Balancing Thermal Performance and Noise in Cooling Systems
A common request from customers has been to reduce system noise while keeping the same amount of thermal cooling performance. One method to reduce fan noise starts with fan selection. It’s important to select a fan that can meet the system’s thermal cooling requirements while balancing the sound produced from higher operating conditions. In a system that has a bit of design flexibility for the size and spacing of the fan, another good option worth exploring is using a larger or thicker higher performance fan and operating this fan at a lower duty.
Comparing 40x40x20mm vs. 40x40x28mm Fans Using PWM Control
In this article, a system using parallel 40x40x20mm fans will be compared to the same system with parallel 40x40x28mm fans. Pulse width modulation (PWM) allows for precise control of a fan’s operation, which would enable the 40x40x28mm higher performance fan to operate a lower PWM duty (68% PWM duty) to match the full operation of the thinner 40x40x20mm fan (100% PWM duty).
Understanding the Three Types of Fan Noise
Fan noise for a typical cooling fan can be categorized into three types of noise: fluid noise, electrical noise, and mechanical noise. Fluid noise would include the movement of the air itself through the fan’s frames and blades. Turbulent air can create even higher levels of fluid noise, as would moving air through a narrow space (e.g., smaller diameter fans versus larger diameter fans). The next category of noise is electrical noise, which includes motor switching and electronics circuit switching. The final category of noise is mechanical noise, such as bearing noise and mechanical vibration of the fan and frame.
Why Larger and Thicker Fans Reduce System Noise
Larger and thicker fans generally have a better ratio of air that’s pushed to the sound produced, leading to reduced fluid noise. At the same operating point, a higher performance larger fan can operate at lower rotation speeds, thus lowering the switching speed of the rotors, lower rotations per minute (RPM) of the blades themselves, and this leads to lower production of sound in the fluid noise category. Using a larger or thicker fan can also have better motor efficiency, which contributes to reducing electrical noise. The mechanical noise is also reduced from the lower rotation of the blades and rotor.
System Test: Evaluating SPL in Two Fan Configurations
With the background of the category of fan noises in mind, next is the actual system test comparing two 40x40mm fans of two different thicknesses.
Two 40x40x20mm 9GA0424P6H001 fans met the system requirements for thermal cooling, and the maximum PWM duty was full operation at 100%. For a single unit of the original 9GA 40x20mm fan at 100% PWM duty at the operating point of 8.4 CFM and 0.23 inH2O static pressure, the sound pressure level (SPL) was 41 dBA. Using the composite SPL equation shown below, the noise of adding another unit of 9GA 40x20mm fan at 100% PWM duty would be equal to 44 dBA, which matched up to the measured results.
Ln : Composite SPL of system
“n” pcs Fan running
Sound of parallel 9GA0424P6H001 fans, L_2 = 41+10 log(2) = 41+3 = 44 dBA.
Results: SPL Reduction with Thicker Fans at Lower PWM
Sanyo Denki engineers evaluated the thicker, 40x40x28mm 9GA0424P3M001 fan to find that 68% PWM duty had equivalent performance as the 9GA 40x20mm fan at 100% PWM duty.
The noise produced by a single unit of the 9GA 40x28mm fan at 68% PWM duty, 8.4 CFM, and 0.23 inH2O static pressure was 37.6 dBA. The noise produced by two 9GA 40x28mm fans at 68% PWM duty at the operating point of 16.8 CFM and 0.23 inH2O static pressure was 40.6 dBA.
Conclusion: 3.4 dBA Reduced System Noise Achieved
In conclusion, using the thicker 40x40x28mm fan at a lower PWM duty was able to reduce system sound pressure levels by 3.4 dBA. The 3.4 dBA reduction eclipses the added fan noise from a parallel, two-fan system as per the composite SPL relationship.
| Fan Setup | SPL [dBA] with Operation at 16.8 CFM and 0.23inH2O System Pressure |
|---|---|
|
2 units of 9GA0424P6H001 @ 100% |
44 |
|
2 units of 9GA0424P3M001 @ 68% |
40.6 |
Written by Jerry Wu
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