This is a solid-state Tesla coil. The primary runs at its resonant frequency in the 41 KHz range, and is modulated from the control unit in order to generate the tones you hear.Video Link. (Thanks, Robert!)
What's not immediately obvious in this video is how loud this is. Many people were covering their ears, dogs were barking. In the sections where the crowd is cheering and the coils is starting and stopping, you can hear the the crowd is drowned out by the coil when it's firing.