Earlier this year I replaced the aux air pump system from stem to stern. The thread about that is here: Aux air pump system replacement (http://www.foreforums.com/index.php?topic=44535)
The one thing that went backwards was the noise level. The new pump, while much faster and worker for a shorter period of time, is a bit noisier than the original pump. A few months ago, Jim - fourdayoff replaced his Thomas pump with the same Air Zenith pump I used. During this process, he mentioned his "muffler" to me. Apparently his coach was built or someone added a muffler to the inlet system.
I got the part numbers from him and tracked it down to a Burgess Manning 44-124-100 unit, which is no longer made. After some more research, I found a Solberg muffler that I thought would do the trick. 1/2 NPT male at both ends, 25CFM rated. I ordered it up from Grainger, along with some 1/2 NPT to 3/4 hose barb fittings.
The inlet to my pump used an oddball M20x1.25 thread that I could not find any adapters for, so I ordered up a 1" hex aluminum bar stock and a M20x1.25 die as I already had a 1/2 NPT die. We made the needed adapter and mounted the muffler on the back bulkhead of the electrical bay. I used the original Air Zenith supplied filter at the inlet of the muffler.
Before installing all of this, I took new readings of the noise level at these increments: 1' from the pump in the bay, 3' from the coach with the bays closed, in the kitchen, in the bedroom and in the bedroom with the doors closed. The pre-muffler measurements were very close to the original measurements taken when the system was installed 6 months ago. The new measurements show the muffler did help. Overall I think I have about $200 in this upgrade, but $50 or so of that was the aluminum stock and die I needed to adapt to my pump. Definitely worth doing in my opinion if you don't have a muffler now.
Parts list:
Solberg muffler: http://www.grainger.com/product/SOLBERG-Inlet-Discharge-Silencers-3TKY6
Adel clamps to mount the muffler to the bulkhead: uxcell 2-1/2"(63mm) Cable Clamp 304 Stainless Steel Rubber Cushioned... (http://www.amazon.com/dp/B07T2JJ3P7)
1/2 NPT Female to 3/4" hose barb fittings: Beduan Stainless Steel Elbow Barb Hose 3/4" Hose ID x 1/2" NPT Female Pipe... (http://www.amazon.com/dp/B07MZKXPL9)
3/4" heater hose.