Double constructions
The mass-spring-mass system becomes efficient with increased sound insulation above its resonance frequency f0. In the schematic diagram the base line (lower limit) represents the sound insulation for the simple mass system (total mass). To obtain the best possible absorption in the cavity, it is recommended to use e.g. mineral wool with an airflow resistivity of at least r ≥ 15 kPas/m2.
Resonance frequency f0 of the cavity in double constructions (mass - spring - mass system):
If the two mass layers are mechanically connected, these connections create sound bridges and reduce the improvement of the double layer construction with cavity. Following diagram shows in principal this influence.
Beside the typical steel connectors which create sound bridges, the insulation material in the cavity can also act as a sound bridge, if the material touches both sides of the double construction. Therefore the use of stiff insulation materials is critical. This is also the reason, that sandwich panels, where the filling material has to transfer the mechanical tensions, are not acting like the here described acoustic double constructions.
A system of two standard marine panels with low density Rockwool infill in the void between the panels can be regarded as a double construction and will perform very well for noise reduction.
Models and software programs exists, able to calculate the total sound insulation for double layer constructions with acceptable accuracy, if all necessary material parameters are known. The problem is to have the right values for Young modulus (to calculate fc) and the internal damping factor for the mass layers, to know the number, kind and stiffness of sound bridges and the absorption properties of the used filling material.
In practice this means, that calculated sound reduction indices R might differ 5 to 10 dB to measurement results obtained in laboratory measurements for the same constructions.
