Resonant frequency is the frequency at which an object will begin to vibrate in excess of the energy that you put into it.
Grant Imahara

Ugh… just typing that makes me cringe.  Grant said it on the Fire vs. Ice episode (no. 121) that aired May 27, 2009.  Hopefully, any statement about energy being created out of nowhere should automatically activate the B.S. detector in your brain.  If you don’t have a B.S. detector, let me know and I’ll send you some blueprints.

Anyway, of course, an object vibrating at its resonant frequency will not vibrate in excess of the energy you put into it.  That would violate all sorts of physical laws.  What it will do is accumulate the energy being put into it, amplifying its response.  Many times, if the forced input continues to match the natural frequency for a good length of time, the energy will tear the object apart; just like Grant’s example of the wine glass.

I wanted to do a swing-set example, but I saw Wikipedia already took care of it, so I won’t duplicate their effort.

The other problem with their experiments is that they often aligned their guns perpendicular to the sound waves that were hitting them.  Acoustic waves are longitudinal waves, which means they transmit energy in the same direction in which they move.  In order to “accidentally” fire a gun, the pin needs to line up with the wave propagation direction.  Oftentimes (most egregiously, in the sound chamber they used), the gun in question is lined perpendicular to the acoustic wave propagation, meaning there is no impetus for the pin to move along its axis.  Think about getting a Shake Weight going… you shake it along the axis, right?  Perpendicular shaking would be all sorts of counter-productive.

What I would have liked to see is for the ‘busters to mount the gun to some sort of programmable shaking device that would vibrate the gun along the firing pin’s axis (i.e. its barrel).  Then they could have figured out what frequency/amplitude it took to get a misfire.  Then, the final step would be to figure out if they could replicate those circumstances with sounds waves in a car.  Done and done.