Computer Noise in the 21st Century

The Silent Front
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This January 1999 article originally entitled Acoustic Noise Emission and Communication Systems in the Next Centuryis reprinted with permission of the author Dan Quinlan, of Lucent Technologies, and the publisher, Electronics Cooling, a magazine about thermal management in the electronics industry. It remains relevant to silent PC enthusiasts today. Mr. Quinlan documented the trend towards greater heat in telecom & computer equipment, leading to increased use of cooling fans and more noise, possibly as much as 10-20 dB in the next 5-10 years.

Within a few months of the article's publication, the first AMD K7-500 burst on the scene, with a then-unheard-of 42W maximum power dissipation, some 35% higher than a similarly clocked Intel P3. Based on personal experience with PC upgrades, Dan Quinlan's prediction of at least 2 dB/year noise increase for the next decade is right on the money so far; I don't need test gear to tell me that my latest 2 GHz system run in stock form is easily 6 dB louder than what I was using in January 1999. If any of the new 50+ cfm fan equipped CPU heatsinks were used, the increase would easily exceed 12 dB.

Mr. Quinlan has been working on a very different set of problems since then, but has a more hopeful view today: My guess is that the graph might well look different since the chip industry seems to have put alot of effort into lowering power dissipation. Manufacturers of laptops, PDAs, and cellphones have been applying plenty of pressure to make improvements. For those industries, power is a major issue. Whether this translates to noise reductions in PCs as well remains to be seen.

Forward by Mike Chin, April 11, 2002

Introduction

Acoustic noise emission is one of several physical design issues addressed during the design of telecommunications and information technology equipment. In most systems, noise is a by-product of the air-movers used for system cooling. The amount of heat dissipated in electronic systems cooled by forced convection is directly proportional to volumetric flow rate. The flow rate, in turn, is directly proportional to the rotational speed (N) of the air-mover. Dimensional analysis and laboratory measurements have shown that when the rotational speed of a typical air-moving device increases by a factor of , the corresponding increase in the sound power level, Lw(in bels), can be estimated using:

Lw = A log (n)

where 5.0 <= A <= 5.5 [1]. For reference, a relatively quiet system will have a sound power value of 3.5 - 4.5 bels, whereas emission levels above 7.0 bels will result in local sound pressures that will be considered to be quite loud, or even harmful at higher levels. In the world outside electronics, a vacuum cleaner might be near 7 bels while a jet engine might be near 15 bels [2].

Given the strong dependence of noise level upon heat dissipation, trends in noise emission are inextricably related to developments in integrated circuit and printed wiring board design. The intent of this paper is to briefly discuss projected trends in chip and board design, and then assess the impact on noise emission.



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