I was playing the opening 16 hertz organ pedal tone from Also Sprach Zarathustra (Theme to 2001: A Space Oddyssey) and taking some SPL ... all » measurements from the far end of the room. When levels reached 133dB @ 16Hz, the carpeting started to float off the floor! The rest of the building was making terrible noises, like joists snapping and other nasty stuff, so I wasn't about to push things any higher, although I had over 10dB of amplifier headroom left. Those Bassmaxx ZR18s never cease to amaze me with the weird things they can do at subsonic frequencies.