The structure works just like smaller pendulum wave toys. Each bowling ball’s string is a slightly different length, with the shortest string mounted at one end of the series, the next-shortest string mounted next to it, and so on until the longest string is found at the other end. For best results, the string lengths should follow a particular formula, as described in this short paper published in the American Journal of Physics in 1991:
The lengths of the pendula are carefully adjusted such that if the longest pendulum executes L oscillations in a time interval T, then each successively shorter pendulum will execute one additional oscillation in that interval.
Although the video does not show it, we are guessing the top of the outdoor structure is slanted to accommodate the different string lengths, while keeping the bowling balls all about the same height off the ground.
Want to make your own mesmerizing physics demo in the forest? You could start with a desktop version, so you can figure things out on a more manageable scale. There are a number of instructions online for making indoor-sized wave pendulum structures. We didn’t immediately find any instructions for a structure large enough to hold bowling balls, however, which would require a little engineering to ensure the frame is strong enough. Let us know if we’ve missed any instructions that are already out there…or if you’ve made this on your own.