Energy in active noise cancellation
Active noise cancellation works by generating a sound with 180 degree phase shift from the original sound. It's effectively an interference effect.

Like any interference effect, it doesn't produce cancellation everywhere. If the phase shift is 180 degrees at one location, it will be 0 degrees at some other location, resulting in a greater sound intensity there. So it doesn't convert the energy to another form, it just moves sound energy from one location to another.