A Study Was Done
Researchers from Austria, Czech Republic, and Sweden conducted a study and released in 2016 their conclusions, which said that Freddie Mercury had a rare and unique singing voice. The study stated that Mercury likely used subharmonics, in which the ventricular folds vibrate along with the vocal folds.
To give you some perspective, most human vocal patterns never use the ventricular folds. Freddie’s vocal cords moved faster than other people’s. While a typical vibrato voice will fluctuate between 5.4 Hz and 6.9 Hz, Freddie’s was 7.04 Hz.