How Big a Hole Until a Port Is No Longer a "Port"?

[toddalin]

My center channel cabinet has two outter 4 cu ft cabinets that are ported to the exterior AND to a cental 2 cu ft cabinet, also ported to the exterior. All ports are 4" dia x 9.5" long.

The intent is to make this a single 4 cu ft cabinet for the W15GTI and a single 6 cu ft cabinet for the center channel's 2235. It's a simple matter to block off the W15GTI's 4 ft cabinet to the center to attain a 4 cu ft cabinet with one 4" dia 9.5" long port.

It should also be easy to open up the other side to the center for a 6 cu ft cabinet for the 2235. The question is, how much material do I actually have to remove from teh interior wall until they become one and the same cabinet?

Certainly the easiest way to do this would be to riddle the internal wall with holes which would retain much the strength of the inner wall. (I have a 3.125" drill that I used for the 075). I just need to know the minimum/optimum number of holes to accomplish the task.

Thanks

[GordonW]

Interesting question, that I don't know has been scientifically evaluated... at least, if it has, results of such testing hasn't been published where I've found them...

That said, My usual take on that situation, is that if the hole between internal chambers is the BIGGER of the two following possibilities, it'll NOT act as a tuning vent:

1) Bigger than HALF the area of the cabinet in cross-section
2) Bigger than the cone area of the driver in the cabinet.

No, that's not scientific credo, but it has worked/measured/sounded fine in practice, for the cabinets I've built in the last 20 years, at least for the most part...

This is one of those things that's on my list, to research/experiment with further. In designing and building my own speakers, I'm finding there's a LOT of minor and not-so-minor variables like these, that CAN make a difference, SOMETIMES...

Regards,
Gordon.

[moldyoldy]

It's my understanding that any signifigant reduction in area between two compartments can cause a Helmholtz-effect cavity resonance by introducing a delay in equalization of compression and rarefaction between compartments. Unfortuneately, I haven't been able to locate the appropriate material to verify or elaborate on it yet.

[duaneage]

The actual room or space the port empties into is a factor. For example, if you have a pickup cab and have large drivers behind the seat in a separate enclosure, opening the windows affects the tuning because the cab of the truck becomes a second chamber.

As to the divider between the two the second area is probably too small to have an effect on the "port" between them. Take into account there are drivers on both sides so that means the port sees action from both sides. Take the port length and area and calculate a resonance based on one or both volumes and the value you come up with would be out of range for any effect.

Since there is pressure on both sides of this"port" it cancels out any resonance that might develop normally.