The first step to building a quiet PC is to buy a quiet hard drive. Most drive manufacturers give detailed specs on their drives, including noise level, either in sound pressure bels or decibels. For the Americans in the crowd, 1 bel=10 decibels. Some manufacturers (e.g. Western Digital) either lie blatantly about their specs or use significantly different test conditions. For instance, my moderately quiet IBM 37GP claims 34dB, while the Fujitsu MPE3173AE claims 35dB. In reality, while the IBM is moderately quiet, the Fujitsu is much, much quieter than the IBM (the Fujitsu is amazingly quiet, and is the quietest drive I've seen). One can depend on sites like Storage Review and Tom's Hardware to get the low-down. I've lost track of which drives are the quietest nowadays, because virtually all the drive manufacturers have announced new low-noise initiatives and technologies recently, but historically, Fujitsu and Samsung were the best, and IBM was pretty decent.
One thing to look out for is that most hard drives get much louder after a few years (sometimes even a few months) use, as they become decalibrated.
Low-power (less than 5 watt) drives can be placed in a special enclosure called the SilentDrive that further reduces noise level. Why the power requirement? The enclosure traps heat in. Since every watt of power used changes into a watt of heat, high power drives would burn up in an enclosure like this. Companies give specs on power usage, but again, since this depends on testing methodologies, it's good to also check an independent reviewer's web site for how hot the drive runs.
I use the aforementioned enclosure, and it does reduce noise very significantly. Molex claims this will work for most 5400RPM drives, but from my experience, it will not. During a burn-in test, my 2 platter 40GB 5400RPM IBM reached 57C after a little over an hour of continuous seeking, and would have overheated had I not cut the test (max temp is 60C). The room was at a low ambient temperature. When I switched the drive to "quiet mode," and reran the test, the temperature started to drop pretty quickly, and it was clear it would operate fine.
If you do use this enclosure, Glenn Garrett at QuietPC can sell you one packaged with stickers that can monitor your drive temperature (he gets those from rswww.com, which didn't ship to the US when I last checked; http://www.tempil.com/ looks to offer similar parts). Very useful devices. I had already bought the enclosure elsewhere, but I managed to get him to sell me a couple separately. He seems like a good guy.
If you have an IBM, also get IBM's software to reconfigure it to run slower but quieter. It does not reduce spin noise, but just seeking noise. I also noticed that when I put it in quiet mode, it's peak power dissipation drops a lot, and I can run it in an acoustic enclosure. In normal mode, it would border on overheating. Apparently, some software for is also available for the Seagate Baraccuda
Several other people have made enclosures:
I recommend just getting a quiet drive that runs cool enough to sit in the Molex SilentDrive. However, if you really want to keep a that 15,000 RPM noise monster cool but still not too noisy, I'd build a muffler case with forced air cooling.
Pretty universally, 5400 RPM drives tend to be much quieter and use a good deal less power than 7200 RPM ones. Several people have suggested looking to notebook hard drives; although significantly more expensive, these use very little power and some of them are extremely quiet (they can plug into a normal motherboard with a cheap adapter cable). The seriously wealthy can buy a completely silent Solid State Drive (though look out for ones with fans). Even a small SSD can allow your normal hard drive to be spun down almost all of the time if you place your system and other commonly used files there.
There are several utilities that can spin down your hard drive after some time of inactivity. Under Linux, this can be done with hdparm -S 180 /dev/hda where 180 is the time after which to spin down in funky units. Macintosh also has utilities to do this, as does DOS and Windows.
I believe system drive usage is currently much higher than it needs to be, and could be reduced by close to an order of magnitude with a couple system software changes. If there are any Linux kernel hackers out there, skim my proposal, and tell me what you think. It'd safe power on notebooks and make server drives more reliable as well.
One person designed a system for Linux that does part of this. I'd be a bit worried about data reliability to use it, since it caches all writes.
When spinning down drives, please be aware of reliability issues. I contacted IBM to find out how rapid sleep/wake cycles would effect reliability:
Next you ask about start/stop cycles. They are indeed a factor in the service life of a drive. The measurements are generally: power on hours; start/stop cycles; percent accessed. All of these are a factor but one is not more significant than the other.
(from Kathy at IBM's amazingly competent tech support)
This matches my limited experience; drives with a sleep time set to 15 seconds (so they would effectively spin up for every access and spin down moments later) tended to fail much more rapidly than other drives.
I haven't looked into quiet floppy drives or CD ROM drives much, since I almost never use either. However, at least several CD ROM drive makers have focused on reducing noise:
According to several sources, one of the best CD-ROM drives for noise is the Kenwood UCR421. It uses 7 lasers instead of one, and so can read data much more quickly at a much lower spin speed. It's at once one of the fastest and quietest CD-ROMs available, though it costs several times more than a normal drive. According to StorageReview.com, though, it has some fairly large problems with shaking the case when seeking, leading to some noise (though not when reading linearly, as would be the case when playing music, and even with that, it was the quietest drive in that particular roundup).
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