soundproofing a floor when installing hardwood

When we did our renovation a large part of the floor about 600 sf had to be replaced and then the downstairs neighbor heard about the work they pretty much insisted on soundproffing the floor. You can ask your GC about it as I don't have details but there is a $$ cost involved. It involves replacing the material or plywood under the boards with special soundproofing material.

On the plus side it does really help and if I had know what an improvement there is over the creaky old floor i would have redone the whole thing.

Thicker wood is better.
Noble Seal SIS membrane or similar is another option.
The way the floors is installed also makes a difference from what I heard.

"...the floor is laid with two layers of crossing plywood to make sure it’s quiet..." From DNA Info article with advice on making your home more quiet:
http://www.dnainfo.com/new-york/20130110/new-york-city/tips-on-making-your-home-quiet-sanctuary-from-citys-bustle?utm_content=coup.d_oeil%40yahoo.com&utm_source=VerticalResponse&utm_medium=Email&utm_term=Tips%20on%20Making%20Your%20Home%20a%20Quiet%20Sanctuary%20From%20City%27s%20Bustle&utm_campaign=Mechanical%20Failure%20May%20Have%20Caused%20Ferry%20Crash%2C%20Sources%20Saycontent

Read more: http://www.dnainfo.com/new-york/20130110/new-york-city/tips-on-making-your-home-quiet-sanctuary-from-citys-bustle?utm_content=coup.d_oeil%40yahoo.com&utm_source=VerticalResponse&utm_medium=Email&utm_term=Tips%20on%20Making%20Your%20Home%20a%20Quiet%20Sanctuary%20From%20City%27s%20Bustle&utm_campaign=Mechanical%20Failure%20May%20Have%20Caused%20Ferry%20Crash%2C%20Sources%20Saycon

do a membrane

Bumping this thread. Can anyone else have experience soundproofing a floor? Thanks