i have designed a 7 cft enclosure for an 18 btl. the volume is with woofer displacement.

how do i calculate a port to 30 hz then?

is this ok?: 4,72" * 19,69" * 29,92"

That port takes up 1.88 cubes, lowering your box volume to 5.12 cubes. The port dimensions make the tune 36 hz.

if i increase port length the net volume would lower under 5 cft.

does the 18 btl sound good with 36 hz? id like some sq but also much of loudness.

If you want, you could get two 6" aeroports and make each 24" long. That would give you net of about 6 cubes and tuned to 30 hz. Aero ports help you need less port area without getting port noise and the ~57 sq. inches with those two 6" aeros is probably enough.

But if you're more after loudness, 36 hz may work. You could try it both ways. Build your slot ported box first and test it out. If you think you want more lows, take the slot port out, replace the front baffle, and put in the aeros. But since the aeros are more expensive I'd save that option for last.

i build a 6th order bandpass for my btl.

front chamber 2,5 cft @ 62 hz

rear chamber 5,2 cft @ 36 hz

i cant tune lower the rear chamber without losing too much volume and the slot would probably reach the sub...

will this box above give good deep bass?

Exactly what frequencies are you calling "deep bass?" If you want anything below 36 hz, that enclosure will not be ideal. Frequency responses in bandpasses roll off very steeply below tuning (not to mention the subsonic it will require).

ok now i changed the port and made it 43" long.

should be around 30 hz now. the front chamber is around 55 hz.