Abstract: An improvement in the mechanical and functional structure of stringed instruments, tending to obtain perfection in the sound of guitars, violins and other instruments of this type, comprises arranging in the interior of the sound box of the instrument an arrangement which diminishes, cancels and even produces a contrary effect to that originated on the sound box by the tightening of the tuned strings and by the force of sound derived from the vibration thereof. Such arrangement has an adjusting element, preferably arranged on the outside, which controls the adjustment of the assembly, in order to obtain an optimum sound of the instrument.