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Starting Voltage
The test for starting voltage is intended to determine the minimum voltage
required to start the fan reliably, unassisted. This is very important; a fan may be very
quiet below its starting voltage, but if it doesn't start reliably at that
level, it's not safe for use in a system. Starting voltage is the bare minimum
voltage that the fan can be used at — give it any less, and it quite
simply won't turn on. In fact, we recommend adding a few tenths of a volt
to our measurements just to be safe. Sample variance, bearing wear or damage,
dust, and even temperature can all affect the starting voltage, so it's worth
having a safety buffer in case of any mishaps.
The test itself is quite simple, since it's more or less just guess and test.
We find out what voltages the fan doesn't start at, what voltages it does,
and then start the test somewhere in the middle. We then hunt for the lowest
voltage where the fan will start reliably at least eight times in a row, adjusting
the starting voltage up and down in 0.1V increments until we find the starting
voltage.
Ambient temperature can affect starting voltage. Some fans need lower voltage to start when the ambient temperature is higher, and higher voltage when the temperature is lower. This means that if a computer is left turned off overnight in a room that gets cold, if the room is not warmed up in the morning before the computer is turned on, undervolted fans in that computer may not turn on reliably. The temperature during our tests is usually 20~25°C, which is pretty typical room ambient in temperate climates.
Rotation Speed
Rotation speed is measured in RPM (Revolutions per Minute). A laser-based
digital tachometer from Neiko is used to make the measurements. The tool is
accurate to single RPM differences, which is not hard to believe. RPM is perhaps
the simplest aspect of fan performance to measure; it's a simple matter of
counting the number of times a single blade (identified by a small square
of reflective adhesive) passes under the laser of the tachometer in a given
amount of time and multiplying by an appropriate factor. Unfortunately, as
mentioned above, rotation speed is also the least useful aspect of fan performance.
We include it only because it gives us some idea of blade efficiency. Better
blade designs should move more air at a given rotation speed.
This digital tachometer measures RPM accurately enough to detect single RPM
variances in fan speed.
Power Consumption
Power consumption is measured using a
home-made power meter that was originally built for measuring power consumption
in hard drives. Power consumption is hardly an important fan characteristic,
but we include it here for sake of comparison to the original specifications.
The amount of power consumed by most fans — and the amount of heat that
they generate — is almost always negligible, especially at the low speeds
that quiet computing requires.
Measuring the voltage drop across a 0.2 Ohm resistor gives us an easy way
to calculate power consumption.
Airflow
Airflow is reported in CFM (Cubic Feet per Minute), but the actual measurements
are somewhat complicated. CFM is used because it is the most commonly declared
fan specification, despite the existence of a more "scientific"
metric measure.
First, an inexpensive windmill anemometer is used to measure air speed.
This reports a value in LFM (Linear Feet per Minute) that approximates the
speed of the air coming out of the fan, but says nothing about the total volume
of air moved by the fan. To determine volume, the LFM value is multiplied
by the amount of open area in the fan, measured in square feet. A quick unit
analysis reveals that LFM (ft/m) multiplied by an area (ft²) yields the
correct unit: CFM (ft³/m).
Using the anemometer is no cakewalk, and its margin of error is unlikely
to be much better than ±10%. Subtle changes in measurement conditions,
such as the angle of the instrument or which side is placed against the fan
can affect measurements severely. In fact, it was the need to standardize
a measurement position for the anemometer that prompted the construction of
the fan harness. For our purposes, the anemometer is always held vertically
as illustrated below, and an orange dot affixed to one side of the anemometer
ensures that the air always blows in the same direction.
Even with these precautions, the position of the instrument relative to the
fan can also affect the measurements, so measurements are taken slowly enough
that time can be taken to find the peak air speed, which is then used for
subsequent calculations. Aside from the audio recordings, this is the most
time consuming part of the test.
Peak airspeed is carefully measured with a windmill anemometer.
Determining the open area of the fan requires a bit of basic geometry. The
area of the motor hub (a dead spot through which no air can travel) is subtracted
from the area of the open circle in the frame. The final formula gets quite
complicated, since all of the measurements have to be converted to feet.
The area of the motor hub is measured, not assumed, since its size can vary
quite a bit. A set of digital calipers is used to make an accurate measurement
to two decimal places.
The fan hub is measured using digital calipers.
[Editor's Note: This particular CFM measurement technique was adopted after several others were tried. The main reason for our choice is that the results are usually repeatable, and they are the closest to the CFM specifications provided for fans from manufacturers whose technical documentation appears reasonably trustworthy. All the other methods gave us considerably lower CFM numbers.]
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