port spacing for dual port design

[Hezz]

Hi guys,

Sorry, I haven't posted here on subwoofers before. I designed this subwoofer for a friend and for various reasons have decided to go with dual 4 inch ports.

A five inch port was the easiest to implement but the hardest to find parts for. I wanted to use PVC or ABS but five inch stuff is special order only in large quantities and six inch stuff which would have worked is very expensive and takes up so much room in teh enclosure that the bracing will be hard to design.

So I'm going the dual 4 inch route. Port velocities are very nice with this solution.

The question I have is port spacing between the two ports. Is there an issue if the ports are tuned to the same frequency. Will they couple if close together or will I get some interference and should I space them as far apart as I can. Here is the current design without the bracing done yet.

[weltersys]

Originally Posted by Hezz

The question I have is port spacing between the two ports. Is there an issue if the ports are tuned to the same frequency. Will they couple if close together or will I get some interference and should I space them as far apart as I can. Here is the current design without the bracing done yet.

Assuming a Fb of 40 Hz, the wavelength of the primary port output would be 28.25 feet long, any placement on the cabinet will still be within 1/4 wavelenght.
The ports as drawn will work fine.

Putting "top hats" on the inner ends of the ports can reduce the length needed fairly considerably.

[Hezz]

Originally Posted by weltersys

Assuming a Fb of 40 Hz, the wavelength of the primary port output would be 28.25 feet long, any placement on the cabinet will still be within 1/4 wavelenght.
The ports as drawn will work fine.

Putting "top hats" on the inner ends of the ports can reduce the length needed fairly considerably.

Thanks for the feedback. The Fb is set at 25Hz so that should give me even greater lattitude.

The software I'm using (Win ISD) calls for the ports to be around 28.8 inches long. When you say "top hats", do you mean a flat flange area at the end of the inner port terminations?

[weltersys]

Originally Posted by Hezz

The software I'm using (Win ISD) calls for the ports to be around 28.8 inches long. When you say "top hats", do you mean a flat flange area at the end of the inner port terminations?

Yes.
On a test cabinet I used a 7 x 7 inch square port with an 11" circular flange.
I was shooting for a 37 Hz Fb, and had made the port only slightly longer than predicted for a standard port of 49 square inches.
The flange ended up lowering the tuning to 20 Hz, I had to cut the port down from around 13 inches to 9 inches to get the tuning where I wanted.

Sawing off an inch (or so) at a time resulted in 25 Hz, 28 Hz, 35, and 37 Fb.