There were tests done...

I think the tests were in one of the extra publications put out by Model Railroader. I have it some where.  It was dedicated to track laying. The idea was to quiet trains.  So, a series of test were performed using different materials available for road bed.  I believe the tests were performed on top of plywood.

One of the reasons expressed for doing this is to reduce the amplified vibrations so the sound decoders are not competing with the vibrations. More sound decoder for your listening pleasure?

My layout has three inches of blue foam on top of the benchmark.  The foam will enhance the vibrations as the train runs across it.  For grades I'm using Woodland Scenics foam risers.   For sub road bed I'm using a 3M product commonly known as camper shell tape.  You can find it a most any automotive parts store.  It is cheaper to buy it from eBay.

On top of that I am gluing the foam road bed you mentioned.  It also comes in black.  Everyone has their own ideas on what to use. My suggestion is not to do it my way, but if possible visit some local layouts to see what was used there.  Ask plenty of questions.  You might setup a few tests of your own.

Den

I found that book.

You might want to see if you can find a copy of "How to Build Realistic Reliable Track."  Looks to have been published in 2009.  Page 22 is the tests called "Quiet road bed, better train sounds."  It is a good reference book on laying track.

Den

...

I stick with Cork.  It's wonderous stuff...

Stay with cork

I can say from what I have worked with it, including layout building...if you have the choice to use cork, do it!..there is way too much give with foam, and I have done both flex and hand lay on it.  It was nothing short of a pain.  I switched back to cork, I will never use foam again. If you find sound is an issue, I have never had an issue with modules with blue or pink foam on top of a sections and had noise.  I use a two level cork road bed for my main and then one level for the siding's, etc.