Acoustic absorbent materials that does not provide sound insulation… It is not the same acoustic absorption as acoustic insulation . In fact, in many occasions they oppose both properties. The porous materials that are introduced into the enclosure chambers (walls, facades, insulation panels, acoustic screens), do not just provide acoustic insulation by themselves , but they play a fundamental role in improving acoustic insulation of the partitions. On many occasions we have heard of and have seen “yellow spongy material”, “rock wool”, “fiberglass”, etc … and installers and even technicians could have refer to them as “THE INSULATOR”. Well, these materials with high porosity and low mass are usually excellent thermal insulation, but not acoustic insulation. In general the materials with good properties of thermal insulation incorporate in their structure much air, reason why they have little mass. This means that they can not impede the passage of acoustic energy through them, that is, they do not isolate acoustically. However, if their porous structure are open cell (open cell fibers or foams), if they can have good acoustic absorption properties , it is the ability to convert into heat some of the acoustic energy that impinges on them.
To clarify the difference between absorption and acoustic insulation, a layer of material with good absorbent properties (rock wool panel) can absorb (convert to heat) 99% of the acoustic energy that impinges on it, letting through it alone the 1 %. We could think that this 1% is very low energy and therefore it isolates a lot, but we already know that acoustics works logarithmically, so this percentage only supposes a 20 dB isolation, is very little. In contrast, a solid concrete wall with a lot of mass will have good insulation, say 60 dB. This implies that it passes through only one millionth of the acoustic energy that reaches it (and our ear is still capable of grasping it!). The rest of the energy that impinges on this concrete wall is almost completely reflected, so it is a poor acoustic absorbentb but not all thermal insulating materials are good acoustic absorbers. Closed cell (expanded polystyrene, polyurethane foam, polyisocyanurate) neither insulate nor absorb acoustically , so acoustically have no function. Thus, fibers (rock wool, fiberglass, synthetic fibers, natural fibers, open cell foams, etc.) do not isolate themselves, however they do improve the acoustic insulation of a multilayer panel (sheet, double wall Laminated plaster wall, etc.), as the material improves the vibrational decoupling between the heavy layers and absorbs the stationary acoustic waves that occur, greatly improving insulation of the whole. In addition, if one of the layers of a multilayer panel is perforated, it helps to improve the acoustic absorption of the room towards which this perforated face is viewed and greatly improves the acoustic environment “acoustic conditioning”.
In summary, all acoustic insulation based on multilayer panels incorporates between heavy insulation layers lightweight porous absorbent materials. These materials are included to eliminate the vibratory and acoustic resonances between panels, thus improving the insulation behavior of the assembly. A porous material by itself has little acoustic insulation capability. In acoustics each material has a definite role and must be selected and installed according to its function.