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how to convert ports

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_Natty_ View Drop Down
Young Croc
Young Croc


Joined: 01 April 2007
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    Posted: 05 October 2016 at 10:56am
Hi 
for a project I need to swap a  
rectangular port with a circle/cylinder... I know is silly but they asking for that :(
I need to fit 18tbx100 so for this  i start from the b&c sub 18" that is a well tested project for this speaker...

I do no trust in programs like bass box etc. and we dont have enough time to try to build up make some measurement etc

what i've imagined is to calculate the lenght, area and volume  of the 2 ports from b&c project.
then put them in system of equation and then try to calculate the equivalent area, volume and leght...
is it a wasting of time?

the internal volume it will be still the same if i not wrong....

thanks to all
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mobiele eenheid View Drop Down
Young Croc
Young Croc


Joined: 15 August 2004
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mobiele eenheid Quote  Post ReplyReply Direct Link To This Post Posted: 05 October 2016 at 1:28pm
Option 1: If you're replacing 2 rectangular ports with 2 round ports, where each port has the same surface area and length, the tuning might be a bit different but in the same ball park (+/- 5 Hz). If that isn't good enough.

Option number 2 would be to trust in a program like bassbox or WiniSD (that's the one I would choose in this situation). Put the length and area of each port (and chamber volume) into the program, while selecting a rectangular port. look what tuning this turns into. Then replicate that tuning but now with round ports. If anything that's one of the best features of WinISD and only 10 minutes of work.

Best regards


Edited by mobiele eenheid - 05 October 2016 at 1:29pm
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outkast View Drop Down
Registered User
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Post Options Post Options   Thanks (0) Thanks(0)   Quote outkast Quote  Post ReplyReply Direct Link To This Post Posted: 05 October 2016 at 6:16pm
A problem you may come up against is that the ports in the B&C design are not really rectangular ports, they are shelf ports - if you look at the drawing you will see that it uses the bottom panel and the side panels of the cab to form parts of the ports, and these panels are not 320mm but the full depth of the cab, i.e. 700mm.

If they were rectangular ports the equivalent round ports would have a diameter of 186mm and a length of 320mm - if I plug that in to WinISD I get a tuning frequency of 44Hz but the drawing says the tuning frequency is 35Hz. That is quite a big difference.

I have played around with a few of my own boxes, and have found that with round ports the difference between WinISD calculation and actual measurements is about 5Hz, but with shelf ports about 9Hz.

I'm no expert but it seems to me that by using the box panels, shelf ports give you some extra port length without stealing any volume from the box, and that shelf ports are not directly equivalent to rectangular/round ports in terms of dimensions.
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_Natty_ View Drop Down
Young Croc
Young Croc


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Post Options Post Options   Thanks (0) Thanks(0)   Quote _Natty_ Quote  Post ReplyReply Direct Link To This Post Posted: 06 October 2016 at 2:29pm
interesting comments give thanks...ill let u know what will happen...
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_Natty_ View Drop Down
Young Croc
Young Croc


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Post Options Post Options   Thanks (0) Thanks(0)   Quote _Natty_ Quote  Post ReplyReply Direct Link To This Post Posted: 07 October 2016 at 9:09am
I did four tubes depth 15 and 0.94 in diameter. So 41.65 cubic cm of volume and 0.693978 cm square of each area
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_Natty_ View Drop Down
Young Croc
Young Croc


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Post Options Post Options   Thanks (0) Thanks(0)   Quote _Natty_ Quote  Post ReplyReply Direct Link To This Post Posted: 07 October 2016 at 2:10pm
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_Natty_ View Drop Down
Young Croc
Young Croc


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Post Options Post Options   Thanks (0) Thanks(0)   Quote _Natty_ Quote  Post ReplyReply Direct Link To This Post Posted: 16 October 2016 at 6:07pm
the thing is done and tested...how we can say if we did a good job without measurement? to my ears best resul crossed at 40hz/200hz but24 db/oct filter. good punchy on the chest and no strange sound even at full power....
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