[WalterBright]

The movie of the galloping bridge looked like resonant frequency to me.

Back at Caltech, the dorm halls had poured concrete walls. Naturally, some students got a signal generator, a power amplifier, some speakers, and installed the speakers at the node of one of the halls (the halfway point).

Then turned it on, and tuned the frequency until it matched the resonant frequency of the hall. The energy in the halls quickly built up until the entire building was going whomp whomp whomp. Except that the frequency was too low to hear.

You just got a feeling that something was very very wrong. Residences would come out of their rooms wondering what the heck was going on.

A fine prank!

Of course, that was exciting the resonant response with a driving resonance frequency, which is not what happened at the Tacoma Narrows bridge, which had a different way of exciting it as already explained.