After several revisions of the Shop-Vac muffler, this is about as cheap and simple as I could get it. After some work, I’ve been able to achieve a noise reduction of around 3 decibels with it. According to the first chart that I found when searching, 5dB is a “clearly noticeable change.” 3dB isn’t too bad either. When combined with some “acoustic panelling” made out of packing material (see this post), maybe I’m getting close to this. At least I’m under the OSHA 8 hour exposure limit.
As seen in the sketch to the right, the final version of this muffler was made by cutting seven layers of foam into baffles and spacers. These spacers (as seen in the gallery after the “read more”) take the place of the PVC pipe used in the last version, becoming the pipe itself.
Although somewhat arbitrary, the diameter of the spacers was drawn by tracing the bell housing from a 4 inch PVC pipe. The inside was traced from the inner diameter of the 4 inch pipe itself. The baffles, 4 in this case, were the same diameter, but were cut with a smaller hole traced from the 1 1/4 inch adapter used. I tried to undercut this hole, giving an interference fit with the adapter and the Styrofoam. This allowed it to simply sit inside the hole and not fly off.
Just doing this gave me a noise reduction of around 2 dB. Noticeable, but just barely. I tried 2 chambers first, then four, and finally settled on three without a huge difference. A bit more noise reduction was achieved when I decided to put cloth on the inside of the baffles. An old towel was cut up and the pieces glued opposite the holes.
After this improvement, the noise reduction was in the range of 3 to 4 dB depending on where it was measured. After running my meter around it, it appeared that most of the noise was emanating from the Shop-Vac itself, not the exhaust. I’d call this a success for now, but I imagine that a piece of noise-panelling in front of the vacuum hung from the shelf it’s under would do a lot of good as well. As with any project that I do, it’s apparently never actually done.
Sound level meter in the video: Pyle PSPL01
I‘ll still probably use earmuffs much of the time, even if it’s supposedly in the safe limits. If nothing else, I can listen to my iPod earbuds under them while working on stuff. This is actually the third revision of this (see the first here and the second here). It’s the cheapest of the three, and when compared to the first, it’s not quite as effective noise-reduction wise, but it’s much cheaper, lighter, and seems to allow more air flow.