Round vs shelf port

[muddycreek]

So, I'm kicking around building a 215 box, solely because I have a couple of old TOA PA cabinets with decent 15's and really crappy crossovers and mid/high drivers. No T/S specs are available, so I figured I'd just design off the current PA boxes for now.

In reverse engineering the tuning of the current boxes, the tuning isn't ideal for the new drivers. The size is fine.

I've only built cabinets with round ports. If I build the cabinet with a shelf port, I can build it to amend the port easily for new drivers. Are there any concerns specific to shelf ports I should be aware of? Or is a shelf port of x area and y length the same as a round port (or two) with the same dimensions?

[billfitzmaurice]

Originally Posted by muddycreek

Are there any concerns specific to shelf ports I should be aware of? Or is a shelf port of x area and y length the same as a round port (or two) with the same dimensions?

It depends on the width to height ratio and how close the duct entrance is to the cab back. High ratios and close distances will both affect the result, and there's no simple formula that accurately predicts the result. If the ratio is in excess of 4:1 and/or the area between the back and duct entrance is less than twice that of the duct you really have to measure the result with an impedance sweep to be sure of the result.

[Bluesy Soul]

Originally Posted by billfitzmaurice

It depends on the width to height ratio and how close the duct entrance is to the cab back. High ratios and close distances will both affect the result, and there's no simple formula that accurately predicts the result. If the ratio is in excess of 4:1 and/or the area between the back and duct entrance is less than twice that of the duct you really have to measure the result with an impedance sweep to be sure of the result.

I know what you stated is correct, Bill; and you stated it much better than I could have. And isn't it true for ports regardless of their shape?

[billfitzmaurice]

Originally Posted by Bluesy Soul

I know what you stated is correct, Bill; and you stated it much better than I could have. And isn't it true for ports regardless of their shape?

Anytime the duct gets too close to the back of the cab complications will arise, though bass cabs tend to be tuned high enough that the ducts aren't that long. Severe width/height ratios tend to be found in shelf ports, but the same caution would apply to an oval.

[standupright]

Originally Posted by billfitzmaurice

It depends on the width to height ratio and how close the duct entrance is to the cab back. High ratios and close distances will both affect the result, and there's no simple formula that accurately predicts the result. If the ratio is in excess of 4:1 and/or the area between the back and duct entrance is less than twice that of the duct you really have to measure the result with an impedance sweep to be sure of the result.

i keep trying to understand this stuff......and everytime i read one of your posts i just want to shoot myself in the face.

I R DUM

[Bluesy Soul]

Originally Posted by billfitzmaurice

Anytime the duct gets too close to the back of the cab complications will arise, though bass cabs tend to be tuned high enough that the ducts aren't that long. Severe width/height ratios tend to be found in shelf ports, but the same caution would apply to an oval.

Then is it correct to say that port shape is not an issue in and of itself; but it may become an issue if the port--of any shape--has a such an unbalanced width-to-height ratio that it requires the port to be longer to meet the required volume, which in turn might place the end of the port too close to the back of the cab? If this is the case, it's easy to see how that issue might arise more easily with shelf-type ports than round ones.

[billfitzmaurice]

Originally Posted by Bluesy Soul

Then is it correct to say that port shape is not an issue in and of itself; but it may become an issue if the port--of any shape--has a such an unbalanced width-to-height ratio that it requires the port to be longer to meet the required volume, which in turn might place the end of the port too close to the back of the cab? If this is the case, it's easy to see how that issue might arise more easily with shelf-type ports than round ones.

It has to do with friction the of air passing through the duct. The higher the duct area to wall area ratio the less friction and the closer the duct will work compared to the predicted result. Round is best in that respect. When you get a very high width to height ratio not only is there more wall friction but also the smaller minimum cross-section can cause turbulence in the duct, increasing friction further.

[Bluesy Soul]

Originally Posted by billfitzmaurice

It has to do with friction the of air passing through the duct. The higher the duct area to wall area ratio the less friction and the closer the duct will work compared to the predicted result. Round is best in that respect. When you get a very high width to height ratio not only is there more wall friction but also the smaller minimum cross-section can cause turbulence in the duct, increasing friction further.

Thanks, Bill, as always. My education continues.