This is the tuning fork. It resonates at a certain fundamental frequency depending on the length of the bars. The two bars in a single fork speak to each other because they are exactly the same. The vibration from one bar shoots into the other bar because they are connected at the bottom. Even if they were not connected, the vibration of the first bar shoots into the air and hits the second bar, making it vibrate anyway. This can also work with two completely different tuning forks. This is known as sympathetic vibration.

What happened in the Spider Man 3 scene mentioned above is a rough portrayal of how it works. The metal bars have to be the same length and sticking out of the ground at the same length to preserve the purity of the fundamental frequency. That is of course, if that's what you are going for. It could be useful to have them at different frequencies but that is obviously too specific/technical for a movie. Certain frequencies cause certain weird things to happen. (2/2) #videos