Not sure, but it sounded like it went from somewhere in the 40s or so and swept down to mid to low 20s, because I can tell by some of the things that rattle in my house - they rattle at very specific frequencies (found that out with test tones). It was the fact that it was a fairly sustained tone and VERY powerful that made it so impressive IMO. It was on par with War of the Worlds for sheer shaking. I rewound it and replayed it about 5 times in a row