When we had the tile installers in to provide cost estimates, we asked about the bathroom subfloor. Since travertine is heavy, we wanted to be sure that the subfloor would be stable enough to support the tile and not lead to cracking.
One installer said they would just put down backer board and the other recommended adding another half inch of plywood before adding the hardiback.
After carrying the travertine tiles around the tile store on Saturday, it emphasized just how much weight would be on the plywood. So, on Sunday, we put down additional plywood.
It is amazing how much firmer the floor is when you walk on it.