Noise and Vibration

Due to their mechanical nature, there are several characteristics of hard disks that aren't relevant to most other PC components. Among these are the noise and vibration created by the disk drive. (Noise and vibration are of course two parts of the same phenomenon.) Since most devices inside the PC have no moving parts, they don't make noise. Hard disks on the other hand have very high-speed motors and actuators, both of which can make considerable noise and generate vibrations. Like cooling, noise issues have become exacerbated in recent years primarily due to the increase in spindle speeds of modern drives. Faster actuators are also part of the issue.

Hard drive noise specifications are normally provided as two numbers. The values correspond directly to the two different mechanical devices that make noise in the drive:

Idle Noise: This is the noise level the drive makes whenever the drive is in operation (and not in sleep or standby mode), regardless of what it is doing; it is caused by the spindle motor and the rotation of the platters.
Seek Noise: This is the noise level when the drive is performing positioning tasks, particularly random seeks. It is caused by the movement of the actuator assembly (as well as the spindle motor and platters of course, since they continue to spin during seeks!)

Hard drive noise is specified by the manufacturer in bels (or sometimes decibels; they are the same unit except that one bel equals ten decibels.) A "bel" is a logarithm of a ratio between two sound values. It is defined as follows:

Noise level (bels) = log₁₀ ( N₁ / N₀ )

Where "log₁₀" means a base-10 logarithm, "N₁" is the noise level of the device being measured, and "N₀" is a reference noise level. (Usually, N₀ is taken as the threshold of human hearing, the quietest sound audible to the average person. The actual value of N₀ is not important as long as all the noise figures are calculated using the same reference point.) The reason for the logarithm is notational convenience; the human ear can detect sound intensities in a tremendous range: up to a factor of 1,000,000,000,000 from the quietest to the loudest. Logarithmic notation expresses these sounds in a range of 0 to 12 bels, saving us all those zeroes.

Note: Be sure to remember, when comparing noise specifications, that the values are base-10 logarithmic. That means a hard disk with a noise specification of 5.0 bels is not 25% louder than one that reads 4.0 bels; it is 900% louder: 10 times louder.

Companies always provide noise specifications, and some hard disk reviewers have started measuring noise levels with noise meters. Frankly, I don't put a lot of stock in these measurements and figures. I would even go so far as to say that they have almost no bearing on real world noise perception and by themselves are not useful for deciding if a drive model is going to give you a noise problem. Here are my reasons:

Noise is a very subjective and personal matter. It's very possible to put two people in a room with the same system and have person A be "driven crazy" by the noise and person B not even realize the PC is turned on.
Some types of noise bother certain people more than others, because humans have different sensitivity to sounds of various frequencies. For example, some people find certain drives have a high-pitched "whine" due to their spindle motors that others can't even detect; some find the spindle noise acceptable but the seek noise irritating, and some are the opposite.
Hard disk noise levels depend a great deal on the case in which they are installed, and the location of the case relative to where the user is sitting. A small desktop case right in front of the user will make noise seem much more significant than a large tower case with the hard disk mounted in the rear, sitting on the floor next to a desk! And if a high-speed drive is installed in a server in a locked, ventilated, insulated computer room, does its noise level matter much at all?
Absolute noise figures don't mean anything unless you take them in the context of the surroundings. If a drive has a high noise rating then it might be considered "noisy", but if you are using it in a crowded office with a lot going on, you're probably not going to hear it.
Noise levels can sometimes vary between specific units even of the same model of drive.
While I trust manufacturers to make accurate sound readings, some of the individual hardware reviewers using sound meters haven't been properly trained in their use. Sound measurement is not a simple thing; you must compensate for the noise of other components and other effects to ensure the readings are accurate.

So fine, look at the specifications for the drive. Clearly, if there is a big difference in the noise values, you are more likely to have a noise issue with the drive that has the higher specification. But in reality, it all depends on your personality, your surroundings, and your ears. If you aren't the type to care about the noise level of your PC, just don't worry about the whole issue. If you are, then you should if at all possible find a PC that has the drive you are considering installed into it, and listen to it yourself. That's the ideal trial step, but not always possible. A second choice is to look on the Internet for a significant number of subjective comments. If dozens of people are complaining about the noise level of a drive, then there is a good chance the drive may bother you; look for one that is commonly used and has many comments saying the drive is quiet.

Another key issue with hard disk noise is that while seek noise specifications just consist of the actuator noise "added" to the spindle noise, they are very different kinds of noise! If you are sitting in front of a PC for hours, your brain will tend to "tune out" the background noise of the spindle. The noise of the actuator is intermittent however, and much louder. It is this noise that often causes the most objections, and again, it's a very personal thing.

To some extent, you must realize that noise and vibration are a price you pay for increased performance. Idle noise is directly correlated to spindle speed, and seek noise is correlated to faster actuators (and hence reduced seek times). It is also true however that manufacturers are constantly improving their products, which means that the generation of a given technology is important. For example, the first 7200 RPM drives were rather noisy and hot; now 7200 RPM is a mature technology and the drives of this speed run much quieter and cooler than their ancestors.

An important final note: to minimize vibration and noise in a PC, ensure that the drive has been properly mounted. If the drive is loose then its inherent vibrations can become amplified by the metal that touches it, making the entire PC seem to resonate in some cases! See here fore more on hard disk mounting.

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