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Forced Air Cooling

Air Performance

Sound Generation

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Values of noise power emission level (NPEL, in bels) and sound pressure (A-weighted decibels) shown in product listings are for fan operation at normal speed against zero back pressure. Sound generation against greater static pressure levels will differ from these values.

Sound generation is greatly influenced by the specifics of fan installation. Physical features of a package that are close to a fan, especially those on the air inlet side of the fan, can have a significant impact on sound levels by introducing non-uniform air flow patterns.

Air delivery and sound generation for fans are subjects of standards for laboratory test methods. The sound test standard is defined in a document called ISO/DIS 10302 or ANSI S12.11, the American National Standard for the Measurement of Noise Emitted by Small Air-Moving Devices. <more>

Sound is almost universally quantified in terms of sound pressure level, the magnitude (in dBA) of pressure fluctuation at some point in space, commonly measured one meter directly in front of the fan. Since fans do not radiate sound uniformly in all directions, a different pressure level may simultaneously exist at another place.

U.S. and international standards call for characterization of sound output as total acoustic energy radiated by a fan.The standard technique is to sum A-weighted measurements of the energy radiated in all directions. The result is a standard sound power rating - the Noise Power Emission Level.

The table below illustrates typical differences in evaluation of fan sound as outcomes of different measurements. The fans radiate total acoustic energy at the same amplitude:

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Forced Air Cooling • Air Performance & Package Resistance