Reducing port size after final construction?

[creyc 10+ year member]

I'm trying to reduce the frequency of a circular port in my sub, to extend my low frequency capbilities. It's currently 6-7/8" and I'll need to take it down to 4-7/8" to get the change I'm looking for. Is there an easy way to do this without major work?

The port is 15" long, the other option I have is to extend this to 32", which is longer than the whole box. How should I go about re-tuning this port?

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[tcguy85 10+ year member]

I'm trying to reduce the frequency of a circular port in my sub, to extend my low frequency capbilities. It's currently 6-7/8" and I'll need to take it down to 4-7/8" to get the change I'm looking for. Is there an easy way to do this without major work?
The port is 15" long, the other option I have is to extend this to 32", which is longer than the whole box. How should I go about re-tuning this port?

can you use a 90 degree bend and add to the port.

 

[JohnEJanowitz 10+ year member]

I'm trying to reduce the frequency of a circular port in my sub, to extend my low frequency capbilities. It's currently 6-7/8" and I'll need to take it down to 4-7/8" to get the change I'm looking for. Is there an easy way to do this without major work?
The port is 15" long, the other option I have is to extend this to 32", which is longer than the whole box. How should I go about re-tuning this port?

It's usually pretty hard to do this after the box is already built. You could go to home depot, get some of the small sonotube they have for concrete forms though and make a sleeve to go inside with a few layers. You can cut it length wise on one side and pull the diameter down to fit as you need it.

Otherwise you could make a plate to screw over the port with a smaller diameter tube in it. Just stick it right into the other one essentially. That way you could unscrew it to get higher tuning and more SPL.

I usually design SPL boxes with multiple ports. That way you can tune high, say 50hz for SPL comp, then plug 2 of 3 ports and be tuned down much lower for everyday use.

John

 

[creyc 10+ year member]

It's usually pretty hard to do this after the box is already built. You could go to home depot, get some of the small sonotube they have for concrete forms though and make a sleeve to go inside with a few layers. You can cut it length wise on one side and pull the diameter down to fit as you need it.
Otherwise you could make a plate to screw over the port with a smaller diameter tube in it. Just stick it right into the other one essentially. That way you could unscrew it to get higher tuning and more SPL.

I usually design SPL boxes with multiple ports. That way you can tune high, say 50hz for SPL comp, then plug 2 of 3 ports and be tuned down much lower for everyday use.

John

Great idea with the plate/tubes. I might have to do just that.

The sub in question is a pre-fabricated kicker box with two 10" L5s. They tune it high to get a nice loud peak around 64Hz, at the expense of a 38Hz F3, a bit high for my tastes. With this port change I should see a flatter response and a lower F3. Taking cabin gain into consideration only magnifies both of these effects.