where does the port end?

[xplod1236]

Where does the port end, at the red line or the blue line?

[BillFitzmaurice]

It would be the blue line, but that's not technically a port. That is a line, the beginning of which is where the dimension of the line is smaller than that of the chamber before it.

[xplod1236]

I know that it could go either way, so would it be acceptable to draw the line in between the two for figuring out port length/box volume?

[Svante]

The simplification to divide the system in a cavity and a port (Isn't this a TL, anyway?) is useful for understanding the principles of its operation, and for an approximate dimensioning. But since the model is not exact, there is little hope of success in having an exact dimensioning from the simplified model.

If you really want to predict the response of this system, you'd have to go down the FEM path, or similar. The other way would be to build it and measure. A third way would be to realise that it probably does not matter that much. A fourth way would be to ask this forum for practical experiences. (which I don't have)

[Jennice]

To me as a beginner in the DIY speaker area, it looks like a TL, which leads me to a question re. the the picture in the first posting. (sorry if it seems slightly off-topic).

It isn't the first time I've seen some debate about these edges, but why couldn't one make an "inner lining" with the kind of flexible metal tube used for kitchen ventilations etc., and fill the remaining volume with expanding foam?

This way there would be no edges and reflections. Kinda like the B&W Nautilus (?) snail shape....

Jens

[Artmaster]

Originally posted by xplod1236
Where does the port end, at the red line or the blue line?

gday!
I think it's a red line, if you wish to calculate confined space for phase inverter port

[MJK]

There is no port. This is a continuum and will exhibit standing waves like a TL. A bass reflex like motion will not occur.

My MathCad worksheets can be used to model this fairly accurately if you calculate an equivalent single driver midway between the two holes on the front baffle in the picture. The MathCad worksheets will allow you to calculate the acoustic and impedance response for the frequency range where the wave motion is along the axis of the path. All of the info you need to perform the calculations can be found on my site. Look under the "General Speaker Related Articles" to see how to model an equivalent single driver in the MathCad worksheets.

A finite element model will calculate all of the standing waves in the enclosure but will not calculate the acoustic and impedance response directly without a fairly complex and elegant model. I have used ANSYS acoustic FEM to double check the MathCad model predictions for standing waves in TL's and the correlation was very good over the frequency range where the MathCad models were intended to be used.

[Artmaster]

have not contradiction.
anyway this is way for sound from driver to outlet. right?
xplod1236 ask about end of this way.
therefore I think - it's red line on the picture

[BillFitzmaurice]

Originally posted by xplod1236
I know that it could go either way, so would it be acceptable to draw the line in between the two for figuring out port length/box volume?

It depends on what you're trying to calculate. I'll make the assumption that you're aware that this is basically a TL design, and that you're trying to calc the line volume. That being the case the line volume is everything within the box, including the driver chamber, exclusive of the space taken up by the driver. But if you're calculating the line length, that which results in the 1/4 wavelength, then you want to conside the driver chamber as a separate entity in this particular case. When figuring the effective length of a bent/folded line/duct/horn it's customary to use the measurement down the middle, rather than to 'hug' either the inner or outer wall. 'Down the middle' splits the difference. In any event you'll only get an approximation, so if you're concerned if the length is enough better to be too long than too short.

[xplod1236]

Originally posted by Svante
The simplification to divide the system in a cavity and a port (Isn't this a TL, anyway?)

No, it's not a TL. It is a bass reflex enclosure for two 12"s with more than 1 inch of excursion each. The port is 4" x 17" to minimize port noise. The air velocity thru the port would be about 17m/s.

Originally posted by MJK
There is no port. This is a continuum and will exhibit standing waves like a TL. A bass reflex like motion will not occur.

Then what am I supposed to do to get it to work as bass reflex?