Fine tuning my sub cabinet

I finally finished my sub cabinet! The 2242H in my 8+ cu.ft. box, tuned to the recommendations of Giskard and the like (thanks guys) sounds great!
I did a sweep and it seems to be tuned to 32Hz, though the 2242H isn't broken in yet.
When I play CDs through it I don't hear much air coming from the ports. I suspect it's because there isn't that much 30Hz stuff on most CDs.
Since I want to use this sub in a PA application, with the graphic EQ rolling off stuff below 40Hz, would it make sense to raise the tuning of this cabinet?
How much? Would 40Hz be a good point? Will shortening the 3 ducts accomplish this?

Any input will be appreciated.

Tom

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Originally Posted by Tom Loizeaux

I did a sweep and it seems to be tuned to 32Hz, though the 2242H isn't broken in yet.

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Mine took around 100 to 200 hours of playing(low wattage-under 100 watts), but moves some serious air now with as little as 10 to 20 watts!;)

I love mine and never regret getting it!

Ron

Unless you're going to fairly "brick wall" filter it below 40 Hz or so, I'd be inclined to leave the vent down at 32, or a max of like 35 Hz. It'll make sure that any low-frequency "rumble" that make it through, doesn't unload the cabinet and bottom out the woofer. Last thing you need, is to mushroom the bottom of the voice coil!

Regards,
Gordon.

The port damps the driver and it's optimum frequency depends on the driver and the volume of air it is mounted in. Raising the port frequency will stiffen the air in the box at too high a frequency causing poor transient response. At the same time the drive will not be controlled at 32 hz where it should be. Port tuning should not be used for frequency response shaping or in response to anticipated use.

Thanks for the responses guys.
I gave my 18" sub a "field trial" last Friday night. I helped a DJ fill in his speaker system in the low end. I used my 2242H on one side and a Gauss 15 on the opposite side. I crossed the subs at 90Hz to a pair of JBL 2-15 w/ horn cabinets and drove the subs with a Crest CA12.
Well, there IS 30Hz material in disco CDs! The ports were pumping serious air - and it sounded good! I'm glad I left the cabinet tuning at 32Hz. The ports helped reduce cone excursion at the low end, giving the driver a chance to do what it's supposed to do. There was plenty of low bass behind the speaker as well. By the end of the night I began to see why people use multiple 18s in larger systems.

Tom

THe neat thing about ports are at the resonant frequency the port radiates MOST of the sound and the driver hardly moves. For an 18 inch woofer you should have at least a 6 inch port if not multiple ports to bring it up to the amount of an 8 inch port. Really high power applications recommend port areas equal to the driver. A shelf port built into the cabinet is the most efficient way to get serious port area although they are harder to tune.

Large ports reduce port compression at high power (which raises the tuning frequency dynamically). Smaller ports will test ok at small signals but above 50 watts the port frequency starts to shift.

The drivers might heat up a bit, further raising the Qts and requiring a different tuning frequency. If your going to be running them constantly at high power I would consider dropping the tuning frequency a little to 31 or 30 and trying that out.