Doom metal не знаю что такое, а вот сладж представляю что:

Sludge is a term generally more associated with an atmosphere, hateful, slow and noisy music, than a precise kind of music. It's almost like black metal where the atmosphere plays a huge part. But here, it's only the atmosphere that counts. For exemple, I think there are elements of Sludge in Starkweather, even if the music they play is closer to metal hardcore.

если ты конечно знаешь английский...)