HVAC Duct Sound Reduction Using Fibre Glass Insulation Materials

One of the options is to reduce the velocity in each duct.

Sound in the duct is most likely attributable to poor design...Insulation of metal duct is usually done by gluing 1/2" fiberglass to interior before fabrication...Your best bet, as mentioned above, is to lower air velocity, or possibly install flex duct section(expansion joint) at supply to minimize vibration....

http://www.indiamart.com/unijoint-fabrications/expansion-joints.html

http://www.soundproofingamerica.com/soundproofing-products.asp

Something that - may or may not - work would be to apply stick on sound absorbing panels to the outside of the duct. But this would be very easy. An asphalt based product gluing over full area would have best chance - not a rock or mineral wool panel with mounting tabs.

May be available from an automotive supplier.

Check if duct is rigid, or resonating / vibrating.

SP, is the duct resonating or vibrating?

Or is it just air noise from the air movement?

There may be a cheaper solution.

Usually sound attenuators are provided at the out let of AHU. As already mentioned reduce the velocity of air . this will increase the duct size and cost. You can use good insulation materials of 75 to 100 mm thick with chicken wire mesh arranged in wood pattern as sound absorbing method. For more details you can refer ASHRAE Handbook or any reputed HVAC consultant.

My A/C ducts have urethane foam adhered to them on the inside, but inside or outside the result is a quiet airflow - no drumming/resonance. If I remember correctly it is about 10 to 15mm thick.