This doesn't help too much with sound from my experience, but it is amazing for reducing the blower effect. When cleaning up my show the blower effect often pushed around dust that was behind me, meaning that I had to hit some areas twice. This fixes that problem.
I thought that this was a great idea when I purchased it. However, it is advertised as being a muffler and the noise reduction, which has always been an issue at our house, is minimal if anything at all. I can't hear any difference with or without it on the machine and that's my wife's vote also. The only thing it seems to do is slow the exhaust air flow down slightly.
What you sent me differs from the picture of the item. There is NOTHING across the back end of the muffler/diffuser. This creates not just one but two problems. First off, it diffuses nothing! The exhaust air still just blasts straight out rearward without being diverted in multiple directions. Second, there is nothing to hold the foam insert in place. It gets shot right out the back the second the vacuum is turned on. What you sent me is a useless piece of junk.
This shop vac muffler/defuser is a must when vacuuming indoors. Less Noise and defuses the blower. No more dust flying around the house.