Sound diffuser accessory, suitable to be associated to a sound source, such as an electric or electronic or digital musical device or instrument and suitable to diffuse sounds or sound signals deriving from said sound source, wherein such accessory includes at least one sound board, and support structure for the sound board, wherein the sound board includes a board or panel having a flat or slightly curved shape and including a first exposed surface, a second surface, opposite to the first surface and a thickness (S), wherein the sound board includes or is connected with at least one transducer of the electromechanical and/or piezoelectric and/or piezoceramic and/or magnetodynamic and/or magnetostrictive type and wherein the support structure includes at least one element having a substantially strip-like shape and spacer means having a height (H2), suitable to constrain and space the at least one sound board and the support structure.