There was a guy back in the old neighborhood in the early 70s who built a subwoofer (back then, just called a 'bass speaker') by forming and pouring concrete.

It sounded great, as did the rest of his system.

But wait...he then decided to take his system to wherever the party was.

I'll never forget the guy and his buddy, with the concrete sub strapped to it's own 'travel dolly', luggin' that monster up a flight of stairs.

Still not happy with the results, he removed cinder blocks from his basement walls and dug into the earth and built 2 concrete subs back in there.

He then had to remodel the basement so the parties could come to him.

Unforgetably cool.