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Assessing Vibration Noise
Measuring vibration is a complex task. Since vibration is essentially motion,
a proper measurement of vibration must measure motion and acceleration along
each of the three spatial axes. Additionally, because vibration is oscillating
motion, frequencies must be specified to make the measurement
complete. Our lab is not equipped to make measurements of this
complexity, which require investments in expensive specilized test equipment that will have no other usefulness.
After much thought and experimentation, we realized that it is not necessary
to measure the vibration directly. Instead, we can gauge the effects
of vibration, the structure-borne acoustics. To this end, an
old aluminum electronics project box (43cm x 25cm x 10cm) was pressed into
Aluminum box, open side up on test bench. One of the PC systems used for testing is in the background.
Placing a powered drive on the sound box immediately makes its vibrations
audible. The box acts as a sounding board and resonating chamber for the HDD
vibrations. It exaggerates the vibration-induced noise to make it easier for
us to hear and evaluate. The air inside the box resonates along with the
thin aluminum panels at the primary spin frequency of the drive (as well as
its harmonics). We experimented with placing the HDD at different positions
on the board and found that the greatest noise was produced near one edge.
We applied duct tape on that spot to prevent short circuits and extraneous
mechanical noise, but the box is otherwise unadorned. The box actually has
only five sides; the sixth is a cover that we left off. The box is placed
with its open side down on a somewhat resonant test bench.
Musical drives: SPCR's highly resonant aluminum sound box, set up to amplify hard drive noise.
It is possible to measure the SPL of a HDD on the box. But this is a measure
of sound, not vibration. Our sound box is an artificial
means of producing sound from HDD vibrations; it does not produce noise that
is directly comparable to how the vibration will sound when the drive is used
in a real situation. Furthermore, any SPL measured in this way would include
the direct acoustic noise of the drive as well as the vibration noise. Thus,
we cannot equate drive vibration with the SPL of our test box.
So, how do we assess vibration? The only way we can: Subjectively, by listening carefully. We may
not be able to report vibration according to standard units of measurement,
but we have many HDDs on hand that we can compare. And, if we can tell which drives vibrate more than
others, we can put them on a scale. This is exactly what we did.
We listened to and compared over a dozen drives, and ranked them on
a 10-point scale. 10 represents no vibration at all; this will probably not
be reached with any drive that has moving parts. The ranks of 1 is
reserved in case a drive reaches new vibration high in future. All
the notebook drives we tested scored either 8 or 9; we expect most notebook
drives to be in this range. The lowest vibration 3.5" drive received
a 7, and it was barely audible when placed on the sound box.
In practical use, the audible difference in vibration noise between 8 and
10 should be minimal. Most of the time, the drive's airborne noise will
overpower its vibration noise at this level, but there will be exceptions if
the drive produces very little airborne noise or if it installed in
a particularly resonant case. Although our test setup amplifies the vibration
noise enough to discern between low vibration drives, differences in this
range are unlikely to be audible under ordinary circumstances. Most users
will be happy with anything above 7.
MP3s of vibration noise are recorded 7.5 cm (3") from the side of the sound box.
We also tried recording our vibration tests. We experimented with many different mic positions before settling
on 3" from the sound box, centered on the side farthest from
the HDD. At this position,
- the resonant vibration noise is most emphasized, and
- the direct acoustic noise of the drive is more reduced than in other positions, allowing the vibration noise to be heard more clearly.
Our intention was to make these vibration sound recording available for download, but after a couple of weeks of experimentation, we decided against this idea. The reasons are many and complex, but in a nutshell:
These HDD vibration recordings do not
represent the exact acoustic characteristics of the drives; the vibration induced noise is exaggerated and the acoustic noise deemphasized.
These recordings are meant to be compared
against each other. As explained previously these sounds will be
at the same frequencies: 70, 90, 120, 167 or 250 Hz depending on spindle
The problem is that these lower frequencies are not accurately reproduced by most PC audio playback systems. What you hear from these vibration noise recordings will be more seriously affected by the fidelity of your audio system than with the broadband noise recorded and posted at SPCR till now. We ran into trouble confirming our own vibration assessments (based on the live sound) when listening to the vibration recordings. This experience was enough to convince us that the vibration recordings are useless by themselves for those who cannot actually feel the HDD vibrations in their hands and listen to the sound that the HDD induces on the box.
But to satisfy your curiosity and perhaps entertain you a bit, here are a few HDD vibration box recording samples. Note the difference in tone (pitch, frequency) of the 5400 rpm (90 Hz) notebook drive compared to the 7200 rpm (120 Hz) 3.5" drives.
For many complex reasons discussed later in this article, we will not be taking temperature measurements of the drives. However, power consumption testing is done. The power drawn by a HDD essentially determines the amount of heat it must dissipate. The exact temperature seen at various points on a drive will depend on its mechanical design, the materials used in the drive, and other factors, especially how it is mounted in a case and where it is positioned. (That is a hint to why temp measurements are not being done.) However, total power draw of the drive is the starting point for any serious thermal considerations about a hard drive.
Standard 3.5" hard drives are powered from both the 12V and 5V lines. Standard notebook drives draw on only the 5V line. To measure the power drawn by a drive, a simple circuit interrupt was devised. It inserts 0.25 ohms of resistance in the +12V line and 0.2 ohms of resistance in the +5V line. The basic concept is the same as the one used in A $5 DIY Power Meter, but without the complications of AC current; this is DC, to which Ohm's laws can be simply applied.
HDD current measurement rig.
Current measurement rig at work.
Step 1: Determine the current for each voltage line. The voltage drops across the small resistances are neglible, well under 0.3V for the +12V line and under 0.2V for the + 5V line when a 7200 RPM 3.5" drive is in idle. The amount of voltage drop allows us to determine the current drawn on that line by using simple Ohm's law:
I (current) = V (voltage) ÷ R (resistance)
The current is calculated by dividing the voltage drop (measured across the resistance) with the resistance. This formula is used to determine the current draw on each of the two voltage lines.
Step 2: Calculate the power. Once the current is known, then we can apply another simple variant of Ohm's law:
P (power in Watts) = I (current) x V (voltage)
The current was determined in Step 1. The voltage here is the voltage seen at at the terminals of the HDD for each of the lines. Because of the voltage drop caused by the resistance, the voltage at the HDD is usually a bit lower than 12V or 5V, but for the sake of simplicity, we're just going to use 12V and 5V. Any error introduced here will be consistent for all the drives we compare, and it will be no more than 0.25W in the worst case.
The original plan was to measure power draw at startup, idle and seek/write. The latter two tests were retained for the final test methodology, but we gave up trying to measure power draw at startup because it is simply too dynamic and dependent on the response time of our multimeters for the results to be reliable. For what it is worth, most 7200 RPM 3.5" drives showed a spin-up peak power draw of ~20W. None were significantly lower; a couple were slightly higher. This figure has little bearing for heat / noise in normal PC operation and is only really useful if you are trying to perfectly match the power needs of the system with the power supply.
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