This is a subsidiary page to the Fan Selection & Application Guide, discussing matters relating to fan noise; where it comes from, how it can be dealt with, and general correlations with other device parameters.
Mechanical vibrations have numerous origins, among them worn bearings, mechanically-imbalanced moving parts, and torque ripple; the amount of mechanical force exerted on the motor shaft is not perfectly constant throughout a mechanical rotation. Though improvements in manufacturing and design can reduce it, mechanical vibration remains a matter not of ‘if’ but rather ‘how much.” The physical attachment of a fan to some larger system creates a mechanical path through which a fan’s vibrations can travel in search of a more effective way of becoming acoustic noise; most anything in the mechanical system that can move or vibrate at frequencies near those produced by a fan will tend to do so, and act as mechanical amplifiers in the process. Especially if some sort of mechanical impact results, for example a sheet metal enclosure wall vibrating against an internal support. Aside from securing the various parts of the system against vibration, using elastomeric (rubbery) materials in the mounting hardware to dampen transmission of vibrations from fan to system is suggested as a means of reducing noise of a mechanical origin for a given fan/system combination.
Relation to fan parameters
Noise due to air movement is largely a result of turbulence–when instead of flowing smoothly along a surface or through some restriction, the flow becomes disordered and irregular.
Fluid velocity is one of the key ingredients for turbulence; in general, faster airflow means more turbulence and thus more noise. Figures 1-3 below show manufacturers’ published noise figures for fans carried by Digi-Key at the time of writing, as a function of nominal rotational speed, nominal flow speed (rated flow divided by frame area) and maximum static pressure. Clear delineations in noise ratings can be seen as a function of rated speed and frame size; larger, faster fans make more noise than smaller or slower ones, as a general rule. Viewed as a function of nominal fluid flow rate through a fan’s aperture or maximum deliverable static pressure however, noise levels appear to become a generally logarithmic function of either with the influence of frame size growing much less distinct.
Figure 1. Nominal noise levels vs. rated speed for tubeaxial fans. Note the distinct banding in the plot by fan size.
Figure 2. Noise vs. nominal flow speed through the fan. Note the lack of significant distinction based on fan size.
Figure 3. Nominal noise ratings vs. maximum static pressure. Though not particularly distinct, some grouping by fan size does appear.
From intake/exhaust ports
While a fan itself is clearly a potential source of flow-related noise, in electronic systems they’re often moving air through an enclosed space, thus all the air that they’re moving must pass through an exhaust (or intake) vent at the other end of the system. Increasing the size of this vent reduces the speed at which air must move through it to allow a given volume of flow in a period of time, and also the amount of static pressure required to drive that flow. Use of a vent having an area at least 1.5 times the fan frame size is recommended to reduce noise and static pressure drops occurring from this source, as well as the amount of electrical energy expended in moving air through the system.
Figure 4. A large exhaust/intake port (L) allows for low flow speeds through the port, low pressure drops across it, and quiet flow, whereas a small port (R) results in large pressure drops and fast, noisy air flows.
Some sort of barrier to protect fingers and such from a rotating fan (or vice-versa) is a common necessity, and often integrated with some type of filtering. Whether implemented as a discrete component or fabricated directly into an enclosure wall, their contribution to noise and static pressure drops within a system can be significant, and shouldn’t be overlooked.
As might be expected, the proportion of a fan’s aperture that is obstructed by a grate or guard is a major factor in the amount of excess noise and pressure drop it causes. It is not the only factor however; the cross-sectional shape of a guard’s structural members, mounting distance from a fan, and geometry of its openings are all influential. Predicting the acoustic behavior of a given guard is a complex topic, but a few concepts go a long way towards developing an intuitive sense in the matter.
First, corners and sharp edges are more likely than a rounded surface to induce turbulence in a stream of air flowing across them, and turbulence translates into noise. For this reason, a guard of a bent-wire sort is likely to be less noisy than one stamped out of sheet metal & having otherwise equivalent geometry.
Second, a bad fan guard has a lot in common with a good siren. The basic concept behind a mechanical siren is to use what’s essentially a fan to pressurize air and release it in interrupted fashion by enclosing the fan in a shroud with openings that are blocked or opened by the passage of the fan blades. To avoid stumbling onto this effect:
- Mount guards at a distance from the fan
- Beware of geometries that place obstructions parallel with the nearer blade edges
- Beware of geometries that allow multiple fan blades to pass behind an occlusion simultaneously
Figure 5. A deliberately-noisy fan guard, created by doing the opposite of the concepts above. Note the change as the distance between guard and fan is increased.
The purpose of a fan filter is to allow the passage of air while restricting the passage of other things, particularly airborne dust, lint, and other detritus that reduces the effectiveness of forced-air cooling by forming a thermally-insulting buildup on surfaces to be cooled. A filter placed at the air inlet to a system can capture a portion of such materials, avoiding their deposition deeper inside a system where removal is less convenient and potential for other problems (short circuits, etc.) is greater.
Fan filters can also serve to reduce acoustic noise, in a few ways. The static pressure drop that occurs across them reduces flow rate and consequently flow speed through a given system, reducing flow-related noise generation throughout. They can also behave like a flow diffuser, breaking up larger flow streams (e.g. those through each opening in a guard/grill) into a number of smaller ones whose acoustic effects are more likely to cancel each other out or be found less objectionable. To a limited extent, filter media can also function as a sound dampening curtain, reducing noise transmission from one side to the other.
Whether filtering dust or noise, the porosity characteristics of a filter have a strong influence on effectiveness and performance over time. Filter elements with larger pores pose less opposition to airflow, allow more undesirable stuff to pass, and become clogged less quickly. Regardless of filter selection, maintenance is important to achieving best system performance and service life. It’s also a nuisance… Where feasible, use of temperature monitoring and speed-controlled fans to reduce airflow to the minimum required for effect minimizes excess filter loading, extends the time between required filter services and enables compensation for filter loading and system health monitoring.
Finally, fan filters are also useful for restricting passage of radiated signals. Filter elements made of a conductive metal mesh are an effective means of keeping high-frequency radio signals on their assigned side of a system’s enclosure, when a hole required for airflow/fan mounting would otherwise look like an open door or (worse) an antenna for such. Note that close electrical contact with a device’s chassis is required for maximum effectiveness in such use.
Figure 6. (L-R): 09325-F/30, 09325-F/45, LFG80, and 06325-SS, an assortment of filters for 80mm tubeaxial fans. Note the varying degrees of porosity in the filter element.