[Bent]

In my case, I didn't specifically chose 18 hz, This was my first project, I had lots of time and selected an existing design. ThomasW and John Marsh had chosen to make a larger version of a 12" Ariel Acoustics commercial subwoofer, and wound up with what they called the AS-15 (Ariel-Stryke 15" subwoofer). They used a HE 15 driver manufactured by TC sounds for what was then called Stryke Audio in what was then considered a large(ish) enclosure.
They determined via testing as well as Unibox predictions prior to building that they would acieve an 18.56 hz tuning frequency.

Mine was built with an ondoard Adire Audio ADA 1200 plate amp, and I extended my enclosure by approx 1.5 inches - and in confirming my tuning with a Fluke 187III and instrumentation grade adjustable frequency current/voltage source, I determined mine was also 18 Hz.