There is a very different sound in air you have know what to listen for
I don't mean to get too technical, but I feel it's the pitch of the scream. I can compare it with the three last wands I've owned,
mach15
prochemquad with a
glide
mach12
Starting with the mach12 it's a high pitched scream that hurts your ears. That tells you that it's sucking a lot of air, but it's also struggling to get more. The same goes with the
prochem quad. But with the mach15, it's a deep sounding scream that's loud, but doesn't hurt the ear drum. I like to compare it to darth vader breathing very loud, lol. The water recovery is noticeably different in the three wands as well