Sound Solutions (acoustical engineers) evaluated the noise from HVAC systems at a new flight simulator facility for the US Marines Base in Yuma, Arizona. The flight simulator facility has many training rooms, flight simulator rooms and conference rooms where a certain background noise level needed to be met. Sound Solutions (acoustical consulting) measured the noise in all of the noise sensitive spaces. Where the noise level exceeded the noise criteria (NC), the path of the noise was determined and noise mitigation measures were recommended.
In evaluating the most effective acoustical treatment, it is important to understand the path of the noise. Some noise is duct-borne (traveling down supply or return ducts and out the diffuser or register). Some noise is breaking out of the supply or return ducts (going through the duct and ceiling system into the room of concern). Some noise is radiating from the mechanical unit through the roof or mechanical room wall.
When the path of the noise is know, the noise mitigation treatment can be determined. For example, if the primary noise source is from duct-borne noise, effective treatments may include: internally lining the duct with fiberglass ductliner, adding a silencer, or adding additional duct work between the unit and the diffusers. If breakout noise is the primary source, effective treatments may include: lagging the duct with loaded vinyl or gypsum board, adding fiberglass and gypsum board above the ceiling (along with lined boots on the return grills), adding a silencer, or adding additional duct work on the roof or between the unit and when it enters the room of concern. If noise radiating from the unit to the room of concern is the primary noise source than improvements to the mechanical room wall or the ceiling may be needed. If the unit is located above the space of concern, it will be important to determine if the path is airborne or structural. If it is structural, than vibration isolation will be most effective. If the path is airborne, then improving the roof/ceiling path will be most effective (if the unit can not be relocated).