I recently read a lot about guitar sustainers, for those who don't know what are they - it is basically an electromagnet with an amplifier. It takes an input from pickup, amplifies it and then generates the magnetic field that vibrates the string.
I have the question - if we need magnetic field that changes corresponding to specific frequency in order to vibrate the string - why would we put a permanent magnet underneath the core? Wouldn't it be pointless, or even destructive? What's the point of it hanging around there? How does it help?
I probably missing some basic laws here, but can't find out what it is...
The picture below from one of many patents on this kind of invention(http://www.google.co.in/patents/US5932827).
Other patent I found also mention permanent magnets, but none of them describe in clear language what is the point of it.