Case Basics & Recommendations

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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
RPM
Primary (Hz)
Harmonics (Hz)
4,200
70
140, 210, 280...
5,400
90
180, 270, 360...
5,900
98
196, 294, 393...
7,200
120
240, 360, 480...
10,000
167
333, 500, 667...
15,000
250
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 "string".

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.


AIR RESONANCE

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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