Ray Finch wrote, "Should they strike a xylophone bar briefly and bounce
off, much like a piano hammer hitting a string?"

I built a electro-mechanical xylophone operated by solenoids and what
I found out was that the shorter the contact time between the key and
hammer the longer the note would ring. That meant if you wanted a very
short note you would hold the hammer against the key.  If you wanted a
long note you would hit the key and release it immediately.

Robert Lang
http://www2.netdoor.com/~rlang/xylotron/xylotron.htm

 [ I believe that, in Mr. Lang's design, the solenoid armature strikes
 [ the bar directly, without rebound, and so he can control the damping
 [ via the duration of the solenoid current.  This is quite different
 [ from the usual beaters at the end of a long stick.  -- Robbie