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Circlotron 12th February 2011 10:55 AM

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?

Circlotron 12th February 2011 11:22 AM

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...

Circlotron 12th February 2011 01:34 PM

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???

tb46 12th February 2011 05:24 PM

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?


Circlotron 13th February 2011 12:30 AM

Hi tb46.

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

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

full specs:

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 :D
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.

tb46 13th February 2011 02:05 AM

1 Attachment(s)
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.


Circlotron 13th February 2011 06:11 AM

3 Attachment(s)

Originally Posted by tb46 (
An isobaric pair would require 1/2 the volumes, and longer tuning ducts.

That's odd. The details I gave WERE for an isobaric.:confused:
Have a look at the pics.

tb46 13th February 2011 02:51 PM

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?


tb46 13th February 2011 06:18 PM

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


Circlotron 13th February 2011 10:26 PM


Originally Posted by tb46 (
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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