String Vibrations Interesting thing I noticed just now playing my ukulele. 
For those who don't know how a ukulele works, it has four strings: a high G followed by a lower C, E, and A. Holding down frets causes the strings to play progressively higher-pitched notes. Now, it is possible for two or more strings to play the same note. I've noticed that when this happens, playing one string will cause the others of the same pitch to vibrate, even without actually playing them. However, it's only when they are exactly the same note. If it's not the same note, it doesn't vibrate (at least not perceptibly). Why? If it's caused by the vibration from the string spreading outward, why would it matter what pitch the strings are relative to each other?
Perhaps it's a property of the nylon in the strings? After noticing this, I checked on my steel-stringed acoustic guitar, which definitely does not (perceptibly) do this.
 A: The sound of a ukulele (or any similar instrument - guitar, violin, etc) does not come from the strings themselves, but from the whole body of the instrument vibrating and moving the air which is in contact with it.
The vibrations are transmitted from the strings to the body of the instrument mainly through the bridge.
If you stop two strings so that they produce the same pitched note, the vibration of the bridge will cause both strings to vibrate if you pluck one of them.
This will also happen on an acoustic guitar, but in an electric guitar the sound is not physically produced by the body of the instrument vibrating. Instead, by magnetic pickups sense the vibration of the steel strings and the electrical signals are then amplified and sent to a loudspeaker.
The same effect happens in instruments like the piano, but the steel strings are at a higher tension than in plucked instruments and there is no visible vibration of the strings (except perhaps for the lowest bass notes) in normal playing.
You can prove this does happen on a piano with an experiment like the following: press the middle C key slowly, so the note does not play, and hold it down. Then play a short loud note on the C an octave below. You will hear the middle C sounding quietly after the lower note has ended. In this case the second harmonic of the lower C is the same frequency as the fundamental frequency of the middle C. Holding the note down silently lifts the damper from that note's strings, so they are free to vibrate.
