These tips for reducing fan noise work

Latest update time：2023-06-01 15:28

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Q: Common sources of fan noise

Fan noise is the noise produced by the fan when it is working. To solve these noises, it is recommended to first understand some common sources of fan noise.

If you would like fan selection and application guidelines, you can find them here .

mechanical noise

Mechanical vibration has many causes, including bearing wear, mechanical imbalance in moving parts, and torque fluctuations; the mechanical force exerted on the motor shaft is not completely constant throughout the mechanical rotation. Although mechanical vibration can be reduced through optimization of manufacturing and design, mechanical vibration is still not a question of "if it exists," but a question of "how serious it is." The physical connection of the fan to some large system creates a mechanical path through which fan vibration can propagate and find a more efficient way to become acoustic noise; most items in a mechanical system can move or vibrate at the approximate frequency at which the fan generates noise. , will do this, acting as a mechanical amplifier in the process. In particular, it is the result of some kind of mechanical impact, such as vibration of the sheet metal housing walls against the internal supports. In addition to securing various parts of the system against vibration, it is recommended that elastomeric (rubber) materials be used when mounting hardware to dampen vibration transmission from the fan to the system and reduce mechanical noise for specific fan/system combinations.

aerodynamic noise

Relationship with fan parameters

The noise caused by air movement is largely the result of turbulence - instead of flowing smoothly along a surface or through some kind of restriction, airflow becomes disordered and irregular.

Fluid velocity is one of the key factors in turbulence; generally speaking, faster airflow means greater turbulence and greater noise. Figure 1-3 below shows the manufacturer's published Digi-Key fan noise figure at the time of this writing as a function of nominal speed, nominal flow rate (rated air volume divided by frame area), and maximum static pressure. A clear breakdown of noise levels can be made as a function of rated speed and frame size; generally speaking, larger, faster fans produce more noise than smaller or slower fans. However, as a function of the rated fluid flow rate through the fan aperture or the maximum deliverable static pressure, the noise level appears to become a general logarithmic function of both, with the effect of frame size becoming less pronounced.

Figure 1. Nominal noise level versus rated speed for tube-axis fans. Note the clear banding by fan size in the image.

Figure 2. Noise versus nominal flow rate through the fan. Note the lack of significant differences between different fan sizes.

Figure 3. Nominal noise rating versus maximum static pressure. Although less obvious, some grouping by fan size does occur.

Intake/exhaust port

While fans themselves are obviously potential sources of flow-related noise, in electronic systems fans often move air in an enclosed space, so all the air they move must pass through the exhaust (or intake) vents at the other end of the system. Increasing the size of this vent reduces the velocity of air passing through it, the amount of flow required to achieve a specific volume over a period of time, and the amount of static pressure required to drive that flow. It is recommended to use vents with an area at least 1.5 times the size of the fan frame to reduce the noise and static pressure drop caused by this source and the power consumed by moving air through the system.

Figure 4. Large exhaust / intake ports (left) enable low flow rates across the port, low pressure drop, and quiet flow, while small ports (right) result in large pressure drops and fast, noisy air flow.