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Fanless System Design
Key computer components today generate enough heat that cooling fans are almost inevitable for stable operation and to avoid jeopardizing product reliability or longevity with high temperatures. The handful of commercial computers with a serious claim to be "silent" are mostly fanless, with custom cases that are in part massive external heatsinks to allow passive cooling of the hot components. This means there is no fan noise, which is a big part of typical computer noise. However, this does not eliminate all sources of audible noise.
There is the hard drive, an electro-mechanical device spinning at high speed, often more than one in many systems. Hard drives have a wide range of acoustic output and also add vibration to the case, which usually causes a host of other audible effects, including harmonics and intermodulation. They also make quite different noises when seeking compared to when they're idle, and the change is very noticeable for anyone who listens. The acoustic effects of the hard drive must be neutralized if the costly removal of cooling fans is to be effective in achieving inaudibility.
There are still other noise sources: electronic parts such as capacitors and inductors can emit mid/high frequency noises, especially of a tonal nature, and often intermittent. These parts are found mostly on power circuitry and they can be truly annoying even when at very low measured loudness. With conventional computers, such tonal noises are often not heard directly because they are masked by the roar of fans and hard drives. In a fanless system, this noise is plain to hear. It is far more common than you'd think. It can sound like CRT monitor high frequency whine, which most people have heard. It can also sound like a buzz or hum. Often this noise is too low in loudness to appreciably affect any conventional SPL or sound power measurements. But they are perfectly audible for users with normal hearing, as many a frustrated user can attest. Only careful selection of parts and good circuit design can ensure that such noise problems don't arise.
What all this means is that in designing a fanless system for low noise, any one of many factors can lead to failure, to nasty noise, unless the primary design target is kept firmly in mind: Human perception.
Fan Cooled Quiet System Design
A different approach to low noise computers using carefully selected, high quality, low noise fans in more conventional cases is usually cheaper to implement. Although the absolute measured "loudness" of such fan-cooled systems might come in a bit higher than for completely fanless systems, the perceived audibility may be just as low. In many conditions, the residual broadband airflow noise of the fans can provide a smooth masking effect over tonal aspects of the acoustics that can lie at very low loudness levels. Keep in mind that serious tonal or intermittent noise factors will still be easily heard by noise-conscious users, and hard drive noise still has to be well managed.
Furthermore, the issue of fan speed changes in response to rises in component temperature (due to high load or hot weather) also must be managed well. Too much of a speed up (or even down), especially in a short period, is heard as an annoyance by most users. Lower power components, especially those meant for mobile computing where the drive to maximize run time on batteries has created highly power-efficient parts, can make noise optimized fan cooling a practical and viable way of building inaudible computers.
Carefully designed fan cooling can also be used to create high power computers that are audible but have a benign acoustic signature that makes them unobtrusive in most environments, for most people. A broadband random sound like softly falling rain can actually measure fairly high, yet rank very low in perceived "loudness". Combined with care around the other noise factors above, such a computer can have excellent acceptance among noise conscious consumers.
Despite the name of our web site, a silent computer may be scientifically impossible... but "inaudible" or "audible but benign" computers are well within reach. Careful system design is necessary to ensure that all the potential pitfalls are avoided, not just "low measured loudness":
- sharp tonal aspects
- intermittent sounds
- rapid changes in noise
- harshness (caused by intermodulation and harmonics)
- vibration induced noise
Keep in mind that all of these various aspects of noises can be identified using sophisticated audio measuring equipment, the same equipment need to test for sound power.
A Need for People-centric Metrics
In light of these various factors, the long upheld ISO 7779 standard for measuring computer acoustics is clearly lacking. By focusing only on sound power and a single half meter SPL measurement, ISO 7779 manages to ignore the sound quality aspects so important to human perception, leaving only a machine-language definition of overall noise. The fact that so few companies actually use this standard and its results for promotion is actually something of a relief. It would only lead to greater confusion and consumer dismay.
Articles of Related Interest
The State of the Industry, March 2006: Through Silent Eyes
Noise in Computing: A Primer (Oct 2003)
External Link: Product Sound Quality from Perception to Design (PDF document)
by Richard H. Lyon, RH Lyon Corp, Cambridge, Massachusetts
March 2003 Article in Sound & Vibration magazine
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