AFMG Network Forum

wsjk · Member

Hello everybody, I have such question - comparing two variants of structures I came to an interesting conclusion.

My aim is to compare Rw of two different structures fixed on a concrete wall.
As a body of a wall I chose 140 mm - "Concrete heavy 40", alone it has Rw=51 dB.

Structure 1: Gypsum board(12mm)+Absorber(2000kg/m3, 4 mm)+Gypsum board (12mm)+Basalt Wool(40kg/m3, 5 mm)-->Concrete wall 140 mm.
Structure 2: Gypsum board(12mm)+"Silt,sandy hard"(12mm)+Basalt Wool (40 kg, 5mm)-->Concrete wall 140 mm.

According to SoundFlow, Structure 1 has Rw=51 dB, Structure 2 - 54 dB. Does it mean that "delta Rw" which gives each structure is 0 dB for Str.1 and 2 dB for Str.2? Is it a mistake or not? In some combinations Rw for such structures Rw becomes LOWER than for a wall without extra isolation... Shocked

Bruce · Member

Hello, I am not quite sure how you would use the silt next to wool in Structure 2 as a wall. Also the Absorber in Structure 1 needs additinal information besides the Density such as Flow Resistivity. As a result I get different values for Rw than you have reported.

To answer your question, if you mean by "extra isolation" the addition of another layer, the main difference I see between the two structures is the addition of a plate in Structure 1 that actually reduces the overall TL because of the added mass between the other layers.

Please send the 2 structure files to support-us (at) afmg.eu so we can see what you used for the modifications to the AFMG Example absorbers.

Peter Patrick · Member

Don't forget coincidence effect. You can easily produce a worse transmission loss by adding layers of the same material separated by air gaps et al. Triple pane glass windows perform worse than double pane glazing and so forth.