Zero Level
This is a standard reference audio level used when discussing relative levels. Commonly the zero levels are chosen to be 1 mW. This corresponds to 0.775 volts RMS across a 600 Ohm resistance.

A musical instrument composed of strings mounted upon a resonate frame, strings can be plucked with fingers or struck using small mallets.

Lean your body forward slightly to support the guitar against your chest, for the poetry of the music should resound in your heart.”
  Andres Segovia