Alpha waves ( light meditative state ) are created by two singing bowls that are 7.5 - 12 hertz apart.
Theta waves ( Deep meditative state, light sleep ) are created by two singing bowls that are 4 - 7.5 hertz apart.
Hertz is a way we measure sound. Hertz are how many cycles per second the sound travels. Think of a slinky and it's coils. Theta waves travel 4 - 7.5 coils per second. Cool eh?