Simple and cost effective methods of reducing noise from a ventilation duct come down to either introducing Sonex into the duct work or building a treated outer air flow 'manifold'. In both cases, we employ Sonex to absorb noise energy and covert it to heat via open cell foam.

The usual culprits in ventilation systems are fan noise and duct flow noise. Fan noise is usually a high frequency (calculated by the number of blades x the speed of the fan rotation). The duct air flow noise is typically caused by a large 'velocity' of air flow going through a small pipe, causing a whistle or a whine.

In the duct solution
If you can, lining the air duct with a thin layer of Sonex such as 1" Mini-Sonex can often reduce the noise. This is best achieved by lining the inner duct from the 1st 90º joint to the exit port. The reason for this is that corners, when acoustically treated are extremely efficient at absorbing noise as high frequencies tend to be more direct than lower frequencies.

One concern here is reduced air-flow. Ideally, a larger pipe should be installed so that the air duct size remains constant after introducing acoustic absorption. The other concern here is dirt and contamination. Sonex fire rated melamine foam will not produce contamination like fiberglass. (We think this is good!) If the duct will be used for both heating and air conditioning and the temperature levels can be extreme, it may be worth the added expense to invest in a coated product such as Sonex with Hypalon. This breathable coating resists water and chemicals.

Out of the duct solution
This is often the best 'fix' for air ducts and vents as one can often attach a 'manifold' or 'labyrinth' at he vent outlet with excellent results. The concept is simple: Provide a path for air flow while reducing the air velocity.

Building this device is simple. A 'U' channel or tunnel covers the opening, air is diverted to two outlets with 90º ports thus redirecting the air (and the noise) while transforming the air movement from a 'small-fast' vent to a 'large-slow' vent. The larger the final duct opening, the better the air will be distributed and the more noise will escape. As a rule, one would 'at least' double the size of the manifold exit vents.