quiet drain noise
 

Ok, so I know cast iron does a better job of reducing drain noise for vertical drops. But what about the horizontal sections? We've got a master bath that's going to be over a large family room. I don't want to be hearing the tub, toilet and shower downstairs. There will be a number of 5 to 7' stretches of drain to get from the various fixtures to the vertical pipe. Should these also be in cast iron?

Sure, there's an added cost, but I *really* do not want the noise.

I've used Schedule 80 ABS in these situations. Smaller id, same od, thicker wall which helps to lessen the water noises.

Adding some insulation and using 5/8 sheetrock on that wall will also help.

Insulation is good. Also consider sch 40 or 80 pipe instead of foam core DWV pipe. Cast is pretty spendy and heavy- I'd almost consider copper instead....
There are commercial grade pipe insulation systems available. Perhaps you can find a pipe wrap company in your area and cut a deal with him.