 The possible variations of the mode of emission, adequate for a sufficient acoustic response of the resonator (shell), are limited and with scarce modulability.The basic frequency, obtained by the tension of the skins, upper and lower, each bound to the edges with 16 mechanical tie rods, is influenced by the non-homogeneous distribution of the tensioning forces which contributes to render complex the spectrum of the real modes.Experiments were made at the Centro Ricerche Musicali - CRM in Rome and at the Istituto Gramma in L'Aquila with the aim of achieving these objectives