(Classic )Heavy Metal is the next level on the "heavy" scale. See the career of Judas Priest as an example of leaving "rock" behind and define the heavy metal genre.
Thinking of Heavy Metal, great singing is characteristic!!
And Geoff Tate, Michael Kiske, Rob Halford, Simone Simons, Harry Coklin, Devon Graves, Stu Block, Todd La Torre, Floor Jansen, thousand others... can sing Wheresthedug ?