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VIBRATION FROM HARD DRIVES
Vibration-induced Case Noise cannot be eliminated with heavy panels and solid construction alone. Hard drives are normally tightly coupled to the case with steel screws. The vibration of a hard drive occurs at the primary frequency determined by spin rate, as shown in the table below, and harmonics (multiples) of the primary frequency.
Frequency of HDD Vibrations
140, 210, 280...
180, 270, 360...
196, 294, 393...
240, 360, 480...
333, 500, 667...
500, 750, 1K...
Such vibrations can cause the entire case to vibrate you can feel it
when touching any part of a normal PC case. They also cause low frequency acoustic
noise the humming, thrumming and growling
types of sounds that are lower in level than typical fan noise but there contributing
to the overall noise. The harmonics can cause noise in the mid-band where human
hearing is most sensitive. Buzzing and whining are
apt descriptions of the kinds of noise HDD harmonics can cause.
The main solution against vibration-induced panel noise is to stop the vibrations
from getting to the panels in the first place. An acoustically inert and mechancially
solid case can help to keep such vibration from turning into a major source
of noise, but cannot eliminate it completely. The best solution is to use low
vibration components, and to use effective mechanical decoupling
of the noise making components. This is best done by using soft mounting techniques
for fans and hard drives. Rubber bushings and grommets that insulate the fan
or hard drive from the chassis can be used, as well as various forms of elastic
The article Hard Drive Silencing: Sandwiches
& Suspensions covers one example of elastically decoupled mounting
for HDDs. There are many more in the storage section, and in the forums: HDD
vibration & noise reducing methods - ranked and HDD
Suspension... Show your pics!
Many cases offer rubber grommets but some have been only marginally effective,
as the rubber used is often much too hard, and the amount of decoupling achieved
is minimal. True elastic suspension for hard drives can be found on the Antec
P150 / Solo and Solo II.
With low vibration HDD, vibration-induced noise from the HDD can be completely
eliminated in this case. Some cases also offer good soft rubber grommets for
decoupling hard drives.
Since SPCR began examining cases and HDDs back in 2002, there has been much
evolution in both. HDDs are generally far quieter now than they used to be,
and they tend to exhibit a lot less vibration. Still, as our reviews show, typical
7200rpm 3.5" drives remain major sources of noise and vibration. New generations
of 5400rpm and 5900rpm drives, especially for NAS applications, are slightly
slower but notably quieter than 7200rpm drives, and usually have far lower vibration.
High areal density, found in the latest high capacity drives, has also closed
the performance gap between the lower spinning drives and the 7200rpm models.
Solid State Drives are the obvious answer to eliminating HDD noise entirely,
and they virtually elminate heat altogether. They are faster than the fastest
HDDs, and arguably more reliable. They are so light, cool and silent that they
can be mounted almost anywhere in a case, even with a single screw if necessary.
A good balance of price, performance and low noise can be had by combining a
lower capacity high performance SSD for the operating system, and a low-vibration,
extremely quiet, sub-7200rpm HDD of high capacity.
A little understood aspect of PC noise is air resonance. The
air in a closed (or mostly closed) space exhibits resonances
centered at certain frequency points. This is true of concert halls as well
as computer cases. Any noise that falls close to these resonance points become
accentuated and amplified. This is not panel vibration, but vibration of the
air in the enclosure. A good explanation of resonance concepts is available
at the Sound and Hearing section of the HyperPhysics web site.
Air resonances occur regardless of the construction materials used. Using non-parallel
panels could help by reducing standing waves, but this is an impractical solution
for a PC case. The common effect of air resonance in a typical mid-tower computer
is a <250 Hz boom or hum accentuated by hard drives and fans. It is usually
low in level, often not noticed until the components have been quieted or when
very low noise components are used; it becomes audible only when your system
is approaching true silence (below ~15 dBA@1m): It's a high class problem. One
solution to air resonance in the case/system is to remove the main side panel;
if the space is not enclosed, there can be no air resonance.
The application of acoustically absorbent materials on the inner
surfaces of the case may help reduce air resonance effects. Such damping materials
can reduce standing waves, and reduce the level of higher frequency noises.
(But don't look to such damping materials to block noise from
getting out: Below ~200Hz, very little can be done to limit sound transmission
from a PC case; there's simply not enough room for the mass, density and thickness
of walls needed to block the lower frequency noise.)
Some acoustic damping materials were developed especially for use in computers.
We've done reviews (years ago!) of some of these materials here in the Cases
and Damping Section. In general, damping materials are most useful with
computers that have already been optimized for low noise, including soft mounting
of all major noise making components. Good acoustic damping can reduce the noise
by a few decibels, and more at higher frequencies. But the need for airflow
leaves at least a couple of open fan holes through which fan and HDD noise always
pass. The best approach is to reduce the noise of those components to a minimum,
and then consider whether acoustic damping might help further.
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