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ShinOBIWAN 29th July 2006 05:44 PM

Effects of non standard boundary loading a port exit?
Hi all,

I'm about to start work on a subwoofer design that was proposed by ScottG and I worked through the finaries and adapted it to my own needs.

Virtually everything has been discussed and agreed upon apart from the issue of the port and how it relates to the base. What am I talking about? Good question, a picture speaks a thousand words, so:

The upper part of the diagram is a cut away side view of the cabinet and the bit I'm interested in is the base which is black. The blue bit is the port and this fires into a cavity in the base (light grey) and this cavity exits from the front of the cabinet.

The lower portion of the diagram is again the base but looking down overhead, the blue circle is the port exit and the light grey the cavity.

What would be the effects of loading the port this way? Is there any benefit to tapering the cavity as shown in the diagram?

johninCR 29th July 2006 06:39 PM

I'm not sure the effects will be very predictable. You've essentially got a port terminating in a cavity. I would stick with something more predictable and just go with a slot port from beginning to end.

Svante 29th July 2006 09:12 PM

The tube will appear slightly longer. The narrower passage, the longer it seems. I could imagine that one builds this box and measures fb and tunes it by changing the length of the internal tube.

I think the design has potential to behave better than just a straight tube, and that it would help to round the corners at the transision between the tube and the outlet.

ThomasW 30th July 2006 04:29 PM

In an ideal situation you want the port terminus no less than it's diameter away from any boundary. Doing this significantly decreases the potential for chuffing.

Hopefully you're going to use a flared port given the low tuning you propose. So it might be best to have no cavity, and elevate the bottom of the cab with legs. That ensures there's minimal resistance near the termius.

Ron E 30th July 2006 06:31 PM

Koonce calls this style of port a diffuser port.

If you put a flare on the port and an inverse flare beneath it, you have what Polk Audio calls a Power Port.

There's Potential advantages, but you may want to test it, or consider what happens to air that exits in different directions from the port...nature likes symmetry...

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