Scientists have studied the voice and vocals of one of the greats of pop music, Freddie Mercury of the band Queen. And the results?
1) He used subharmonics in a way that very few people can. Other than Tuvan Throat Singers, that is:
(Also, listen for the subtle melody that accompanies the low-throated growl in this clip).
That subharmonic vibration also helped give him his "growl," which he used quite frequently.
2) His vocals chords simply moved faster than those of most people. Most vibratos (the oscillation between pitches that great singers employ, especially when holding a note) are between 5.4 and 6.9 Hz. Mercury's was 7.04 Hz, and that is close to what would be considered a "vocal tremor," which creates a wavering and unsteady voice. Yet he had amazing control over that same voice, even as it came close to careening out of control. It really was like he pushed his voice to the absolute limits of what it was physically capable of doing, riding that edge but not going over it. It makes the vocals more emotionally compelling precisely because he's on that edge.
3) It's been widely reported that Mercury could sing through 4 octaves, but this study could not back that up; at 12 semitones per octave, that would be 48 semitones. They could only detect an impressive 37 semitones in the recordings that exist. I mean, it's still possible he could have hit 48 semitones, it's just that there is no recorded example of him reaching that far.
Just listen to his amazing scat-style singing in the "isolated vocals" version of "Under Pressure" in the clip below, which Mercury's band Queen recorded with another great vocalist, David Bowie.
Especially the part at 02:00 — man, the dude had some pipes.
The science behind all of this is in an article posted on Taylor Francis Online in the Logopedics Phoniatrics Vocology section.