A soundproofing assembly intended, in particular, to soundproof a motor vehicle. The inventive assembly includes four stacked layers, consisting respectively of: a first layer having an air resistance of between 500 and 2,000 Nm-3.s; a second porous, acoustic spring-type layer having a resistivity of between 10,000 Nm-4.s and 50,000 Nm-4.s; a third viscoelastic, airtight, heavy mass-type layer having a density that is greater than or equal to 1,500 Kg/m3 and a surface density of between 0.2 Kg/m2 and 9 Kg/m2; and a fourth porous, acoustic spring-type layer having a resistivity of between 10,000 Nm-4.s and 50,000 Nm-4.s.