1st time hearing a [>=<]. Ears rang afterwards.

[Robert Gift]

The SD-10 are ALL dual tone with a perfect 4th interval.

That creates a resultant tone a fifth below the lower note:
If
hi = c2
low = G1
Resultant is C1 5 notes below the G1

So, I presume the two c notes would yield a prominent C sound.

[Robert Gift]

I could feel the energy in my feet, and could see pebbles skimming across his driveway.

What caused pebbles to skim across the driveway?
Thanks,

[SirenMadness]

The vibrations caused the pebbles to skim across the ground. Square waves tend to push quite well.