No-hub cast Iron definitely reduces the sound of waste water but it is also important to avoid horizontal runs over occupied spaces and to use toilets that flush quickly and quietly like most Toto toilets. Vertical stacks in good enclosures are not a big problem. This is a common issue so your designer should be on top of it.

It might help you to know that the term "drop" refers to the slope of a horizontal drain pipe. A vertical main drain is called a "stack".

Good plumbing design is the most cost effective $$ spent. You can't make up for poor design with cast iron.

We have cast iron drops and Toto toilets in our new house. Had cheaper toilets and no cast iron drops in the old house. It makes a huge difference. We did add sound dampening insulation around our powder room as it is next to the den and we wanted the added privacy. It seems to help also.

This is all great to know. We are going with toto. So that will help. Once the plumber is ready to rough in I will talk to him about the vertical drain. There is only one bath that is the real issue. My husband would be very unhappy to hear a toilet flush while he is in his office since he works from home. He needs absolute quiet and is even installing quiet rock on his ceiling and a walls. He is a speaker and makes recordings.

We just want to make sure this extra measure will ensure absolute silence in his space.

" You can't make up for poor design with cast iron."

You cannot quiet the noise of drains well without mass, and using CI adds the required mass.