Converts sphingomyelin to ceramide. a sphingomyelin + H2O = an N-acylsphing-4-enine + H(+) + phosphocholine Binds 2 Zn(2+) per subunit. Lipid metabolism; sphingolipid metabolism. There are two types of sphingomyelinases: asm (acid), and nsm (neutral). Only acid sphingomyelinase has been found in worms. Belongs to the acid sphingomyelinase family.