# Engineering Acoustics/Noise from cooling fans

## Proposal

As electric/electronic devices get smaller and functional, the noise of
cooling device becomes important. This page will explain the origins of
noise generation from small axial cooling fans used in electronic goods
like desktop/laptop computers. The source of fan noises includes
aerodynamic noise as well as operating sound of the fan itself. This
page will be focused on the aerodynamic noise generation mechanisms.

## Introduction

Inside a desktop computer, there may be three (or more) fans. Usually
there is a fan on the heat sink of the CPU, in the rear of the power
supply unit, on the case ventilation hole, and maybe on the graphics
card, plus one on the motherboard chipset if it is a very recent one.
The noise from a computer that annoys people is mostly due to cooling
fans if the hard drive(s) is fairly quiet. When Intel Pentium processors
were first introduced, there was no need to have a fan on the CPU at
all, but most modern CPUs cannot function even for several seconds
without a cooling fan, and some CPU\'s (such as Intel\'s
Prescott core) have extreme cooling requirements,
which often causes more and more noise. The type of fan used in a
desktop computer is almost always an axial
fan "wikilink"), while centrifugal fans are commonly
used in laptop computers. Several fan types are shown here (pdf
format). Different
fan types have different characteristics of noise generation and
performance. The axial flow fan is the main type considered in this
page.

## Noise Generation Mechanisms

The figure below shows a typical noise spectrum of a 120 **mm** diameter
electronic device cooling fan. One microphone is used at the point 1
**m** far from the upstream side of the fan. The fan has 7 blades, 4
struts for motor mounting and operates at 13V. Certain amount of load is
applied. The blue plot is background noise of anechoic chamber, and the
green one is sound loudness spectrum when the fan is running.

```{=html}
<center>
```
![](Noisespectrum.gif "Noisespectrum.gif")

```{=html}
</center>
```
(\*BPF = Blade Passing Frequency) Each noise elements shown in this
figure is caused by one or more of following generation mechanisms.

### Blade Thickness Noise - Monopole (But very weak)

Blade thickness noise is generated by volume displacement of fluid. Fan
blades has its thickness and volume. As the rotor rotates, the volume of
each blade displaces fluid volume, then they consequently fluctuate
pressure of near field, and noise is generated. This noise is tonal at
the running frequency and generally very weak for cooling fans, because
their RPM is relatively low. Therefore, thickness of fan blades hardly
affects to electronic cooling fan noise.

(This kind of noise can become severe for high speed turbomachines like
helicopter rotor)

### Tonal Noise by Aerodynamic Forces - Dipole

#### Uniform Inlet Flow (Negligible)

The sound generation due to uniform and steady aerodynamic force has
very similar characteristic as the blade thickness noise. It is very
weak for low speed fans, and depends on fan RPM. Since at least of ideal
steady blade forces are necessary for a fan to do its duty, even in an
ideal condition, this kind of noise is impossible to be avoided. It is
known that this noise can be reduced by increasing the number of blades.

#### Non-uniform Inlet Flow

Non-uniform (still steady) inlet flow causes non-uniform aerodynamic
forces on blades as their angular positions change. This generates noise
at blade passing frequency and its harmonics. It is one of the major
noise sources of electronic cooling fans.

#### Rotor-Casing interaction

If the fan blades are very close to a structure which is not symmetric,
unsteady interaction forces to blades are generated. Then the fan
experiences a similar running condition as lying in non-uniform flow
field.

#### Impulsive Noise (Negligible)

This noise is caused by the interaction between a blade and
blade-tip-vortex of the preceding blade, and not severe for cooling
fans.

#### Rotating Stall

Click here "wikilink") to read the definition and an
aerodynamic description of **stall**.

The noise due to stall is a complex phenomenon that occurs at low flow
rates. For some reason, if flow is locally disturbed, it can cause stall
on one of the blades. As a result, the upstream passage on this blade is
partially blocked. Therefore, the mean flow is diverted away from this
passage. This causes increasing of the angle of attack on the closest
blade at the upstream side of the originally stalled blade, the flow is
again stalled there. On the other hand, the other side of the first
blade is un-stalled because of reduction of flow angle.

```{=html}
<center>
```
![](Stall.gif "Stall.gif")

```{=html}
</center>
```
repeatedly, the stall cell turns around the blades at about 30\~50% of
the running frequency, and the direction is opposite to the blades. This
seriese of phenomenon causes unstedy blade forces, and consequently
generates noise and vibrations.

#### Non-uniform Rotor Geometry

Asymmetry of rotor causes noise at the rotating frequency and its
harmonics (not blade passing frequency obviously), even when the inlet
flow is uniform and steady.

#### Unsteady Flow Field

Unsteady flow causes random forces on the blades. It spreads the
discrete spectrum noises and makes them continuous. In case of
low-frequency variation, the spread continuous spectral noise is around
rotating frequency, and narrowband noise is generated. The stochastic
velocity fluctuations of inlet flow generates broadband noise spectrum.
The generation of random noise components is covered by the following
sections.

### Random Noise by Unsteady Aerodynamic Forces

#### Turbulent Boundary Layer

Even in the steady and uniform inlet flow, there exist random force
fluctuations on the blades. That is from turbulent blade boundary layer.
Some noise is generated for this reason, but dominant noise is produced
by the boundary layer passing the blade trailing edge. The blade
trailing edges scatter the non-propagating near-field pressure into a
propagatable sound field.

#### Incident Turbulent

Velocity fluctuations of the intake flow with a stochastic time history
generate random forces on blades, and a broadband spectrum noise.

#### Vortex Shedding

For some reason, a vortex can separate from a blade. Then the
circulating flow around the blade starts to be changed. This causes
non-uniform forces on blades, and noises. A classical example for this
phenomenon is \'Karman vortex
street\'.
(some images and
animations.)
Vortex shedding mechanism can occur in a laminar boundary layer of low
speed fan and also in a turbulent boundary layer of high frequency fan.

#### Flow Separation

Flow separation causes stall explained above. This phenomenon can cause
random noise, which spreads all the discrete spectrum noises, and turns
the noise into broadband.

#### Tip Vortex

Since cooling fans are ducted axial flow machines, the annular gap
between the blade tips and the casing is important parameter for noise
generation. While rotating, there is another flow through the annular
gap due to pressure difference between upstream and downstream of fan.
Because of this flow, tip vortex is generated through the gap, and
broadband noise increases as the annular gap gets bigger.

## Installation Effects

Once a fan is installed, even though the fan is well designed
acoustically, unexpected noise problem can come up. It is called as
installation effects, and two types are applicable to cooling fans.

### Effect of Inlet Flow Conditions

A structure that affects the inlet flow of a fan causes installation
effects. For example Hoppe & Neise \[3\] showed that with and without a
bellmouth nozzle at the inlet flange of 500**mm** fan can change the
noise power by 50**dB** (This application is for much larger and noisier
fan though).

### Acoustic Loading Effect

This effect is shown on duct system applications. Some high performance
graphic cards apply duct system for direct exhaustion.

The sound power generated by a fan is not only a function of its
impeller speed and operating condition, but also depends on the acoustic
impedances of the duct systems connected to its inlet and outlet.
Therefore, fan and duct system should be matched not only for
aerodynamic noise reasons but also because of acoustic considerations.

## Closing Comment

Noise reduction of cooling fans has some restrictions:

1.  Active noise control is not economically effective. 80mm cooling
    fans are only 5\~10 US dollars. It is only applicable for high-end
    electronic products.
2.  Restricting certain aerodynamic phenomenon for noise reduction can
    cause serious performance reduction of the fan. Increasing RPM of
    the fan is of course much more dominant factor for noise.

Different stories of fan noise are introduced at some of the linked
sites below like active RPM control or noise camparison of various
bearings used in fans.

## Links to Interesting Sites about Fan Noise

Some practical issue of PC noise are presented at the following sites:

-   Cooling Fan Noise Comparison - Sleeve Bearing vs. Ball Bearing (pdf
    format)
-   Brief explanation of fan noise origins and noise reduction
    suggestions
-   Effect of sweep angle
    comparison
-   Comparisons of noise from various 80mm
    fans
-   Noise reduction of a specific desktop
    case
-   Noise reduction of another specific desktop
    case
-   Informal study for noise from CPU cooling
    fan
-   Informal study for noise from PC case
    fans
-   Active fan speed optimizators for minimum noise from desktop
    computers
-   Some general fan noise reduction
    technics
-   Various applications and training in - Brüel &
    Kjær

## References

\[1\] Neise, W., and Michel, U., \"Aerodynamic Noise of Turbomachines\"\
\[2\] Anderson, J., \"Fundamentals of Aerodynamics\", 3rd edition, 2001,
McGrawHill\
\[3\] Hoppe, G., and Neise, W., \"Vergleich verschiedener
Gerauschmessverfahren fur Ventilatoren. Forschungsbericht FLT 3/1/31/87,
Forschungsvereinigung fur Luft- und Trocknungstechnik e. V.,
Frankfurt/Main, Germany

back to **Engineering Acoustics**
