Strategy to reduce return noise

Figure 8. Strategy to reduce return noise
As air speed increases in duct systems, so does the noise level. Ducts are sized to maintain the
maximum velocity of air without adding noise to the room. The type of air outlet chosen and its
placement in the room will have an impact on the noise levels in the house. Air that leaves an air
outlet at a higher velocity than the outlet is intended to handle will create an undesirable
whistling or hissing noise. Improperly placed or selected air outlets can also create a draft in the
occupied zone, which is a perceived comfort issue. When selecting an air outlet, the
manufacturer’s performance data will list a noise criteria (NC) rating based on a very specific set
of testing data. The NC ratings are based on laboratory testing of the air outlets. It is important to
realize that the listed criteria are for only that specific test situation; however, the data provide a
comparative rating. The NC ratings for residential settings are NC 30 or lower. Table 1 shows
how the NC ratings apply to familiar settings