Replies

1. 
   renosteinke | Jan 25, 2009 08:40pm | #1

   First ... do you KNOW there's a problem? Or, are you just assuming there will be one? No point fixing a problem you don't have.

   Most air movement noise is created by - yup, you guessed it - air moving. This means your primary defense is in your fan and ducting methods. Simply put, eliminate turbulence in the airflow, slow down the air speed, and you reduce the noise.

   Turbulence is broken up by vanes in elbows and other bends. It's also prevented by eliminating rough edges on the inside of the ducts. The fan design makes a huge difference; propellers are a lot noisier than squirrel cage designs.

   Air speed is reduced by using larger ducts.

   The noise reduction possible from things like wrapping the ducts is quite minimal. Insulating the duct will help prevent cooking fumes from condensing within the duct.

1. 
   Yersmay | Jan 25, 2009 09:34pm | #2

   Thank you for the response. Well, you're right -- I don't KNOW there's a problem... but something tells me. I've got four 90 degree bends. I honestly don't know the type of fan... it's a Viking... its output is 600 CFM. I'm curious to know more about vanes in the bends. Are those applied while you're building it? Or do bends come with them?

1. 
   Piffin | Jan 25, 2009 10:01pm | #3

   I had a major problem with one range hood. Can't remember the brand right now.But the flapper would stay open only on high speed. With a low speed on the fan it would thump closed intermittently, and the mid-setting would let it flop open/shut at a rate of about three times a second. That was insane.It was one of those with an option of going out thru the wall or straight up, with that latter being the default. I had to switch it to the wall exhaust so I assume that may have had something to do with it.I had to double up the spring that holds it closed to make up for the back pressure gravity would have provided.Anyways, before discovering the source of the vibrations, I used spray foam on the outside of the ducting to muffle the sound about 30% - which was a lot, considering how bad this thing was. 

    

   Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

2. 
   renosteinke | Jan 25, 2009 10:59pm | #4

   Elbows can be purchased with vanes in them.

   As for the fan type ... look inside. What do you see? If the motor sits in the sirflow, and the fan blade resembles the blade of a room fan, or an airplane propeller, well, that's the noisier propeller type.

   A 'squirrel cage' looks just like that ... air enters a 'cup' like opening, and around the perimeter are small vanes, making it resemble one of those wheels you see in a hamster cage. This is a much quieter type of fan.

   Four 90's? You're setting yourself up for trouble ... and noise is the least of it. I think you're going to need another fan in the duct, or at the end, to help things along.

   The metal tape on the joints is nice .... but I think the preferred method is a mesh tape, with a gooey grey paste - called 'pookey' around here - applied over it. This is also applied to all seams. It does dry .... but will ruin whatever clothing you wear.