In this excerpt from Alex de Grassi Fingertstyle Guitar Method, Alex de Grassi talks about the percussive qualities that a guitar's body has and how to achieve those sounds.
The terms tambor (meaning “drum” in Spanish, tambora in Italian) and golpe (strike) have been used for some time as directions in classical guitar repertoire—most notably South American music (such as tango and other dance rhythms). Typically, the player is meant to strike the picking-hand thumb at one of three locations: directly on the bridge, somewhere on the soundboard, or on the strings just in front of the saddle. Tambor is typically indicated by an X with a stem in the notation (usually on the first space) and by an X in the tablature. The words tambor or golpe are written above the staff.
To play tambor, the picking-hand arm needs to shift somewhat so it rests on the upper bout closer to the back of guitar. The picking hand should be open, hovering about an inch (2–3 cm) above the top, with the thumb extended straight. In preparation, rotate the wrist so the thumb lifts away from the guitar, then rotate the hand back toward the guitar and strike the thumb against the bridge (see above). Then, with the hand even closer to the back of the guitar, practice striking the top at locations behind the bridge and listen to the variations in tone. A somewhat lower, thuddier sound is produced when the heel of the hand rests on the top.
Example 1 shows a typical tango rhythm (approximately 120–132 bpm). Muting the strings by immobilizing them with the fretting hand yields the purest drum sound. However, sometimes it’s desirable to let the strings ring.
Example 2 integrates strummed chords with drumming. Let the first two chords ring and continue drumming till the next chord. Stop the strings abruptly with the fretting hand after the third chord and continue drumming till the repeat. Experiment with striking both the bridge and the guitar top.
Excerpted from Alex de Grassi Fingerstyle Guitar Method.