SPCR's Hard Drive Testing Methodology

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May 11, 2005 by Devon Cooke and Mike Chin

Most people who work with computers know that hard drives make noise. So do the hard drive manufacturers, who are usually good about providing at least some data about their acoustics. Often, the data is very comprehensive, detailing noise at idle as well as in seek. However, anyone who has tried to make a truly quiet computer soon discovers that the specified acoustics don't really predict the noise produced by a hard drive in their own system. We have long been aware of the role of the HDD in PC acoustics, and suggested many ways of subduing their noise, including some commercial add-on products.

Meanwhile, we have been working to create a system of HDD noise assessment that combines traditional "hard" data measurements along with subjective listening and our own unique silencentric methods. This article documents our new HDD testing methodology.

Before you can understand how we assess HDD noise, you need to know something about HDD noise itself, how it is created, what forms it takes and why we often describe it as insidious. The following section about HDD noise is partly a summary of information contained in the article Recommended Hard Drives.


The noise of a disc drive mounted in a case comes in two forms:

  1. Airborne acoustics is what all drive manufacturers currently specify as the HDD noise. It is the sound that comes from the drive through the air to the observer. This value is measured with the drive suspended in space by wires.
  2. Structure-borne acoustics induced by the drive's vibration during idle and seek is not quantified by HDD makers. This vibrational energy is transmitted to the PC chassis and causes the chassis to act much like a sounding board.

Airborne Acoustics

The acoustic noise that a drive produces is a result of mechanical movement that occurs inside the drive enclosure. There are two sources of acoustic noise inside a drive: The motor and the actuator.

The bulk of the noise produced by a hard drive comes from the electric motor that spins the discs. This is the noise you hear when the drive is idling. Except when first turned on and spinning up to speed, the drive is at a constant speed, so the noise at idle is always present. The primary frequency of the HDD noise is easy to predict: Simply divide the motor speed by 60. This is because RPM is the spin rate per minute, while frequency or Hertz is the rate of cyclical movement per second; there are 60 seconds in a minute.

Primary Frequency of HDD Vibrations / Noise
Typical Drive Interface
Frequency (Hz)
Notebook, PATA

Keep in mind that HDD noise is never composed of only this fundamental noise, the first harmonic. Like many sources of noise, there are many harmonics (multiples of the fundamental frequency) at higher frequencies. There is also broadband noise, a hissing sound that seems related to the spinning of the discs themselves, and this noise varies in overall frequency balance and loudness level from drive to drive. Some appear to have hardly any of this noise at all, while others exhibit a fair amount. Generally, the higher the number of platters, the higher the idle broadband noise.

Although it is less constant than motor noise, the seek noise produced by the actuator is louder and usually more irritating. Its irregular character tends to draw attention so it can't be tuned out easily. Seek noise can be reduced by adjusting the speed that the actuator moves across the discs, although this acoustic benefit often comes at the cost of drive latency. Many drives provide this feature as an option in the form of Automatic Acoustic Management (AAM). Generally, the higher the number of platters, the higher the seek noise. Again seek noise varies quite a lot, with each drive exhibiting a unique combination of sounds.

These are the reasons for the unique acoustic signatures of different HDDs of the same speed and number of platters; they have the same fundamental tone, but each has a unique set of harmonics and broadband noise, affected not only by design aspects such as internal damping and bearing type but also production variances including balance. Add the variance of seek noise and you have quite an acoustic range.

Vibration Induced Noise

Mounting a hard drive the way it is designed to be mounted — screwing it directly to a computer case chassis — conducts the vibration to the case. This is referred to by Seagate, the world's dominant HDD company, as structure-borne acoustics.

A case that is vibrating due to conduction of mechanical energy from a HDD produces noise by itself. The large thin metal panels of the typical case act as sounding boards which convert the vibrations into airbone noise. Aluminum cases are especially susceptible — so much so that the issue gets special mention in our article on Computer Case Basics and Recommendations. This is because aluminum has about 1/3 the density of steel, and yet the sheet metal used for aluminum cases is no thicker than in typical steel cases.

A vibrating case can also create additional mechanical noise. A loose screw, a badly fitted side panel, or a cable resting against the side of the case — all these are prone to rattling when the case vibrates.

The sources of vibration in a hard drive are identical to the sources of acoustic noise: Once again it is the moving parts, the motor and the actuator.The primary frequency of the HDD vibrations are 70, 90, 120, 167 or 250 Hz, as mentioned before, along with harmonics, broadband noise, and the complexities of the seek vibrations. Add whatever harmonics the case itself might add to this rich mix of sounds, and you can begin to understand that the total noise of a hard drive depends not only on how it is mounted but what kind of case it is mounted into.

The vibration produced by a HDD in idle does not necessarily correlate with its airborne noise. While some drives are quiet and vibrate very little, there are also many drives with low acoustic noise and high vibration, and vice versa. Seek vibration, by contrast, usually correlateswith seek acoustic noise. A drive with loud seeks in free air usually has even louder seek noise when installed in a case. The AAM feature discussed above is just as effective for reducing seek vibration as it is for direct acoustic seek noise.

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