| Sound Generation and NPEL | ||
| 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 specifics of fan installation. 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 generation 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. |
| Sound Generation and NPEL |  Forced Air Cooling • Air Performance & Package Resistance |
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The standard technique is to sum A-weighted measurements of the energy radiated in all directions. The result is the standardized sound power rating (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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