need clarification regarding dual bandpass design

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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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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.

Face

This section isn't as busy as the home audio area, give it till after the Holiday.

kcoc321

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 lapsed

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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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

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Freaking bump

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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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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.

Jstas

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.

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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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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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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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so far:

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got the carpeting on. its is horrid, but is acceptable for my first.

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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