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Sound |
| 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 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 is almost universally quantified in terms of sound pressure level, the magnitude (in dBA) of pressure fluctuation at some point in space, commonly 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. more... |
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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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