So those Pentatonix guys I linked last time, that guy at (your) lower-right? His name is Avi Kaplan, and he can do something called "overtone singing," and I can almost guarantee that you know what it sounds like even if you don't know what it's called. Here he is:
See? You know that sound, right?
Here's another example of one guy doing it in a bunch of different styles. It sounds kind of like "human bagpipes":
No crazy effects, except perhaps the acoustics of that wall.
The physics department at UNSW has done quite a lot of research with musical acoustics, and they actually have a page with some great diagrams describing quite well what goes on in overtone singing. The short version is, here's what a whispered O looks like:
Mmm, yes, I can see the O sound. Right there.
...here's what a sung O looks like:
"Fundamental" means "the note sung" (in this case, a B♭).
...and here's an overtone:
I bet it didn't occur to you until now that
you're learning about the acoustics of meat.
As it says on the page, "In this technique, one of the vocal tract resonances is made much stronger, while all the others are weakened. The strong resonance can be made so strong that it selects one of the harmonics and makes it so much stronger than its neighbours that we can hear it as a separate note. [Here] it is the eighth harmonic that is amplified. Although the fundamental is only 8 dB lower than the selected harmonic, the fundamental lies in a range in which our ears are much less sensitive, so it sounds much less loud."
So that low "droning" part is the fundamental, and the higher whistling sound is the harmonic resonance - and it's more like whistling than it is like singing, at that point, though I suppose singing itself is just whistling from inside your throat with a specialized organ. And the coolest part is that, as Glenfield demonstrates, the drone and the whistle can be modulated independently, so you can actually sing not just two different notes but two distinct melodies at the same time!
Anyway, that's pretty much all there is to that. Here's another Pentatonix video, because they're just so cool. Crank your bass (or plug in your headphones, if your speakers suck) and listen - this guy's voice gets bone-shakingly low!