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There have been some posts on rec.woodworking in the past reviewing the muffler available for the Ridgid shop vacs. I decided to check one out and see how it performed on my system. My shop vacumn is the 6.25HP 16 gallon model. Its completely stock with the exception of a Gore Tex Clean Stream filter. I normally us it with two hose lengths, but for this test, I only used one. To keep measurement errors as minimal as possible, I looped the hose over it's self like you see in the picture and left it in that position during all the measurements. For the measurements I used a Radio Shack SPL Meter model # 33-2055. It's not a laboratory grade instrument, but it gives very good readings in field conditions. Mine is not calibrated, but then we are mainly looking for relative measurements anyway.
The first reading is the reference of the shop vacuum unmodified. The meter was placed on a specially designed acoustically accurate platform, commonly called a cardboard box. I didn't measure the distance but the meter is about 2 feet from the exhaust port and 45 degrees off to the side. From this location I read 97db spl. Wow, I knew the shop vac was loud, but this is in hearing damage range!
I then installed the muffler and pointed it up in direction. The shop vac and meter remained the same location during all the tests. With the muffler installed I measured 90db. Big change! 7db is a noticeable amount of noise reduction. I'll talk more in about the human perceptions of this change.
It's been discussed a few time to add some steel wool stuffing to the muffler to improve it's performance. I disassembled the muffler and it came apart into three pieces. The intake and exhaust side shells and an internal diverter. From my very limited knowledge of sound reduction, it appears to operate by redirecting and breaking up the air movement. I took a pad of #3 course steel wool and unrolled the pad.
I then refolded the pad so that it would fit into the intake cavity of the muffler. I trimmed off the excess corners so it would fit the round space.
I reassembled and installed the muffler. In this configuration the SPL reading was still 90db. No measurable difference. I then stuffed the exhaust side of the muffler in the same way.
Reassembly and measurement this time showed a SPL reading of 88db. This 2 db increase in performance may be related to the steel wool being on the exhaust side or may be due to the increase in total steel wool to reach a critical mass. So to sum it up, the muffler stock from Home Depot cut the noise level from 97db to 90db, a 7 db improvement. The steel wool in both chambers increase the reduction to 88db for a total of 9db of reduction. Put into human perspective, the average human can hear a change of 2 db. If you imagined a volume control where you could lower the volume by a small amount with each click, this muffler lowered the volume by 4 clicks on that scale. That's a noticeable improvement. Now let's look for a minute at OSHA's standards for maximum exposure to sound pressure levels without protection.
By these standards, I still cannot work round the shop vac for more than 8 hours without experiencing damage. With the unmodified vacuum I could not work around it for 4 hours without experiencing damage. In speaking with a friend who is an audiologist and a woodworker, he recommends much lower levels of exposure and mandatory hearing protect usage around power tools. He also recommended placing the shop vac as far from you as possible. Sound levels decrease over distance, so by doubling the distance between you and your shop vac you can decrease the noise by 6db. So for example, by using a second extender hose on my vacuum, I can get another 6db decrease in noise that my ears hear. Well, I hope this page is of some value to someone out there. If you have any questions, please feel free to drop and an email.
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