The Siren Board

Hacksaw wrote: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.

Elliott wrote: 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,

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