or may not be integrated into the installation by virtue of the simplified method of manufacture and the lower consumption of materials reduction in weight the soundproofing assemblies according to the invention can be manufactured at a much lower cost than the assemblies shown in fig3 and even a little lower than or at least comparable to the cost of the assemblies shown in fig1 it may also be noted that the assemblies of the two embodiments of fig3 and 4 give identical results with structures in which the only difference is the replacement of two layers 32 34 of fig3 by one single layer 36 of fig4 with practically the same thickness therefore it should not be considered that the system of fig4 is a u201c down graded u201d system relative to that of fig3 on the contrary it constitutes an improvement since for equivalent acoustic results it simplifies the manufacture and significantly reduces the cost finally it will be noted that the manufacture of the soundproofing assemblies produced according to the invention is well suited to the formation of holes adapted to the passage of the elements which have to pass through the assembly such as the steering column the brake and accelerator pedals etc consequently by virtue of the use of this foam with a high tortuosity it is possible to obtain properties which are as good as those given by the known system described with reference to fig3 but at a much lower cost than this system by comparison with the known system shown in fig1 the reduction in weight is very considerable and therefore corresponds to a reduction in consumption and pollution of the vehicles in which these systems are used by comparison with the system of fig2 the.