Air moving through fans, diffusers, and registers creates noise in a space. In many instances, this isn’t a problem, while in others it can even be a benefit as background or “white” noise. Different sound levels can be tolerated in various types of occupancies.
The main noise source in a space is air diffusers, including both supply- and return-air grilles. The manufacturer’s literature provides dependable information on sound levels versus airflow rates through their grilles. There’s a trade-off between performance and sound level, because a higher airflow rate increases the throw of air into a space but is noisier than lesser flow rates. Consequently, low-noise applications will have more air-distribution sites in a space, with lesser quantities of air emitted at each diffuser. These factors are indicated in the function
ft3/min = V * A * K
V = outlet velocity, ft/min
A = neck area, in2
K = diffuser delivery factor
An undersized diffuser creates not only a high noise level but also a high velocity. When the rate of air movement exceeds 25 ft/min, the draft is uncomfortable. On the other hand, an oversized diffuser may be quiet, but it results in poor mixing too, which can cause vertical stratification and areas of horizontal stagnation. Widely spaced diffusers and the use of multiple return-air inlets will minimize such effects.
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Excerpt from Energy Conservation Projects.