What
@shadyJ is saying is that your hearing noise when a 5 Hz tone is produced is not a product of your ability to hear low tones, but of your sub's harmonic distortion trying to reproduce that tone. What you heard might've been 10 or 15 Hz, the 2nd or 3rd order harmonic of the 5 Hz fundamental. Exciting the air enough to produce a 5 Hz fundamental tone would take an enormous amount of energy and driver surface area, more than a 12" sub can provide. For reference, each out-in cycle of a driver playing a 6 Hz tone happens at the same rate you can say each syllable of "seven Mississippi". Vibrations that slow are more floppy than audible.