Anyone who has experience with tuning string instruments understands the concept of the Law of Sympathetic Resonance. Wikipedia explains it like this: "Sympathetic resonance or sympathetic vibration is a harmonic phenomenon wherein a formerly passive string or vibratory body responds to external vibrations to which it has a harmonic likeness." To put it in simpler terms, it's when two tones are played at the same time and one can hear the sound waves beating off of each other. The smaller the interval between the two notes, the faster the waves beat against each other. For instance, let's say you want to tune the A string on a guitar. You could play an A on the E string, then pluck the untuned A string and let the two waves collide and bounce off each other. In order to tune the string, you listen to this resonance and slowly turn the tuning peg until the two tones become one note, and are therefore in unison. This is how you can use sound frequencies in order to perfect the sound of an instrument.