[1] Sometimes death growl vocals are criticized for their "ugliness" and inability to be understood without an accompanying lyric sheet, but the presentation of gruff vocals contributes to death metal's abrasive style and often dark, obscene subject matter."[4] Sociologist Deena Weinstein has noted of death metal: "Vocalists in this style have a distinctive sound, growling and snarling rather than singing the words, and making ample use of the voice distortion box.Despite the misconceptions and stereotypes that are associated with them, usually by non-extreme metal fans, death growls require traditional clean/melodic vocal techniques to be done properly.[8] In 2005, professional vocal coach Melissa Cross released the instructional DVD The Zen of Screaming, to teach students the proper way to sing in several styles of rock, metal and hardcore.[9] The low, raspy, aggressive pitch of Lemmy Kilmister of Motörhead, Conrad "Cronos" Lant of Venom and Tom Warrior of Celtic Frost were not unlike the death growl and may be thought of as precursors to the current style.