Reflex Port Equivalents.

george a · 26th November 2002, 11:33 AM

Hi All

Is anybody aware of some formula for calculating what reflex port dimensions can be substituted if you need to change port size.
For example, design calls for 1 port 35MM diameter x 125mm long, what would a 50mm diameter port have to be in length to do the same job.
I assume its not just the volume of the port tube but also must consider frictional losses etc.
Thanks in advance.

Redeye · 26th November 2002, 12:09 PM

It's just the volume of air in the port that's important really (in the electrical analogue it's the moving mass which is the important thing, which is calculated from the volume of air in the port). The most important correction factor is the end corrections due to flanged or unflanged ports which somebody smarter than me will no doubt be able to quote equations for off the top of their head.

In my experience, you normally get the best results with a bit of trial an error on the port length - start with a port a bit longer than you need and cut little bits off until you get the measured response you're looking for (or until it sounds good - i always trust my ears over an analyser).

kelticwizard · 26th November 2002, 02:10 PM

You can also go to Steve Ekblad's site and use the Java-based Port Designer there. Just fill in the volume of the box and go from there. If you are not sure of the box volume, just estimate it, and use the old diameter port. Then adjust the tuning frequency until you find the proper length. Once you find the propler tuning frequency, onsert the larger diameter and in the program and the length will appear.

35mm=1.4 inches
50mm=2 inches
http://www.wssh.net/~wattsup/audio/

Or you can download WinISD. That will calculate the size of any size port in any measuring system.
http://www.linearteam.dk/

By the way, I just ran your port for 1 cubic foot and 2 cubic foot boxes with Steve Ekblad's Port Designer. In both cases, a 35mm X 125 mm port gives the same tuning as a 50 mm X 280 mm port does.

Different programs might yield slightly different results, but they won't be far off.

26th November 2002, 11:33 AM	#1
george a diyAudio Member Join Date: May 2002 Location: oz	Reflex Port Equivalents. Hi All Is anybody aware of some formula for calculating what reflex port dimensions can be substituted if you need to change port size. For example, design calls for 1 port 35MM diameter x 125mm long, what would a 50mm diameter port have to be in length to do the same job. I assume its not just the volume of the port tube but also must consider frictional losses etc. Thanks in advance. __________________ george a

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26th November 2002, 12:09 PM	#2
Redeye diyAudio Member Join Date: Aug 2002 Location: London	It's just the volume of air in the port that's important really (in the electrical analogue it's the moving mass which is the important thing, which is calculated from the volume of air in the port). The most important correction factor is the end corrections due to flanged or unflanged ports which somebody smarter than me will no doubt be able to quote equations for off the top of their head. In my experience, you normally get the best results with a bit of trial an error on the port length - start with a port a bit longer than you need and cut little bits off until you get the measured response you're looking for (or until it sounds good - i always trust my ears over an analyser).

26th November 2002, 02:10 PM	#3
kelticwizard diyAudio Member Join Date: Sep 2001 Location: Connecticut, The Nutmeg State	You can also go to Steve Ekblad's site and use the Java-based Port Designer there. Just fill in the volume of the box and go from there. If you are not sure of the box volume, just estimate it, and use the old diameter port. Then adjust the tuning frequency until you find the proper length. Once you find the propler tuning frequency, onsert the larger diameter and in the program and the length will appear. 35mm=1.4 inches 50mm=2 inches http://www.wssh.net/~wattsup/audio/ Or you can download WinISD. That will calculate the size of any size port in any measuring system. http://www.linearteam.dk/ By the way, I just ran your port for 1 cubic foot and 2 cubic foot boxes with Steve Ekblad's Port Designer. In both cases, a 35mm X 125 mm port gives the same tuning as a 50 mm X 280 mm port does. Different programs might yield slightly different results, but they won't be far off.