Your addressing a crawl space for a raised foundation? if so there are a few things you need to do.
First off, look at the crawl space like a speaker box itself with ports in the sides to let air in and out..big problem...
Here is what I would recommend, take 1/2" cement board / hardie backer and green glue , cut and put up in between the floor joist up against the sub floor.. screw and caulk in... this will add mass and change that boom effect some what. After this step is done we can evaluate the results and see if you need more mass or just adding insulation and sealing up under the room with OSB would be enough..
This is only addressing what is going under and back up...it does not change what is traveling under the walls. It will dampen it some what...if your trying to reduce what is traveling under the walls it needs to be address from the top of the sub floor itself, which means a new isolated decoupled floor.
Feel free to contact me direct.
Randy S.
760-752-3030 ext 104