Right, it doesn't matter. One does hardly hear anything below 20 Hz, but the harmonic distortion in the audible range. People are often confused between hearing a real 20 Hz sine wave, and hearing something when a 20 Hz sine wave is fed into the system. The "something" is often 40 Hz or any higher harmonic generated by mechanical imperfections.
If you want test signals to check these frequencies, have a look here : http://www.audiocheck.net/testtones_subwooferharmonicdistortion.php