"NotMe" hath wroth:

(Please learn to trim quotations)

Actually the human ear can detect a beat note down to a few cycles.

No, you cannot. Figure on 20Hz to 20KHz for human hearing:
http://hypertextbook.com/facts/2003/ChrisDAmbrose.shtml

What happens when you zero beat something is that your brain is
filling in the missing frequencies. As you tune across the frequency,
and the beat note goes down in frequency, most people overshoot to the
other side, and then compensate by splitting the different. As you
approach zero beat, your perception of the sound drops. If the lack
of hearing below 20Hz doesn't make it disappear, the frequency rolloff
in the audio amplifier stages will probably also drop off at about
20-300Hz depending on whether it's a hi-fi or communications radio. I
have a home made DC coupled hi-fi and can see the speaker moving in
and out slowly at very low frequencies. I don't hear a thing.

However, you don't have to hear it to detect infrasonic sounds.
http://en.wikipedia.org/wiki/Infrasound
Your inner ear, which is responsible for your sense of balance, can do
that for you. You don't actually hear the tone, but your body
certainly responds to it. Depending on frequency and level, tones
below about 20Hz will bring on confusion, nausia, disorientation, and
all manner of sensory anomalies. It's been used for effects in music,
sound tracks, and military weapon systems. I've experienced the
effects personally and can assure you that it was not pleasant.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558