"Poort was mostly active during the 80s, and performed voices until the mid-90s. For a long time, he was one of the main singers. He also performed the voices of Hard Head Henry Harris, Harvey Kneeslapper and Oscar the Grouch in the song \"Swamp Mushy Muddy\" (one of the only Oscar appearances of that era)."@en . . "Sjef Poort"@en . . . "Poort was mostly active during the 80s, and performed voices until the mid-90s. For a long time, he was one of the main singers. He also performed the voices of Hard Head Henry Harris, Harvey Kneeslapper and Oscar the Grouch in the song \"Swamp Mushy Muddy\" (one of the only Oscar appearances of that era)."@en .