For live sound, I have tried close miking of acoustic instruments (guitar, piano, violin) by placing a mic on the instrument or inside of the instrument. But I have never been satisfied with the sonic results. The instrument is loud enough, but I don't like the timbre. Comments?
The underlying physics problem is the location of the mic in relation to the instrument. An acoustic instrument, like a flat top guitar, sounds odd when closely miked.
An acoustic instrument is designed and constrcuted to "sound good" as some distance from the instrument. Placing a mic extremely close to an instrument is the equivalent of placing your ear there. You will not hear a blend of the instrument's many timbres; you will primarily hear the instrument's timbre at that exact point.
It is this basic fact of physics that leads to the assertion that a mic should be:
1) placed on the outside of the instrument,
2) not mounted on the instrument as this leads to unwanted physical vibration of the mic, and
3) placed as far as way as feasible to obtain a blend of the instrument's many vibrating surfaces.
Of course, gain-before-feedback in the sound system will not allow the mic to be placed at an infinite distance. A compromise must be made between gain and timbre.
In general, Shure feels that a flat response, condenser mic on a stand sounds better than a clip-on mic or a gooseneck mic mounted on the instrument. Again, the fundamental reason is that the clip-on mic or gooseneck mic is too proximate to the instrument to provide a balanced audio quality.