I had a job running and went out to walk my dog, and when I was coming back to the house I stopped at my neighbor's mailbox to listen (160 feet away), and the sound was very noticeable and unusual coming from a residence. It sounded like a Shop Vac was in my front yard - quite metallic sounding. I was horrified at how loud the blower sounded outside of the house. I have it go to a 4 inch dryer vent with no filter.

Well it turns out that the pot growing industry does not like pesky neighbors calling the Man on their grow house - so there are all sorts of products to deal with this. I ordered this:

Amazon.com: Lighthouse Hydro Duct Muffler Inline Fan Hydroponics Noise Reducer Silencer, 6-Inch: Patio, Lawn & Garden

There are also others, but never tried them:

Suncourt 6 in. In-Line Duct Muffler-DM106 at The Home Depot

Shop DuctMuffler 6-in x 24-in Galvanized Duct at Lowes.com

When it arrived, it was just a 10 inch tube with Sonix(R)-like foam in it and step downs. You could make one for maybe $20 less.

I had to use that metallic tape to connect it, as it did not naturally fit the duct work and elbows that I ordered - though you need that tape anyway to seal it. I turned the blower on and walked back to the neighbor's house. I could not hear it at all. Nothing. I walked closer and closer, and I had to get to about 25 feet away before it sounded about as loud as I remember it did from the 160 feet away before. I actually own a B&K professional sound level meter, but I loaned it to someone, so no dB results. Better yet, it was not just quieter, but the metallic/industrial sound was gone and now it sounded just like a common dryer venting. The air was still rushing out as a powerful blast.

Amazon's listing says "reduces noise by up to 25dB." If my guess is correct and what used to be 160 feet now is at about 25 feet, then that is more like 16 dB. Even so, depending on the frequency, 6dB sounds about 1/2 as loud, so it resulted in a sound level that is almost 1/8 as loud. It worked way better than I expected given that it is only 24 inches long.

You may have to replace the foam periodically if tons of wood dust is building up in it.