vibrate.

    I have seen some designs where ports are run through internal bracing but with 4 inch ports only 9 inches long, there is plenty of material and heft that they shouldn't vibrate at all. You could always use some foam rings glued with like 3M General Trim Adhesive on to ports as spacers. They won't change properties as much as dynamat will.

    As for polyfil, I wouldn't put any in the vented chamber at all. You could use it in the sealed chambers if you like but I wouldn't stuff the outside chambers either. maybe half a pound of polyfill for each sealed chamber at the most.

    One thing though. If you are using different box dimensions, are you sure your ports are sized properly for the frequency that your bandpass is tuned at? If not, you could end up disappointed in performance.

    Yes, the ports are sized according to the manufacturer's specifications according to the dual-bandpass design. Thank you for the reply Jstas and nice going for your 8,000th post !
    I just cut up some of the sides last night. I was using a bad straight edge and my side panels have to be re-cut. One side was out 1/8th inch.
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  • Chase lapsedChase lapsed Posts: 17
    edited January 2010
    Jstas wrote: »
    Adding mass to ports runs the risk of changing their tuning. Unless you know how the adding of mass will affect the resonances of the tubes and/or tube materials, I wouldn't do it. If you are worried about them vibrating against each other, unless they are very close together, the won't really vibrate.

    I have seen some designs where ports are run through internal bracing but with 4 inch ports only 9 inches long, there is plenty of material and heft that they shouldn't vibrate at all. You could always use some foam rings glued with like 3M General Trim Adhesive on to ports as spacers. They won't change properties as much as dynamat will.

    As for polyfil, I wouldn't put any in the vented chamber at all. You could use it in the sealed chambers if you like but I wouldn't stuff the outside chambers either. maybe half a pound of polyfill for each sealed chamber at the most.

    One thing though. If you are using different box dimensions, are you sure your ports are sized properly for the frequency that your bandpass is tuned at? If not, you could end up disappointed in performance.

    Wanted to ask you; what's a good distance apart of each screw ? Is 6 inches sound or maybe more or less ?
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  • Chase lapsedChase lapsed Posts: 17
    edited February 2010
    i am almost complete. need to install side handles and then the top side. put about a pound of polyfill in each sealed chamber. will update with pics and video later.
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  • Chase lapsedChase lapsed Posts: 17
    edited February 2010
    p1050521p.th.jpg
    p1050522.th.jpg
    so far:
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  • Chase lapsedChase lapsed Posts: 17
    edited March 2010
    got the carpeting on. its is horrid, but is acceptable for my first.
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  • Chase lapsedChase lapsed Posts: 17
    edited March 2010
    thinking of going with one of these methods do drive the subwoofers both equally
    http://img30.imageshack.us/img30/7599/scan0001pd.jpg
    http://img16.imageshack.us/img16/4858/scan0002yc.jpg
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