Anatomy of the Silent Fan

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FAN BLADE DESIGN

Looking into propeller, impeller, or fan blade design can sink you quickly into aerodynamics, physics and other disciplines that are arcane and mind-boggling for the average person, certainly for this writer. The topic of spinning axial blades has received intense, prolonged study for over a century, in aviation (propellers for airplanes and helicopters, turbines in jet engines, etc.) and in marine-related applications — just think of the money and intellectual resources lavished on the development of silent propellers for nuclear submarines alone. Not having as much military value, it's not likely that blade design for axial fans has received quite as much attention, but the technology does go back over 120 years.

Elsewhere, we've mentioned the difficulty of trying to correlate aspects of blade design to practical results. That comment bears repetition. The best we can really do is to try and identify the various elements of blade design. Changes in any one of these elements can have an impact on airflow / noise performance, and there are endless variations.

Much thanks go to long time SPCR supporter Neil Blanchard, for creating the following illustrations. Here, Neil compares the variations in the design of two relatively quiet fans.

Neil Blanchard's Blade Design Observations


Intake side

On the intake side, the blades on both fans are swept forward, but to different degrees; the NMB's sweep further forward. The blade area on the NMB is greater, and the pitch (angle of attack) of the leading edge is lower. The Nexus has a larger radius on the leading tip, and the edges of the blades are more blunt. The edge of the hub on the Nexus has a larger radius.

On the exhaust side, the frames are quite different; the struts on the NMB are rounder, but they are nearly parallel to the trailing edges. The struts on the Nexus, on the other hand, intersect with the tailing edges only at one point, which would seem to reduce air turbulence. Also, the strut with the wires is quite wide on the NMB, which probably creates a larger "shadow" in the air flow.


Exhaust side

 

An article by Comair-Rotron on Fan Acoustic Noise identifies several primary causes of noise in fans, including some related directly to the blades:

"Vortex Shedding Broad band noise generated by air separation from the blade surface and trailing edge. It can be controlled somewhat by good blade profile design, proper pitch angle and notched or serrated trailing blade edges.

"Turbulence Turbulence is created in the airflow stream itself. It contributes to broad band noise. Inlet and Outlet disturbances, sharp edges and bends will cause increased turbulence and noise.

"Speed The effect of rotational speed on noise can best be seen through one of the fan laws:

  • dB1 = dB2 + 50 log10 (RPM1 / RPM2)

Speed is an obvious major contributor to fan noise. For instance, if the speed of a fan is reduced by 20%, the dB level will be reduced by 5 dB."

OTHER FACTORS

  • Acoustics in general are affected by all mechanical / structural aspects, and this holds true for fans. The strength of the frame which houses the fan, the strength and precision of all the parts, the internal damping characteristics of the materials from which the parts are formed — all of these factors have an impact on the acoustics of the fan. Suffice it to say that very high precision in parts, good dynamic balance of the rotor and fins, high strength and rigidity in the mechanical structures, and very good damping of all internal resonances are important aspects of good fan acoustics.
  • Commutator Switching Frequency Noise is described by JMC...

    "The stator motion is a square wave that is switched on and off before and after the peak torque position. This motion causes a small amount of undulation in motor torque, producing an audible noise caused by the lower frequency commutation operation. Each small torque causes a minute contracting of the entire fan structure and results in an audible clicking noise while the fan is operating."

This noise is most often heard as a rapid clicking or buzzing. JMC's solution to commutator switching noise is to put the frequency up to 25KHz, where it is inaudible to human beings. This is the main feature of their PWM fans.

  • PWM speed control and commutator switching and work in very similar ways, and they cause similar noise. Pulse-width modulation (PWM) circuits in fan speed controllers were first marketed to PC enthusiasts a few years ago. PWM switches the power to the fan on and off rapidly, which results in a series of pulses. When the frequency of these pulses is fast enough, the fan spins steadily because of its momentum. There are a number of advantages of PWM over linear voltage control, especially for higher power fans. However, the downside is a potential increase in clicking noise, very similar to commutator switching noise.

    Application Note 58 from TelCom Semiconductor explains the issue of Suppressing Acoustic Noise in PWM Fan Speed Control Systems (pdf). (Much thanks to cpemma for finding this document!)

    "Stator excitation is a square wave that is switched ON 45° before peak torque position, and switched OFF 45° after peak torque position. This excitation causes a small amount of ripple in motor torque at the frequency of commutation. Each small torque "burst" causes a minute flexing of the entire fan structure, and results in a faint (but audible) "ticking" noise while the BDC fan is operating (Figure 2A). Acoustic PWM noise is generated in exactly the same way. When the PWM pulse turns on, a step change in torque occurs within the fan, the profile of which matches the rise time of the PWM pulse (Figure 2B). This impulse torque is articulated by the fan structure as audible noise. This is true mostly in larger fans (i.e., fans with operating currents in excess of 300 mA), since they generate a greater amount of torque and have larger size and mass. This effect is more pronounced at low operating speeds (i.e., low PWM duty cycle): the lower the PWM duty cycle, the greater the percentage of time the fan is OFF (quiet), and the more noticeable the acoustic noise caused by the PWM becomes."

    TelCom Semiconductor's solution to PWM induced fan noise is to "slow the slew rate of the PWM drive signal to the fan". In essence, the edges of the square wave pulses are rounded, and the voltage spike reduced. As a result, the acoustic click or spike is dramatically reduced.

  • A larger fan can move more air at the same noise level than a smaller fan. Alternatively, it can be quieter moving the same amount air as a smaller fan. This is assuming that all aspects of the fans except for diameter or depth are identical. It is an outcome that's predicted by the fan laws, and also confirmed experimentally with SPCR's own hands-on, ears-open testing. Larger blades don't have to spin as quickly to move the same amount of air.

    For example, a typical 120x25mm fan spinning at 1,000 rpm can move ~40 cfm. The best quiet 120x25mm fans can do this at <22 dBA@1m. In comparison, an 80x25mm fan has to spin at some 3,000 rpm to achieve the same airflow, and its noise will be a minimum of 30 dBA@1m. Subjectively, the higher pitched tonal aspects of the 80mm fan sound at this speed will make it seem more than twice as loud. In order reach ~22 dBA@1m level, the 80x25mm fan speed has to be reduced to ~1,500 rpm, at which point, the airflow drops to about half of the 120mm fan at 1,000 rpm.

    The broad transition in CPU heatsinks from fans under 80mm size around the year 2000 to fans as large as 120mm by 2005 was driven mostly by the need for greater cooling capability without further escalating noise, which had reached absurdly high levels in "performance" PCs. 120mm (and now larger) fans also appeared as PC case ventilation fans during that period. These developments have helped to lay the foundations for a much broader realization of quiet computers.

EXTERNAL FACTORS

All the factors discussed thus far have been internal to the fan itself. There are external factors that also impinge on fan acoustics. The major ones are noted here:

Fan Load (Electrical) Noise varies as the system load varies. This variation is unpredictable and fan dependent.

Impedance refers to vent openings that may not be large enough to allow 100% of the fan's airflow, restrictive dust or protection grills, or very densely packed heatsink fins that represent high resistance to airflow. Fans make more noise with greater impedance.

Altitude could also be said to be a part of the load. When the air is thinner, airflow is reduced for a given RPM.

Vibration-induced noise may originate from the fan, but can be exacerbated and amplified by resonant panels to which the fan is mounted. Mechanical isolation or decoupled mounting techniques are often required for the quietest fan operation.

FINAL NOTES

It is difficult to sum up all of the various complexities around DC axial fan design and implementation into an intelligent paragraph or two; nonetheless, it's a task expected of the author of an article such as this. Keeping it simple to avoid revealing too many of the holes in my grasp of the topic, I would suggest that the Ideal Silent Fan has the following characteristics:

  • Advanced bearing that allows long life and mounting in any position while remaining extremely quiet, even while spinning as fast as 1,500~2,000 rpm, with a benign acoustic signature that scales smoothly as speed is reduced.
  • Aerodynamic structural design that maximizes laminar airflow and keeps vortex shedding and turbulence to a minimum.
  • High precision parts assembled with perfect dynamic balance from non-resonant materials to ensure minimal vibration.
  • Ultrasonic commutator switching speed, very low start voltage (say 3.5V), built-in silicone vibration decoupling, built-in switchable thermal / manual speed control, availability in sizes from 60x15mm to 140x38mm.
  • And as long as we're asking for the sky, priced at <$10 for the smaller sizes and no higher than $30 even for the biggest.

Any fan makers who have products that come even close, please contact me asap. ;)

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Reference Sources

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POSTSCRIPT overleaf, Nov. 13, 2006:
Dorothy Bradbury on "What determines the rated speed of a fan?"

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Discuss this article in the SPCR Forums.



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