The generally accepted limit of low frequency hearing is either 20hz or 16Hz. The answer actually depends on how loud the sound is.
Below is a graph of seven different studies on hearing thresholds which extend the Fletcher Munson curves to much lower frequencies. We have been able to verify these measurements in house. Our hearing does not stop at a defined low frequency limit. At 16Hz you need about 85dB Sound Pressure Level, at 10Hz you will need about 100dB SPL and at 5Hz the threshold is about 110dB SPL. This is the threshold, in other words the sound is just barely perceptible, not loud at all. For example most would judge 110dB at 30Hz to be fairly loud. At 5Hz 110dB is just noticeable as a sound, the same loudness as you would perceive 30Hz at about 60dB. In reality, to make 5hz meaningful you need 115dB or better capable woofer at that frequency. Other than the Eminent Technology TRW-17, no other commercial woofer even comes close.
More important to understand is the distortion requirements to reproduce these signals accurately because of the slope of our hearing threshold. The Ged Lee metrics suggest it is high order distortion that matters.
What we perceive as low bass is not really low at all. It is mostly centered between 20 and 60Hz. Very low bass, well below 20Hz, is entirely different in both perception and physical experience.
A cone woofer runs out of an ability to couple with the air at very low frequencies. This combined with our lack of hearing sensitivity at very low frequencies result in the belief that we cannot hear much below 20Hz. If you want to experiment with this try our downloadable DVD that has musical notes and octaves down to 4hz. Sound levels are raised as the frequency goes down to try and match our hearing threshold. With this you will probably get a good idea of how low or loud your woofer goes at a given frequency but be very careful with the volume control.
The hearing threshold graph came from research out of Switzerland, we would like to give appropriate credit, unfortunatley the link for that research is no longer valid. .