I love Freddie Mercury. I love Robert plant. I once heard Robert Plant sing a Freddie Mercury song at a tribute concert. Plant sang the song beautifully, but it was obvious that the song was different from the original -- it was obvious that Freddie Mercury was not singing the song.
From what I understand, sound can be described as a sinusoidal wave. Now, based on the assumption that both singers can hit the exact same pitch with the exact same intensity, what other parameter can we modify in a sinusoidal wave that would allow us to distinguish between the two voices?