I went to trade school for small engine repair (lawn mowers, chainsaws, and sitch) and one of the tools commonly used is a vibration tachometer, which translates vibrations of an engine into the RPMs of the engine. Basically it's just a wire wrapped around a dial.
Well I was getting to thinking about it, and I remembered that besides measuring RPMs, it also measured hertz. I wondered how accurate it was at measuring hertz so i downloaded me some test tones and decided to check it out. I first placed it on my enclosure, and I didn't get any kind of vibration whatsoever. I have a bandpass box so I decided to place it on the plexi and that yielded better results. After using a whole bunch of test tones, I came to the conclusion that it's pretty accurate plus or minus a few hertz which really isn't that bad for what it is in my opinion. I thought it was pretty cool.