When you pluck a guitar string it moves back and forth hundreds of times every second. Naturally, this movement is so fast that you cannot see it – you just see the blurred outline of the moving string. Strings vibrating in this way on their own make hardly any noise because strings are very thin and don't push much air about. But if you attach a string to a big hollow box (like a guitar body), then the vibration is amplified and the note is heard loud and clear. The vibration of the string is passed on to the wooden panels of the guitar body, which vibrate back and forth at the same rate as the string. The vibration of the wood creates more powerful waves in the air pressure, which travel away from the guitar. When the waves reach your eardrums they flex in and out the same number of times a second as the original string.