Port length

[buzz_cdn]

I am planning a project and the JBL design (#6 in the pro enclosure guide) calls for two 4" ID x 6.5" lg port tubes. If I use outside flared ports would I measure the 6.5" from where the curve of the flare meets the straight tube or consider the flare to be part of the length? Or include half the depth??? Or is it just better to square butt the tube to the cabinet?

Thanks!
Bernard

[89-300ce]

If you calculate the area of the port tube vs. the flare you should see the area increase very quickly as you enter the flare. I would use the port length without the flare in your measurements and it will probably still work out as shorter than the original due to the reduced resistance of the flare entry.

For air flow the most resistance for an inlet is an abrupt tube inlet in free space (Co=~1.0) like the traditional port entry inside the box. Much better is an abrupt inlet on a flat surface, like a traditional port entry at the baffle (Co=~0.5). The flare or bellmouth entry in free space is approx. the same (Co=~0.6). The flare or bellmouth entry on a flat surface (baffle) is the most efficient (Co=~0.06).

I would use an abrupt entry at the baffle (traditional) and a flare entry in the box.
With a flare at the baffle and an abrupt entry in free space inside the box you would theoreticaly be pressurising the box within a few cycles.

Is this significant? Probably not.

Jorg

[buzz_cdn]

Thanks very much. I had been thinking the the bell mouth would just eliminate noise. Of course it has to do with how freely the air flows. Given that the JBL design has some proven history behind it what do you think that changing the efficiency of the ports will do to the expected performance? The drivers are E145-8's and I am looking for accuracy and not trying to boost the bottom.

Thanks again,
Bernard

[duaneage]

Cut the tubes an inch longer than you need, then test the box tuning frequency and cut them as needed. It is difficult to install tubes of a given length into a custom cabinet and get proper results. Cabinet volume is a big variable that is affected by bracing material, stuffing, even wall flex. I've never hit it right without some tuning. Another method is to use blocks of wood or fiberglass to lower or increase the cabinet volume respectively. By keeping the duct tubes a constant length and varying the box volume you can hit the proper tuning frequency. Unless you are precisely duplicating a tested design, your going to have to tune it.

IF the system is for high power long term use tune the box a few cycles lower. The driver will heat up and this usually raises the tuning frequency after time. Failure to account for temperature increases can cause problems.