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Forced Vibrations

The tendency of one object to force another adjoining or interconnected object into vibrational motion is referred to as a forced vibration. 

If you pluck a guitar string on an electric guitar that is not plugged in you will hear a noise; but the noise would not be anything like the sound that string could produce if it was on an acoustic guitar.

When the string is attached to the sound box of an acoustic guitar, the vibrating string is capable of forcing the sound box into vibrating at its own natural frequency. The sound box in turn forces air particles inside the box into vibrational motion at the same natural frequency as the string. The entire system (string, guitar, and enclosed air) begins vibrating at the natural frequency of the string and forces surrounding air particles into vibrational motion too. 

The fact that the surface area of the sound box is greater than the surface area of the string means that more surrounding air particles will be forced into vibration that if a lone string vibrated. This results in an increase in the amplitude and therefore the loudness of the sound heard.

Add to that the natural frequency of vibration of the guitar body and you get resonant sound that results in tonal qualities of the note.