You mean you've never evaluated his vocal talent? Or are you saying you don't think he's a singer?
Fair enough, but in terms of sheer ability, he's one of the better singers in rock history period. I've seen countless classically trained vocalists try and fail miserably to replicate the drive, power, and tone he hit all his best notes with. They'll reach the note, but they won't manage it nearly as well as he does. One of the finest high register capacities I've ever seen on a vocalist.
He's most certainly a singer, his screaming doesn't make that untrue. Robert Plant and Rob Halford do plenty of screams, but they're still very much singing. It's not like a black or death metal singer where it's more in key primal wailing and growling, he just has a really gritty timbre when he sings AC/DC material, and when you mix that with the generally higher registers AC/DC songs are sung in (and the fact that Brian is a baritone, and baritones aren't supposed to be able to sing that high that well), it makes that sound. I could understand where you'd get the idea that he's not singing in the traditional sense from the raw, primal edge it has to it, but he is. Axl, as great of a singer as he is, is no Brian Johnson, he just doesn't have the unique vocal sensibilities for it, and thus his tone and everything sounds like crap paired with AC/DC's music. And I can confirm they're enjoying themselves, I caught them when they came through my city this year and Brian looked like he was having the time of his life, very jovial mood.
