I figured I'd share my stealth exhaust muffler with you guys. It doesn't cut out the noise 100% but it definitely cuts it down a lot. I wish I remembered to take pictures while I was building it, but it's easy enough to explain. I went to home depot and got a small rubbermaid tote and some cork flooring underlay. I took a hole-saw and cut a hole in the bottom of the tote the same size as my exhaust fan's outlet. Then I cut a rectangle section out of the front (or back) of the tote for the air to get out. I cut pieces of the underlay to line the sides and bottom of the tote as well as the lid. Then using a couple pieces of scrap metal I had, I made a ramp inside the box with the underlay in order to break up the airflow. Finally I stuck some regular batting insulation in to further reduce noise at the sides of the bottom of the ramp. This is a basic design and I don't claim to know everything about mufflers, this is just one man's attempt and it works well enough for me. It cuts out the high pitched sound of air rushing so you no longer hear it from outside the room. Here are a few pictures of the finished product.
inside the armoire.. the plastic bin sits ontop of the exhaust hole. The bin bounces / redirects the air around before it exits, just like a resonator in a car exhaust. well done BTW, you could try baffling the inside walls of the bin to further absorb sound.