If you're looking for a way to reduce the noise coming from your HVAC system, duct sealing is a great option. You can apply sealant inside the vents and ducts, which will expand to fill the space and dampen the sound that passes through the metal. This is usually done by placing a damping material, such as a soft rubber or gel-like material, in the duct. A manufacturer has a demonstration video in which they place a piece of a few square centimeters on a cymbal and the sound turns into a dull noise. The most effective way to eliminate airborne noise in ducts is to add 90-degree turns inside the ducts.
The noise that circulates in a straight line inside the ducts will not hit any barriers that block its passage. However, if that noise is forced to rotate 90 degrees, the sound coming out the end of the duct will be reduced by about 6 dB. We recommend adding two or three 90-degree turns for optimal soundproofing. You can also take one or two bags (enough to fill the cross section of the duct to a depth of a few feet), try to keep them intact and insert them into the duct. This will help reduce airborne noise that can pass through ducts and “seep” from the next room. By taking these steps, you can significantly reduce noise from your HVAC system and create a much quieter environment in your home.
