The rumbling engine of my HT system, capable of going as low as 15HZ without issues.
Can generate local earthquakes with clean sound and unhearable distortion.
It has a 120L braced enclosure for lots of low frequencies.
Crossed at 40hz with the front, 40hz with rear, and 120hz with height speakers.
82DB @ 1 watt, 1 meter
220w rms amp (Behringer A500)
Reconed Shark XSUB12 (thanks to FixMySpeaker by PSI)
15-120hz frequency range
End result photo of the finished subwoofer: