Thought I’d try out one of the exhaust silencers on the ShopVac to see what difference it could possibly make.
The design is pretty basic – a tube with an opening and exit the diameter of the exhaust of the vac, and a wider diameter between with a foam tube inside.
When fitted to the outlet of the ShopVac, there was a noticeable difference, although not particularly significant. The noise is more the motor than the exhaust it seems. Nor does the silencer work particularly well in its as-supplied state.
And yes, given how simple the design is, you could easily make one at home.
What I am thinking is to block the outlet, and cut slots around the circumference instead, and see if that makes more difference. I’ll research that another time.
Out of interest, I measured the sound levels in the workshop.
Outside (at the time): 40dB
Inside workshop: 38dB
1m from Sherwood dust extractor: 77dB
1m from ShopVac (no silencer): 86.5dB
1m from ShopVac (silencer): 84dB
Stu's Shed 
Hi Stuart, just remember that sound measurements are logarithmic. Therefore 3dB reduction is a 50% reduction in power. Your silencer almost halved the audio output.
If you measure the output power of an audio amp and you turn down the volume so that you can just hear a change- chances are good that your power meter would show that you’ve halved the power.
I was hoping (unreasonably) for closer to a 10x reduction in power, which would have made more of an audible difference.