Why do you want to hit 18Hz? You know that you rather feel those low frequencies than you actually can hear them? What you're hearing with most subwoofers at these frequencies are the way too much raised distortions.
Another point to consider is the listening room. With single subwoofer configurations this is often the limiting factor. In most cases you will not get a reasonably reproduction of frequencies under your first room mode. I say this from my personal experience. I myself had different subwoofers able to play down to 20 Hz and lower. But in nearly every room I tested them I wasn't able to hear these frequencies only in rooms with one dimension taller than 8 meters it was possible. A way around this would be a dipol subwoofer, double bass array or a single bass array with huge back wall absorption. Just mentioning...
Referring to your question: With your given enclosure dimensions you can reach good 19Hz (at the -3db point). If you're planning to run the Dayton with its rated 500 watts the port should be big enough in order to avoid audible port air velocity. I would choose at least the following internal port dimensions so you can keep the air velocity under 20m/s at 24Hz and up (with a peak of 27,5m/s at 17,5Hz): 35,36cm (wide) x 5cm (tall) x 50cm (deep). And no need to worry I already considered the port volume (liters) in my calculations and subtracted it from the box volume of 222,5 liters. Here is the max possible spl within the xmax of 14mm and power rating of 500 watts (sorry for the spaces in the link that's because my account is still limited):
Another point to consider is the listening room. With single subwoofer configurations this is often the limiting factor. In most cases you will not get a reasonably reproduction of frequencies under your first room mode. I say this from my personal experience. I myself had different subwoofers able to play down to 20 Hz and lower. But in nearly every room I tested them I wasn't able to hear these frequencies only in rooms with one dimension taller than 8 meters it was possible. A way around this would be a dipol subwoofer, double bass array or a single bass array with huge back wall absorption. Just mentioning...
Referring to your question: With your given enclosure dimensions you can reach good 19Hz (at the -3db point). If you're planning to run the Dayton with its rated 500 watts the port should be big enough in order to avoid audible port air velocity. I would choose at least the following internal port dimensions so you can keep the air velocity under 20m/s at 24Hz and up (with a peak of 27,5m/s at 17,5Hz): 35,36cm (wide) x 5cm (tall) x 50cm (deep). And no need to worry I already considered the port volume (liters) in my calculations and subtracted it from the box volume of 222,5 liters. Here is the max possible spl within the xmax of 14mm and power rating of 500 watts (sorry for the spaces in the link that's because my account is still limited):
Code:
pic-upload . de / view-26142476 / Dayton_RSS390HF_BR_01 . jpg . html