[ff_]

The most impressive thing about this is not the overtone singing - one can learn that given some practice time - but to me it's how LOUD the quartet sounds.

I'd recommend being in the same room with them if you can, the feeling is just incredible, and videos and recordings absolutely do not render how powerful this kind of singing can get with the years of practice that they put in.

[YeGoblynQueenne]

You can try that out with your friends. Get two or three of them, stand in a circle, facing each other, and start making "MMM" or "AAAAH" sounds, trying to "find" each other in tone and pitch. When you manage to tune in to the group sound, it will sound as loud as a brass orchestra.

Speaking from experience. Although it works better with male singers (because louder voices).