are orientated by construction as shown by the double arrow 8 the result is that the passive spherical skullcap is urged along its marginal radii which causes movement thereof parallel to axis xx the broken line 6 shows the low position of the radiating structure and the dash dot line 7 shows the high position although it is not active the spherical skullcap sweeps a relatively high volume for the transducer effect is concentrated in the conical suspension with a maximum sensitivity for deformations along the meridians so as to obtain better mechanical compliance of the active peripheral suspension the circumferential stiffness may be reduced as shown in fig3 this result is obtained by special shaping which consists in creating radially orientated protuberances 11 which alternate with active sectors 12 each protuberance 11 provides sealing of the radiating structure so as to counteract the acoustic short circuiting between the radiating faces of the vibrating piston if offers however no circumferential stiffness able to prevent the active sectors 11 from following the translational movement of the central dome since the central dome plays a passive role and since it may undergo bending it may be formed from another material than the truncated cone shaped active suspension or with another wall thickness by acting on the piezoelectric parameters and by proportioning the ratio of the active surface to the passive surface taking into consideration the opening angle.