Port lengths for 2-chamber 2-port bandpass - diyAudio
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Old 12th February 2011, 10:55 AM   #1
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Default Port lengths for 2-chamber 2-port bandpass

Starting to think about building a new sub.
Fiddling with WinISD 0.7 and altering the tuning for minimum cone excursion over the range 22Hz - 50Hz while using the option "force Flat Response". Now it just so happens I have a maximum =total= box volume of 450 litres that I can split any way I want. For 23.1Hz and 39Hz I can have 333.5 litres and 117 litres respectively and this gives me a port length of 62.7mm for both chambers.

Is it a good approach to have identical ports, or is it better to have them proportional to frequency as per the chamber volumes?
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Old 12th February 2011, 11:22 AM   #2
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As a summary of chambers and ports so far:

for identical ports 102mm diameter:
333 litres, 62.7mm, 23.1Hz
117 litres, 62.7mm, 39Hz.
450 litres total

For chambers inversely proportional to frequency:
282 litres, 87.8mm, 23.1Hz
167 litres, 21.6mm, 39Hz.
449 litres total

Both give identical responses.
Something tells me the ratio of port length to chamber volume should be the same for both sides...
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Last edited by Circlotron; 12th February 2011 at 11:34 AM.
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Old 12th February 2011, 01:34 PM   #3
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Equal ratio of port length to chamber volume, for ports 102mm diameter:
4mm per litre.

304 litres, 76mm, 23.1Hz
145 litres, 36.25mm, 39Hz


Which of the above three methods is best???
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Old 12th February 2011, 05:24 PM   #4
tb46 is offline tb46  United States
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Hi Circlotron,

4"Dia. ports sound a bit small for such a big box. It would be nice to know what driver - and application - you are looking at. And just a dumb question: Do you prefer the 6th order over the 4th order bandpass, and why?

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Last edited by tb46; 12th February 2011 at 05:27 PM.
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Old 13th February 2011, 12:30 AM   #5
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Hi tb46.

It's sort of a mixed bag, this project.

This driver:
Altronics - Your One Stop Audio Visual & Electronics Supplier

full specs:
http://www.altronics.com.au/download...ns/c/C3033.pdf

1/ I have to fit the box into a very specific space and match existing furniture.
2/ I got two drivers (isobaric setup) at a very good price
3/ having only two port tubes and nothing else facing the outside world solves LOTS of problems re appearance and domestic acceptability
4/ I always wanted to build a 6th order bandpass.

I mostly want to use it for home theatre from about 22Hz to 50Hz. Going low is more important than sounding accurate.
From previous experience with other subs, 4 inch ports should be big quite enough for the moderate sound levels I want. I can appreciate that other people would want larger though.
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Last edited by Circlotron; 13th February 2011 at 12:40 AM.
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Old 13th February 2011, 02:05 AM   #6
tb46 is offline tb46  United States
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Hi Circlotron,

Sounds like a fun project. I simulated a single driver in BP6, and it likes about all the volume you are talking about. An isobaric pair would require 1/2 the volumes, and longer tuning ducts.

Regards,
Attached Images
File Type: jpg Altronics_C3033_All_small.jpg (130.2 KB, 124 views)
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Old 13th February 2011, 06:11 AM   #7
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Quote:
Originally Posted by tb46 View Post
An isobaric pair would require 1/2 the volumes, and longer tuning ducts.
That's odd. The details I gave WERE for an isobaric.
Have a look at the pics.
Attached Images
File Type: png Driver.png (24.9 KB, 114 views)
File Type: png Box.png (22.6 KB, 113 views)
File Type: png Driver specs.png (13.7 KB, 112 views)
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Old 13th February 2011, 02:51 PM   #8
tb46 is offline tb46  United States
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Hi Circlotron,

I don't have any opinion on WinISD. I tried this driver in Boxplot and Hornresp. Boxplot doesn't allow for BP6, but will do BP4 isobaric (Fs=18.2Hz Vb=152.7L Vf=42.3L Vent=10.1cm_Dia/28cm long). Hornresp doesn't do isobaric (you would have to measure the isobaric pair and insert the compound values to make Hornresp think it is looking at one driver), but it will simulate a BP6 just fine.

On the WinISD driver editor picture you attached it says "...Voicecoils 1...", is that correct?

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Old 13th February 2011, 06:18 PM   #9
tb46 is offline tb46  United States
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Hi Circlotron,

I downloaded the WinISD Pro Alpha Version 0.50a7, and it gives a popup: "Warning! ...can't calculate optimum box parameters ...". It popped up some enclosure values around 100L/20Hz-26.2L/63.47Hz, I reset these to something closer to your values:

Vr=300/F=20Hz/Vent: Dia=10.2cm/L=12.91cm
Vf=55/F=50Hz/Vent: Dia=10.2cm/L=10.31cm

With a bigger vent diameter of 15.2cm (6") it likes:


Vr=300/F=20Hz/Vent: Dia=15.2cm/L=34.11cm
Vf=43/F=55Hz/Vent: Dia=15.2cm/L=30.62cm

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Old 13th February 2011, 10:26 PM   #10
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Quote:
Originally Posted by tb46 View Post
On the WinISD driver editor picture you attached it says "...Voicecoils 1...", is that correct?
Yep, that means 1 voice coil per driver.
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