Always a Good Idea:
To double the floor under a washer and dryer. You do this by nailing 1-inch plywood down in that area. Then you put finish flooring on top of the plywood, and trim the edges with a moulding that covers the edge of the plywood. Done right, it's not ugly at all, in fact it's barely noticeable. And it makes a huge difference in the noise and ahake. This is an old mobile home trick that is becoming more necessary in new houses, which despite their fanciness, are lightly built. Even in older houses, it can help a lot.