Bonjour, Voir le brevet américain (US Patent): April 5, 1932 DE HART G. SCRANTOM patent USA: 1,852,793 "apparatus for amplifying and distributing sound waves" http://patft.uspto.gov/netahtml/PTO/srchnum.htm search for : 1852793 Cordiales salutations, Jean-Michel Le Cléac'h [ quote="Davide69"]Hello, ...