Based on research, because I will use partially natural stone I need two layers of underlayment - 1/2" first layer plywood and 5/8" second layer plywood. Then put third layer 1/2" cement board, and then add Ditra possibly, and then finally tiles. As the tiles I like are 3/8" thick, the total floor thickness will be 2" at least without counting the thickness of Ditra. Wouldn't the floor be too high? What is the standard height for flooring?

You'll want to get a lot of samples of both the marble tile and the porcelain. Because the marble is a natural stone with infinite variations. The porcelain will likely not look good when next to the real stone. Possibly a non marble porcelain may work. Lay out a number of each possible choice in your space. For sound reduction an underlayment will not be enough. I'd remove the study room ceiling and try as many tricks as possible. including sound reduction insulation, How do you soundproof a ceiling with drywall?Adding a second layer of 5/8-inch drywall below the existing ceiling drywall will limit sound transmission. Spreading a liquid soundproofing product between the two sheets further limits sound by dissipating vibrations. Soundproofing compound costs about $45 to $60 per sheet of drywall (two tubes per sheet).

Make certain your existing floor joist system can handle the weight you are adding with all this new plywood & waterpoofing. And plywood underlayment will not help with sound transmission.

@ millworkman

This is the joist system underneath the first floor (ceiling of unfished basement). I assume the joist system of the second floor would be the same as this. Each of the joist interval is between 15"-24" I guess. Does it look strong enough to support the additional weight for renovation?

@ dan1888

That's a new perspective. I've seen the Home RenoVision DIY youtuber doing it in one of his videos. But I hoped that I can handle the noise issue while the master bath floor is gut open so I don't need to make two different jobs. Do you think spreading the liquid soundproofing product between the subfloors or between the subfloor and underlayment feasible?

@ Creative Tile Eastern CT

Thanks for correcting me.

Among the options you mentioned, "a subfloor of 3/4 and a better uncoupler such as Blanke Per Mat will allow you to go directly over a single layer for stone" sounds like the least work process so very attractive.

So did I understand you correctly that I do: 3/4 plywood subfloor, top Blanke Per Mat, and install stone and porcelain, then I can achieve my goals to reduce noises and to provide a sound support for additional weights?

Is the 170.5 Sound & Crack Isolation Mat the product that you recommend using as an uncoupler? Or is it a product to use in addition to Blanke Per Mat?

Can a liquid soundproofing product be used between the plywood subfloor and Blanke Per Mat? Would the liquid soundproofing product work as an adhesive? If not, how can I apply both a liquid soundproofing product and an adhesive together?

The only way to get the proper answer would be to contact the manufacturers directly. Visit their websites. They all have a technical department available to specifically address project questions. Gathering well intended opinions on an open forum is risky at best. Manufacturers will provide their approved products and methods.

That is how I learned over the years. All the knowledge is available for free! I only wish more Tile Contractors would take the time and do the same.

@ Creative Tile Eastern CT: Will do, thank you!

@ millworkman: I calculated floor deflection of the 1st floor joist. It turned out to be L / 226 which is only good for vinyl or wood flooring. Can the joist system be partially different in the same house? The current floor of master bath is porcelain tile of 80 sqf.

" Can the joist system be partially different in the same house? "

Not usually unless only a portion of the home is two story or flat roof designed for a roof deck.

Try this.

https://www.johnbridge.com/vbulletin/deflecto.pl

That's where I could calculate the deflection for my house.