The earthshock drills could bore through any substance , even at low speed.  But they had one other feature:  when in use, they produced a loud hum.

"If I can get them going fast enough," he thought. "Hit just the right frequency, maybe..."

The drills became a blur, whirling faster and faster.  The hum went from painfully loud into the ultrasonic range.  Whenua and Nuju both felt certain their heads would split open.  Cracks began forming in the walls and ceiling.   Whenua pushed as hard as he could to increase the speed, then pushed  a little harder, doing his best not to scream from the strain.


~Greg Farshtey, Trial by Fire, pg 22