The main source of sound in an acoustic guitar is the string, which is plucked with the finger or with a plectrum. The string vibrates at a necessary frequency and also create many harmonics at various different frequencies. The frequencies produced can depend on string length, mass, and tension. The string causes the soundboard and sound box to vibrate, and as these have their own resonances at certain frequencies, they amplify some string harmonics more strongly than others, hence affecting the timbre produced by the instrument.
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