In order to increase the isolation of a simple barrier, its mass can be increased. But an increase to double the mass leads to an increase in the isolation of 3 dB. In order to be able to add the insulating power of two barriers, separatiopn between both should be tried, in order to create an an air chamber between the two walls.
Nevertheless this air chamber is like a spring between the two barriers, and the system presents resonance points in which the isolation decays drastically.
For that reason, between multiple partitions an absorbent element is placed (wool of rock, fibber glass, etc) so that it acts as damping and it diminishes the effect of these connections.
In automotive applications, because of the need for mass reductions, barriers with insulating functions are usually multilayer panels, in which alternate material and absorbent insulators are used to obtain a high overall isolation.