End correction formula for rectangular port ?

philippe_95 · 28th February 2007, 03:34 PM

I'm building vented speakers with a rectangular vented port. The port is a part of the walls cabinet (it uses 3 walls of the cabinet, the fourth side must be adjusted at the port length needed).

Of course I can use some dedicated software in order to calculate (estimate) the port length.

But, for my intellectual curiosity, I would like to know how to calculate or estimate the end correction of such a vented port.

I can find some rules in the bibliography, but it's always for a round port. Nothing is clear for rectangular port. And it's less clear for a port using walls cabinet.

Can somebody be abble to provide me some informations ?

Many thanks in advance.
Philippe.

Ron E · 28th February 2007, 05:08 PM

I am afraid if you are trying a special situation, it is cut and try. There are some rules of thumb out there, like subtract half the height of the slot when using a cabinet wall, but I am afraid the best method is to make it too long to begin with, then measure and adjust. There is nothing really wrong with a port tuned too low, they only become a problem when tuned too high...

Svante · 1st March 2007, 01:15 PM

That is my feeling too, and this is sort of problematic because rectangular ports are typically more integrated with the box design, making the length harder to adjust.

The end correction for circular tubes comes from the radiation impedance of a baffled piston, and I could imagine that it would be possible to find an analytical expression for the radiation impedance of an infinitely long bar. Has anyone seen such an expression?

GM · 1st March 2007, 06:03 PM

Greets!

Does this help?: http://www.aplac.hut.fi/publications...-corrected.pdf

GM

Svante · 1st March 2007, 08:07 PM

Quote:

Originally posted by GM
Greets!

Does this help?: http://www.aplac.hut.fi/publications...-corrected.pdf

GM

I don't think so. As far as I see from a quick look the article is about the air between two surfaces, not outside the port.

GM · 1st March 2007, 08:16 PM

Greets!

OK, then what is all the end correction math all about?

GM

Svante · 2nd March 2007, 03:25 PM

Quote:

Originally posted by GM
Greets!

OK, then what is all the end correction math all about?

GM

Ah, sorry, I did not read carefully enough. Yes it is there, interesting!

philippe_95 · 3rd March 2007, 01:17 PM

Yes very interesting.

But is the end correction formula only for one side of the port ? In this case, the correct value should be the double.

In the paper it's write than the expression is for a rectangular duct ended with an infinite baffle. But in a loudspeaker enclosure, on side of the port is ended with a baffle, but the other side has an unflanged opening (or quite because of the walls of the cabinet on 3 sides).

ente · 3rd March 2007, 03:18 PM

... maybe this is what you are looking for

Its good for a first calculation
but measurements with LIMP are much more reliable -->
http://www.fesb.hr/~mateljan/arta/download.htm

Regards
Heinrich

philippe_95 · 3rd March 2007, 05:33 PM

Great ! Many thanks !

But by what K should be multiplied ?

For a cylindric port, 0.732 is the coefficient for the diameter.

But for another shape, is it the equivalent diameter that we have to use ?

I have already LIMP. It's a very convinient software. You are right, the calculation is for a first estmation.

28th February 2007, 03:34 PM	#1
philippe_95 diyAudio Member Join Date: Nov 2006	End correction formula for rectangular port ? I'm building vented speakers with a rectangular vented port. The port is a part of the walls cabinet (it uses 3 walls of the cabinet, the fourth side must be adjusted at the port length needed). Of course I can use some dedicated software in order to calculate (estimate) the port length. But, for my intellectual curiosity, I would like to know how to calculate or estimate the end correction of such a vented port. I can find some rules in the bibliography, but it's always for a round port. Nothing is clear for rectangular port. And it's less clear for a port using walls cabinet. Can somebody be abble to provide me some informations ? Many thanks in advance. Philippe.

28th February 2007, 05:08 PM	#2
Ron E diyAudio Member Join Date: Jun 2002 Location: USA, MN	I am afraid if you are trying a special situation, it is cut and try. There are some rules of thumb out there, like subtract half the height of the slot when using a cabinet wall, but I am afraid the best method is to make it too long to begin with, then measure and adjust. There is nothing really wrong with a port tuned too low, they only become a problem when tuned too high... __________________ Our species needs, and deserves, a citizenry with minds wide awake and a basic understanding of how the world works. --Carl Sagan Science is a way of thinking much more than it is a body of knowledge. --Carl Sagan

1st March 2007, 01:15 PM	#3
Svante diyAudio Member Join Date: Feb 2004 Location: Stockholm	That is my feeling too, and this is sort of problematic because rectangular ports are typically more integrated with the box design, making the length harder to adjust. The end correction for circular tubes comes from the radiation impedance of a baffled piston, and I could imagine that it would be possible to find an analytical expression for the radiation impedance of an infinitely long bar. Has anyone seen such an expression? __________________ Simulate loudspeakers: Basta! Simulate the baffle step: The Edge

1st March 2007, 06:03 PM	#4
GM diyAudio Member Join Date: Jun 2003 Location: Chamblee, Ga.	Greets! Does this help?: http://www.aplac.hut.fi/publications...-corrected.pdf GM __________________ Loud is Beautiful if it's Clean! As always though, the usual disclaimers apply to this post's contents.

1st March 2007, 08:16 PM	#6
GM diyAudio Member Join Date: Jun 2003 Location: Chamblee, Ga.	Greets! OK, then what is all the end correction math all about? GM __________________ Loud is Beautiful if it's Clean! As always though, the usual disclaimers apply to this post's contents.


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3rd March 2007, 01:17 PM	#8
philippe_95 diyAudio Member Join Date: Nov 2006	Yes very interesting. But is the end correction formula only for one side of the port ? In this case, the correct value should be the double. In the paper it's write than the expression is for a rectangular duct ended with an infinite baffle. But in a loudspeaker enclosure, on side of the port is ended with a baffle, but the other side has an unflanged opening (or quite because of the walls of the cabinet on 3 sides).

3rd March 2007, 03:18 PM	#9
ente diyAudio Member Join Date: Aug 2004 Location: NNW of Rome	Vent ... maybe this is what you are looking for Its good for a first calculation but measurements with LIMP are much more reliable --> http://www.fesb.hr/~mateljan/arta/download.htm Regards Heinrich

3rd March 2007, 05:33 PM	#10
philippe_95 diyAudio Member Join Date: Nov 2006	Great ! Many thanks ! But by what K should be multiplied ? For a cylindric port, 0.732 is the coefficient for the diameter. But for another shape, is it the equivalent diameter that we have to use ? I have already LIMP. It's a very convinient software. You are right, the calculation is for a first estmation.

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