text: Muffling Dust Collector Noise
  We are now running a 20hp blower inside, but built a sound enclosure around it, and it is still too loud. Previous shop had a 5hp in a separate insulated room with a muffler, and you could easily talk with it on. Next shop will locate one outside with a sound enclosure. The goal will be to have it so quiet we'll have to have a running light to see if it is on or not. Ringing ears is a bad sign. I hear the birdies 24/7/365 for 10 years now. Take my word - you don't want it.
  We have a 20hp in the building. When we put it in it was 96db. That is loud! It is in a corner. We built two walls - a stationary wall and a hinged wall. We built the 2 x 4 walls with 5/8 drywall. We insulated the inside of the wall with acoustic fiberglass. We also applied acoustic egg crate foam on the wall facing the collector. The hinged wall has two wheels on the bottom and is hinged to an existing wall. We can move the wall to empty the collector. From the ceiling between the beams, we hung a special rubber sound barrier. The space below the wall where the wheels are lets the return air out. It works great - we dropped the sound down significantly. Before we did it, you couldn't even talk within 50 feet of it. Now you can have a conversation within a foot of the wall.
  We also have a neighbor with a similar situation to yours. They built a box with egg crate foam lining the inside to cover the exhaust. The top of the box is open with a piece of egg crate forming a flapper to let air out. It doesn’t sound like it would work, but it really does work well.
  First of all, get your safety glasses and hearing protectors on or you'll be staying after school and explaining in writing why I should allow you to continue attending this class if you ignore shop rules. Sorry, I used to be a teacher.
  I also wanted to reduce dust collector noise. I had a large cyclone outside and the return air came into a 30 bag Dustcop cabinet. I added an attenuator (muffler) to the return line and got a measurable reduction in dbs but not much on the audible side. I think a larger return line would have made as much of a difference. I added a rubber section to both inlet and outlet ducts and this helped with the vibration noise that was transferred through the ductwork. I also tightened up rattling gates. There were a certain number of gates that, if all left open, also helped considerably. This was a large shop - 16" trunkline, two 14" saws, two 10" saws, two 16" joiners, two 8" joiners, four bandsaws and on. Some noise is a result of the orifice at each machine, so I'd keep the small ones closed unless needed.
  In my house I have a remote exhaust fan that not only exhausts the bathroom but also some closets. I have round four inch grates in the ceilings that at first whistled. I figured that the small veins in the grates were causing the noise and removed a couple. The whistling stopped. Your return air apparatus, whatever it is, could be doing the same to you. What that flows through and into should be looked at.
  You, as an employee, have a right to have the noise level looked at. You fall under OSHA regulations; your students don't. The noise may also affect adjoining classrooms. Woodshops are noisy places. Both you and your students should have thorough training in hearing protection. The school should help to reduce your exposure.
  I worked in a shop that had the same problem - very loud return air. We ended up making a muffler out of a large wooden box with a chicken wire tube or wire bent to make a cylinder and then filled the outside of the wire with old socks (real close to a sock factory, so they were free). This made a very large difference. It is basically the same as a car muffler. Next, we lined the elbow on the end where it "dumped" with some old carpet to also deaden the air as it turns.
  There are two sources of noise in DC, and you'll need to address them separately.
  1. Shroud/housing. If you touch it, you'll find it vibrates a lot. A few square feet of metal vibrating with large amplitude and frequency make a lot of noise.
  Solution: buy acoustic barrier. The best, in my opinion, is wispermat wm2 (see http://www.silentsource.com/barriers-whispermat1and2.html). wm2-16-16 (1/4" acoustic foam + vynil barrier + 1" foam) is what you need. At $3.40/sqf it's hard to beat - in fact I'm afraid shipping will cost more than material. FYI, a typical blower needs 5-8 sqf; installation will take an hour or two because of complex shape.
  You should glue it 1/4" foam side to metal (may sound counterintuitive, but that's the way to go). Believe me, it works as advertised (exceptionally well), and it'll virtually eliminate noise from the shroud.
  2. Exhaust flow out of impeller. This is much harder to fight. Remember that, typically, 6 blades clear exhaust port about 3400 times a second, sending a burst of compressed air at 6x3400 = 20KHz. That low frequency roar you hear is the result of interference of 20KHz sound waves.
  Ideally, exhaust air should go into long (about 10-20x impeller diameters, typically 8-15 ft) straight pipe lined with 1-2" of acoustic (open cell) foam from inside, preferably with wavy surface across the flow to introduce slight turbulence into air flow and smooth out bursts of pressure. An almost non-restricting muffler. Pipe diameter should be about 30-50 sq. in. per 1000 cfm of airflow. Ideally, outside of the pipe should have acoustic barrier too.
  Low cost solution is the use of 8" flexible insulated HVAC pipe available from any major blue box (something like $40 per 50 ft box of 8" ID pipe). It won't restrict the airflow and will reduce exhaust noise dramatically (please notice that exhaust air from about 25' of HVAC insulated pipe won't be too loud, but if pipe doesn't provide enough acoustic insulation to suppress noise near the impeller, and if blower is close to the building, you still want to add acoustic insulation to first few feet right after the exhaust - e.g. by wrapping HVAC pipe with R25 fiberglass insulation).
  Please notice that everything mentioned in p. 2 will only work with cyclones when air leaving the blower is almost clean air. It won't work with airflow carrying dust because of abrasiveness.
  Since it sounds like you have a cyclone separator, you may use both 1 and 2. It shouldn't cost more than $150-200 max (well, plus your labour). It won't make the setup dead silent, but will reduce the noise by 20-30 db.
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