Sylvia Lenore Massy is an American record producer, mixer, engineer, instructor and author. Massy is renowned for her multifaceted production/mixing and engineering skills, with her first major breakthrough occurring with 1993's Undertow, the full-length triple platinum-selling debut for Los Angeles alternative metal band Tool as well as her work with System of a Down, Johnny Cash, Red Hot Chili Peppers, and Brazilian band South Cry.

Source: Wikipedia (https://en.wikipedia.org/wiki/Sylvia_Massy)