I am familiar with the "E-Bow" which requires no tuning. I have always
suspected that the signal used to drive the magnetic field was pink
noise.  Pink noise contains all of the audible spectrum and would
excite the strings without needing to "be in tune".  Think of the
sound a bell makes in a sand storm.

An impulse function also possesses the property of containing all of
the frequencies in the audio spectrum, and if you could create a big
enough impulse, you'd have your attack problem solved. I think that the
magnetic field needed to excite a string in this way would take a lot
of energy to produce, though a good mathematician might prove me wrong.

My first time posting to this list,
Chuck Luciano <chuck@internut.com>