Because I am a cabinet maker of well over 40 years and have total access to a well equipped shop with discounted materials, I have built a fair share of layouts. I have tried acoustical sound board, acoustical caulk, polystyrene, convoluted foam and homasote on 3/8” , 1/2” and 3/4” plywood. The bench work can act like an acoustic guitar and so can your rolling stock. Lee Willis did an extensive study and found that sound deadening material placed in rolling stock can reduce the decibel level considerably. The thickness of the plywood did not seem to make a huge difference although the more ply count seems to help. Sound travels along the grain, so thinner ply and higher count does seem to reduce sound transmission. The best results that I have achieved were using 1/2” Baltic birch plywood with 1/2” homasote ( the real stuff, not the builders board) and homasote roadbed. Yes that equals 1 inch of homasote. Then after the wiring is finished place convoluted sound absorbing foam under the benchwork. The only expensive part is the plywood, which is about 2 to 3 times the cost of bcx pine. Polystyrene ( pink or blue ) does not seem to reduce the sound but it is nice for creating landforms. The type of track does make a difference with the plastic roadbed being the loudest and Atlas being the most quiet. There is a product called resilient Chanel which stops sound transmissions dead in there tracks but it is only sold by the case and only wholesale. I have used it once in a commercial application and it is amazing.
