The invention relates to a box of high notes and a box of low notes, said boxes being the same height and laterally connected by a separator (13). The two boxes (11, 12) respectively have a square-shaped resonating cover (14) on the face or upper surface thereof, and preferably acoustic outlets (15) on the lower surface. The inner face of the resonating covers (14) is provided with metallic bars (22) connected to the covers by the ends (21) thereof or by means of perforated L-shaped pieces of metal.
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