Adding different mass together for sound reduction?

If I was using the equation 20log(fm)-47 to find the sound reduction of a wall and was using more than one material would I need to add the masses together? For example, if I was placing a heavy mass vinyl (4kg/m2) and some plywood (14kg/m2) would the equation be (at 120Hz) 20log x (120 x 18) -47 ???? (18 because this is the 4 and 14 together) Am I totally wrong or is that right???? Cheers, james

"kg/m2" is not a measure of mass: it is a measure of surface density. What exactly are you trying to calculate with that equation? - Stuart -

Please update you profile with your location. Do a search for Eric Desart's most excellent MSM spreadsheets on the studio tips forum. There are both metric and imperial versions.

I want to work out the decibel reduction that a certain wall material would give at a given frequency. There is a general rule called the mass law, which predicts the approximate sound reduction in a panel of mass per unit area (m) kg/m2 at frequency (f) Hz <---- something I was given in some university text handouts So for instance: (this is the example given to us in our text) 12mm plywood has a surface density of 17kg/m2. If I wanted to find the decibel reduction at 60Hz I would do this 20log(fm)-47 = 20log(60x17) - 47 = 13.17dB reduction (approximate of course) my question is: if I had that plywood at 17kg/m2 and the plywood had a sheet of heavy mass vinyl at lets say 4kg/m2 glued to it (let's forget about the surface density of the glue to simplify things) would I add these surface densities together so that the calculation would be 20log(60x21) - 47 ? Does this clarify what I'm after? I'm doing a university course so I can't just write down XYZ wall will give XdB reduction I need to show some calculations etc. I've tried looking at the studiotips forum but can't access it at the moment. Did have a look at the other sections of the website but couldn't see eric desarts MSM spreadsheet. Could anyone post a link for me? Thanks for all help. James

at a fundamental level, yes, mass law applies but you also have to factor in MSM in terms of coincident resonance and so to fully appreciate the isolation levels of your design. from a purely empirical perspective, the best source is the NCR IRC documents which show comparative wall assemblies and their measured performance. here's some of the documents which help guide us in figuring out all the various things to be considered: http://www.nrc-cnrc.gc.ca/obj/irc/doc/p ... /ir761.pdf http://www.nrc-cnrc.gc.ca/obj/irc/doc/p ... /ir832.pdf and some other material which is very helpful as well: http://www.nrc-cnrc.gc.ca/obj/irc/doc/ctu-n66_eng.pdf http://www.nrc-cnrc.gc.ca/eng/ibp/irc/c ... s-n13.html http://www.nrc-cnrc.gc.ca/eng/ibp/irc/ctus/ctus-n1.html http://www.nrc-cnrc.gc.ca/eng/ibp/irc/c ... s-n27.html http://www.nrc-cnrc.gc.ca/eng/ibp/irc/c ... s-n25.html http://www.nrc-cnrc.gc.ca/eng/ibp/irc/c ... s-n35.html http://www.nrc-cnrc.gc.ca/eng/ibp/irc/c ... s-n39.html http://www.nrc-cnrc.gc.ca/eng/ibp/irc/c ... s-n50.html http://www.nrc-cnrc.gc.ca/obj/irc/doc/pubs/bpn/44_e.pdf http://www.nrc-cnrc.gc.ca/obj/irc/doc/pubs/bpn/56_e.pdf

For whatever reason, forum.studiotips.com hasn't been accessible for a few days...studiotips.com...but not the forum.

Yeah, I've noticed that. I keep on trying to get through, but the forum seems to be dead right now. I hope it's nothing serious, since that is another great resource for studios and acoustics. - Stuart -