I did wonder about this, I've played around creating binaural beats stuff on protools in the past and I understand where you're coming from, it makes sense to use white noise as it is all audible frequencies played at equal power and the aim in the ganzfeild method is to have an (audio) field free from structure, though using binaural beats or isochronic tones would almost certainly bring a faster induction into an altered state, I'm sure.