I remember when I worked at Peavey back in the mid-80s, the guys in the speaker lab were testing a new subwoofer transducer design. They had an enclosure built that was about the size of a standard refrigerator. They hooked up a signal generator and dialed it down to about 18Hz and cranked it up. At that frequency you don't really hear the sound, you just feel the pressure waves. Through the whole building. Where I was sitting it felt like a freight train was rumbling through the place.