Vibrating Objects I'm not sure if this belongs in Engineering, but if it does, please migrate the question.
What methods are used to vibrate objects extremely quickly? What are the most common?
 A: Very often, in the audio range people will use speaker coils. Audio speakers are designed to give displacements on the order of a few mm over a range of 20 Hz to 20 kHz - with lower amplitudes at the higher frequencies. Since they are mass produced, they are quite cheap and robust. Take off the membrane, and you have an instant vibrator. Just add AC current... You want to pick a coil that is suited to the frequency range you have in mind - woofers for the lower frequencies, tweeters for the high end, and mid-range speakers - well, for the frequencies in between.
An example of one of the most beautiful experiments I know that used this method (many years ago) was the classical experiment described in Squires' book on "Practical physics" (I have quoted this one before...). There, the coil was used as part of the suspension mechanism for a reflector that had to be isolated from vibrations. In essence, they created a mechanism that allowed a short spring (1 m length) to behave like a long spring (1 km length), thus damping most vibrations. I reproduce here a diagram that shows the voice coil being used:
 
I am not associated with the publisher or author of the book - although I was lucky enough to be taught by him as an undergraduate, many many years ago. So I have heard him talk about this experiment first hand...
