Low - in my opinion is flat frequency response to 10Hz in a car using the cabin gain of the vehicle. I've only auditioned a half-dozen subs that do it right.

My old PPI Pro 10 did it great in a .9 cube sealed enclosure.

If you want to build a ported enclosure, and the sub will handle it, then tuning in and around 23 Hz would get you 'low'. It won't be as loud up around 50 though...

 

^^^x2,with a ton of power

 

Indeed - Sealed is the domain for infrasonics.

 

Well...

Subsonic: adj
- of less than audible frequency
- flying at less than the speed of sound

Infrasonic: adj
- Generating or using waves or vibrations with frequencies below that of audible sound.