Ducting design to reduce Airflow noise

[Jeremy]

In my conversion I am retaining the original bus's heating system, which consists of a heat exchanger and fan mounted below the floor on each side of the bus, with a duct running the full length of the bus on both sides to deliver warm air to the feet of each passenger.

I have removed the full-length ducts, and plan to replace them with much shorter ducts directing the air out of vents in the base of various cabinets and items of furniture. This evening I fired up the system (with no ducting fitted at all), and was surprised by how much noise was made by the air rushing through the heat exchangers - it made me realise that it would still be quite noisy even after my 'short' ducts were added, unless I do something clever to combat the noise. Are there any recognised tricks using baffles or bends or filters to reduce the noise transferred through an air ducting, without impeding the air flow too much? I imagine it must be a common concern in air conditioning systems and so on.

Anyone got any tips to share?

Jeremy

[buswarrior]

HVAC industry has lined ducting, the lining acts as a sound deadener.

Sound doesn't like to turn corners, so an elbow or turn in the duct, with a less than reflective surface inside the bend, will knock a lot of noise down.

happy coaching!
buswarrior

[Chuck Newman]

BW is right on track.  I haven't checked this with a sound meter though I recently purchased one, but I've found that for each 90 degree turn, sound drops about 50%, measuring with two pretty good ears.  As BW mentioned, you will also need a sound absorbing liner to greatly reduce the echo effect of sound bouncing off a hard surface. 

Chuck

[Jeremy]

Thanks, that's interesting. Would I be right in assuming the 'sound absorbing liner' is a foam-type material? I cannot use off-the-shelf ducting for my short ducts due to the very specific size and shape it has to be, but I could certainly put some bends into it and stick foam sheet to the inside surfaces

Thanks for the info.

Jeremy

[bevans6]

The material comes to the ductwork sheet metal shops in sheets and is cut by them to make ducts.  Every one is custom, so they could make yours for you based on drawings.  More expensive than DIY, but it's out there.  I had a furnace installed a while ago, the guy showed up with no ducts.  He did a few measurements, called his shop, gave detailed directions over the phone, and by the time he had muscled the furnace into place a truck showed up with his ducts, custom cut and formed, in about 30 minutes, and they fit perfectly.  I was frankly astounded!

Brian