Sound takes the easiest path through a wall so it is difficult to do much to reduce the sound transfer through a wall with a door in it. If there were no door you could use two layers of drywall on one side of the wall and seal the boards to the studs and the plates to the subfloor, etc. making sure the wall board continue to the borrow of the subfloor above and that there are no electrical outlets or recessed cabinets or duct openings in the wall. Some insulation would help but not much.

But first you must install a perimeter sound seal to the door frame and perhaps recess an automatic drop-seal in the bottom edge of the door. Of course, the door should be solid core.

Google some sound proofing wall details. The more you do and spend, the better it will work.

Hang a heavy material(like an area rug) on the inside of the door.

Been there, done that ... applying a 2nd layer of drywall outside the bathroom made a huge difference.

Remove door trim and baseboard.

Make sure old drywall is correctly installed. Seal any large gaps between drywall and floors.

1 - Green Glue soundproofing compound between layers of drywall, following instruction video carefully. Do the easiest to access side, which is probably the non-bathroom side.

Make sure the drywall is cut to fit snugly - no areas for sound to leak!

2 - Green Glue's noise reducing caulk around all edges of wall and door framing to seal leakage paths.

3 - HEAVY SOLID CORE door ... correctly fitted and perhaps with weatherstripping to minimize sound leakage around it

4 Install wider door door trim and re-install the baseboard.

5 - If you need it, add a small amount of sound reduction foam inside any shared air ducts, right where sound would enter or exit.

Here is a link that might be useful: Green glue

Thank you so much for the quick replies. This is great advice.

I will also tile the wall on the bathroom side so hopefully that helps as well.

I would rather not put another layer of sheetrock but I am considering doubling up the sheetrock between the joists on the inside - although I have no idea if that would make a difference.

Is there any particular type of insulation that I should put in the wall?

I am perplexed as to what to do with the gap below the door. I know it is needed for the exhaust fan but I imagine that it will be the biggest sound pathway. Any ideas? Has anyone minimized the gap and created another vent for the air somewhere?

Thanks again.

This post was edited by Lumine on Wed, Sep 24, 14 at 20:36

Insulation is of no perceptible value in reducing sound. Neither are extra layers of drywall unless attached with Green Glue or resilient channels. However, unless you make great effort with the door and seals, don't bother with the walls.

Source City of Vancouver, B.C. "Noise Control Manual."

Add the automatic drop seal I mentioned earlier and leave the door ajar when the exhaust fan is operating (put the fan on a separate timer switch so it can run for a while after you leave the room). The only practical purpose of the exhaust fan is to remove moisture from the room after a shower and during the time it runs you will have no acoustical privacy.

Here is a link that might be useful: automatic door seal

If your concern is the sound of the toilet flushing it would be cheaper and easier to replace it with a quiet toilet. There are models that you can barely hear flush even when standing next to them.