This may sound strange but some folks advocate the use of drywall as a base because of its density to avoid the vibrations that transmit noise like a sounding board. Ever try to run a train on a concrete floor? There is so much mass that nothing is transmitted in the air. Add your grass mats and you'll be ready to go. Although I don't know whether it matters, but some people also put loose fiberglass insulation within the cavity of the Fastrack.
