Sympathetic vibration
I too have heard the marching-over-a-bridge theory, but I don't believe that would be sympathetic vibration as it is defined. However, a common, and benign (no H.G.Wells stuff) everyday use of sympathetic vibration is in tuning instruments.
To tune a piano, organ or harpsichord (and they need tuning more often than most people would think, especially a harpsichord which can go out of tune in a half hour) you don't need a tuning fork or electronic pitchpipe for each note in the scale. A friend of mine who bought a harpsichord asked our organist how to tune it easily. The answer: use a tuning fork to tune middle C; then tune C one octave above and one below, then tune E, G, and B-flat from there, all by holding down those notes and striking hard on C. You tune the octaves of C by listening to the beating of the two notes. Once you have tuned middle C with the tuning fork and you know that one is in tune, you tune the octaves from the beating sound you hear when you play both together. The slower the beating, the farther off; as you get the one more into tune with the first, the beating gets faster, and when you don't hear beating, they are in tune. The beating comes from the sound waves being out of synch and colliding with each other in the air before they reach our ears.
