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Computer Noise in the 21st Century

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Dan Quinlan of Lucent Technologies predicted in January 1999 that hotter chips and the accompanying need for forced air cooling would increase noise in electronic equipment by “10-20 dB in the next 5-10 years.”

Based on personal experience, his prediction of at least +2 dB/year noise increase is right on the money when it comes to PCs. I don’t need test gear to tell me that my new 2 GHz system (in stock form) is easily 6 dB louder than what I was using in 1999. If any of the new 50+ cfm fan equipped CPU heatsinks were used, the increase would easily exceed 12 dB. Mr. Quinlan’s prognostic article from Electronics Cooling is reprinted with permission.

This January 1999 article originally entitled Acoustic Noise
Emission and Communication Systems in the Next Century
is 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.

The Present
Over the past few decades, many electronic system design groups have incorporated
noise control measures into their products. The primary design changes were
the integration of fan speed control, system flow impedance reductions and improvements
related to air-moving device inflow/outflow conditions [1]. It is not unusual
for such changes to provide 1-2 bels (10-20 dBA) of noise reduction.

When designing telecommunications or information technology
equipment, manufacturers rely on a variety of benchmark documents. For the telecommunications
industry, one such document is a recently published standard created by the
European Telecommunications Standards Institute (ETSI) [3]. The ETSI document
specifies a set of sound power limits for equipment sold in Europe. Similarly,
a few individual countries have produced similar documents for the computer
and business equipment industry.

For example, the Swedish Agency for Administrative Development
(Statskontoret) produced one of the first of these several years ago [4]. While
many relevant noise documents are designed to protect the health and safety
of workers [5]., the European documents are principally intended to ensure that
equipment does not excessively impact speech communication, task concentration,
and other perceptual factors in spaces where equipment is installed. In general,
the limits in these documents have not been primary design drivers for manufacturers
since the limits could often be met using conventional noise control techniques.
As a result, noise emission has usually been a relatively low-priority design
issue.

The Future
With very few exceptions, the general trend in telecommunication and information
technology system design has been toward increasing heat loads. Consequently,
for systems that must be cooled using forced-convection, noise emission is also
rising. These trends are driven primarily by customer demand for increased system
throughput, and decreased enclosure size [6,7]. For these industries, the dramatic
increase in Internet access has rapidly driven data rates up.

Over the period from January, 1996 to January, 1997, the number
of addressable Internet domains quadrupled [8]. Soon, as convergent systems
become widely deployed, single networks will carry data, voice and video. The
speed at which these changes will occur is likely to mean that many design assumptions
and specifications will need to be constantly revisited. System power (i.e.,
heat) dissipation is driven by performance trends at both the integrated circuit
and printed wiring board level. The next two sections discuss trends in chip
and board design, and the likely effects on noise emission.

Integrated Circuit Design
Because of the financial and social impact of the electronics market,
trends in semiconductor design have been well documented. Similarly, there has
been a concerted effort by industry groups and government agencies to provide
projections regarding future integrated circuit designs. With regard to heat
dissipation, there are important market drivers toward both lower and higher
dissipation values.

There are two key drivers toward lower dissipation values.
The first is a desire to avoid pushing dissipation to levels that will require
the use of non-traditional cooling schemes. Figure 1 contains dissipation and
power data for a number of different high-end processors, as well as data from
computational studies [9]. As can be seen in the figure, the generally accepted
chip dissipation limit for forced convection cooling is in the vicinity of 1
W/sq.cm. 1

1. Various published papers use this limit but
there is clearly a range in the design community’s view. In systems where extremely
high reliability is required (e.g., military avionics systems), forced convection
cooling is used up to a limit of roughly 0.3 W/sq.cm. Conversely, there are
commercial forced-convection cooled systems where 2 W/sq.cm. chips are being
used.)


Figure 1: High-end processor dissipation and cooling
technology map.

The second driver away from increased dissipation is a broad
push to reduce power consumption. This trend is driven by a broad array of forces
ranging from general DFE (Design For the Environment) strategies to the need
to reduce the restrictions that battery life puts on portable computing/communication
devices. For large scale electronic systems (e.g., telecom transmission and
switching facilities), reduced power consumption translates into operating cost
reductions which can be significant over the lifetime of the systems.

In opposition to these effects are a number of drivers which
are pushing chip dissipation levels up. The two most significant drivers are
the continued demand for higher throughput and for increased on-chip circuit
densities. Projections for clock speed and circuit density growth, as published
by the Semiconductor Industry Association (SIA), are shown in Figures 2 and
3.2. The clock speed data is specified for two categories of microprocessors:
“high” performance and “cost” performance.

2. The SIA report was composed with the support
of NIST; NSF; the Departments of Commerce, Defense and Energy; Semiconductor
Research Corporation and SEMATECH (an industry consortia); and various universities.[10])

Figure 2: Projected on-chip clock frequencies for “cost
performance” and “high performance” integrated circuits.


Figure 3: Projected transitor densities for high volume
?-processors and low volume ASICs.

For the period 1995 to 2005, the clock rates for these devices
are anticipated to grow by factors of 3.2 and 2.9, respectively. The density
data is specified for low cost, high volume microprocessors and for low volume
ASICs. Over the same time period noted above, the growth factor for both types
of ASICs is projected to be 8.5.

Based upon these projections, and with consideration of all
other design drivers, the SIA report provides power dissipation estimates for
high performance microprocessors and ASICs. This data is shown in Figures 4
and 5. Again using the period from 1995 to 2005, the dissipation growth factor
for microprocessors is 1.8. The ASIC dissipation is expected to double between
1995 and 2001, and then remain flat.


Figure 4: Projected power dissipation values for high
performance microprocessors.


Figure 5: Projected power dissipation values for high
performance ASICs.

Printed Wiring Board Design
With regard to circuit board design, the primary dissipation drivers
are similar to the chip drivers noted above. While there is a corresponding
desire to use conventional cooling techniques and keep power consumption low,
there are also similar pushes in terms of board throughput and circuit pack
densities. These figures tend to be proprietary so industry data is scarce.
Figure 6 contains current and projected density data for 11 different boards
used in Lucent products. The ten-year growth rate factors for these products
range from 3.2 to 9.9. Assuming that this data is generally representative,
circuit pack densities are clearly rising at a rapid pace.


Figure 6: Actual and projected circuit pack densities
for 90 printed wiring board designs.

Lucent has also collected board heat dissipation on 90 different
board designs. The data is relatively flat from mid-1991 (i.e., the start date
of the data set) until mid-1996. The average power dissipation over that period
is 0.035 W/sq.cm. In contrast, some new designs are at 0.2 W/sq.cm., a 5.7 factor
increase in dissipation.

Acoustic Noise Emission
Since chip dissipation levels and board densities are projected to increase,
it is clear that system dissipation levels will continue to rise well into the
next century. Unfortunately, there are no available projections for system level
dissipation growth rates. However, given the chip and board level trends discussed
above, it would be conservative to say that the dissipation levels will grow
by a factor of two in the next five-to-ten years. This would result in a 1.5
bel (15 dB) rise in noise emission level. At the other extreme, a four-fold
increase in dissipation would result in a 3 bel (30 dB) rise in level.

In general, system noise level increases of 1-3 bels would
be problematic. First, the increased noise would adversely impact many people,
and purchasers of the equipment could be expected to require manufacturers to
reduce noise emission. Second, many systems would no longer be in compliance
with national and international noise emission/immission requirements. If such
a scenario unfolds, noise emission will become a primary design issue, and significant
effort will be needed to prevent noise emission from becoming a limit on overall
system performance. The first systems where problems will occur will be those
where throughput demand is growing the fastest.

Outdoor Equipment
The trends discussed above apply primarily to equipment intended for indoor
installation. The situation for outdoor equipment is less clear. One problem
is that well-defined sets of outdoor noise emission limits do not exist, leaving
designers without appropriate benchmark information. ETSI and other standards
bodies are attempting to create a set of limits, but there are difficult technical
and political obstacles to overcome. A second problem is that there is very
little outdoor-product noise-emission data currently available. Thirdly, wireless
telecom equipment will be the dominant equipment type to be deployed outdoors
in the near term, and the designs are rapidly changing.

While these problems make it difficult to create a rational
prediction regarding outdoor equipment, there needs to be a general recognition
that when limits are set, they could be quite low. The primary concern that
will drive the limits down will be the possibility of installations on (or near)
apartment buildings, hospitals and other dwellings. As wireless cell sizes decrease,
the likelihood that equipment will be installed near sleeping areas will increase.
In the noise control arena, sleep interference problems generate the most vehement
responses. If the equipment radiates levels that are above nighttime ambient
sound pressure levels (typically 20-40 dB below daytime levels in densely populated
areas), there will be a steady stream of complaints. If the radiated noise contains
prominent tones, the tolerance will be even lower.

Issues such as these add a level of complexity for regulatory
bodies and manufacturers since wide variation in installation environments must
be considered. It is unclear how these issues will be resolved.

Conclusion
Power dissipation data clearly points toward increasing thermal loads for
telecommunication and information technology systems. This rise is occurring
because the drive toward higher system throughput with decreased cabinet footprint
is stronger than the drive toward reduced power consumption. As these heat loads
rise, there will be a corresponding increase in acoustic noise emission. The
design trend data indicates that noise emission is likely to rise by 1-2 bels
(10-20 dB) over the next five-to-ten yeaqs.

Given the proliferation of desktop workstations, it is also
important to note that fans in these systems control the background noise levels
in many office environments. While people have grown accustomed to this added
noise, the negative effects of fan noise will likely become more apparent as
hands-free desktop conferencing becomes more widespread. Very little noise research
has been done on small cooling fans since they are low-margin commodity items.

However, it is clear that the conventional techniques will
soon be inadequate. While active control may seem at first to be a promising
solution, there are several technical and financial problems that may be impossible
hurdles [11]. So, new approaches to noise reduction will probably come from
radical changes in cabinet design unless a concerted effort is made to initiate
research on the basic understanding of the noise generation mechanisms in small
air-moving devices [12].

The above discussion assumes that systems will continue to be predominantly
cooled using forced-convection. Despite the fact that other thermal management
technologies exist, forced-air cooling will remain the predominant technique because
it is relatively cheap, highly reliable and designers are well versed in its use.
However, given the dissipation trends in chip design, it is clear that hybrid
designs that couple forced convection system cooling with “local” cooling
of high dissipation chips will become more common. It may be that local cooling
technologies such as spray cooling, jet impingement, heat pipes, thermoelectric
cooling, and liquid cooling may finally see more widespread use.



Acknowledgments
Many people within Lucent and other organizations have been kind enough to
provide assistance during the preparation of this paper. The author is grateful
for their input and cooperation.
D.A. Quinlan
Bell Laboratories
Lucent Technologies Inc.
101 Crawfords Corner Way, Rm 1H-511
Holmdel, NJ 07733-3030, USA
Tel.: 732-332-5386
Fax: 732-949-8797
Email: dbq@lucent.com

References
1 G.C. Maling, “Historical developments in the control of noise generated
by small air-moving devices,” Noise Control Engineering Journal 42(5),
159-169 (1994).

2 L. Beranek and I. Ver, Noise and Vibration Control Engineering, (New
York: Wiley-Interscience) 1992.

3 ETSI ETS 300 753,  “Equipment Engineering (EE): Acoustic noise
emitted by telecommunications equipment.”

4 Statskontoret Technical Standard 26:2, “Noise of computer and business
equipment.”

5 U.S. Occupational Safety and Health Administration (O.S.H.A.) 29 CFR
1910.95 (CFR=Code of Federal Regulations).

6 C.J.M. Lasance, “The need for a change in thermal design
philosophy,” Electronics Cooling 1(2), 24-26 (1995).

7 W. Aung, Cooling techniques for computers, (New York: Hemisphere Publishing)
1991.

8 “Runaway Internet Growth,” NetworkWorld 14(11),
1, 1997.

9 D. Tilton, A. Partha and F. Borchelt, “Advanced thermal
management for multichip modules,” Electronic Packaging and Production
(1995).

10 Semiconductor Industry Association, The National Technology
Roadmap for Semiconductors
(1994).

11 D. Quinlan, “Application of active control to axial flow
fans”, Noise Control Engineering Journal 40(1), 95-101 (1993).

12 D. Quinlan and P. Bent, “High frequency noise generation
in small axial flow fans,” Journal of Sound and Vibration (in press –
1998)

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