297 lines
7.1 KiB
HTML
297 lines
7.1 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
|
|
<HTML
|
|
><HEAD
|
|
><TITLE
|
|
>Noise Control and Heat Dissipation</TITLE
|
|
><META
|
|
NAME="GENERATOR"
|
|
CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
|
|
REL="HOME"
|
|
TITLE="The Unix Hardware Buyer HOWTO"
|
|
HREF="index.html"><LINK
|
|
REL="PREVIOUS"
|
|
TITLE="But What If I'm Economizing?"
|
|
HREF="economizing.html"><LINK
|
|
REL="NEXT"
|
|
TITLE="Special Considerations When Buying Laptops and Netbboks"
|
|
HREF="laptops.html"></HEAD
|
|
><BODY
|
|
CLASS="sect1"
|
|
BGCOLOR="#FFFFFF"
|
|
TEXT="#000000"
|
|
LINK="#0000FF"
|
|
VLINK="#840084"
|
|
ALINK="#0000FF"
|
|
><DIV
|
|
CLASS="NAVHEADER"
|
|
><TABLE
|
|
SUMMARY="Header navigation table"
|
|
WIDTH="100%"
|
|
BORDER="0"
|
|
CELLPADDING="0"
|
|
CELLSPACING="0"
|
|
><TR
|
|
><TH
|
|
COLSPAN="3"
|
|
ALIGN="center"
|
|
>The Unix Hardware Buyer HOWTO</TH
|
|
></TR
|
|
><TR
|
|
><TD
|
|
WIDTH="10%"
|
|
ALIGN="left"
|
|
VALIGN="bottom"
|
|
><A
|
|
HREF="economizing.html"
|
|
ACCESSKEY="P"
|
|
>Prev</A
|
|
></TD
|
|
><TD
|
|
WIDTH="80%"
|
|
ALIGN="center"
|
|
VALIGN="bottom"
|
|
></TD
|
|
><TD
|
|
WIDTH="10%"
|
|
ALIGN="right"
|
|
VALIGN="bottom"
|
|
><A
|
|
HREF="laptops.html"
|
|
ACCESSKEY="N"
|
|
>Next</A
|
|
></TD
|
|
></TR
|
|
></TABLE
|
|
><HR
|
|
ALIGN="LEFT"
|
|
WIDTH="100%"></DIV
|
|
><DIV
|
|
CLASS="sect1"
|
|
><H1
|
|
CLASS="sect1"
|
|
><A
|
|
NAME="noise"
|
|
></A
|
|
>6. Noise Control and Heat Dissipation</H1
|
|
><P
|
|
>An increasingly critical aspect of machine design is handling the
|
|
waste heat and acoustic noise of operation. This may seem like a boring
|
|
subject, but cooling is a centrally important one if you want your machine
|
|
to last — because thermal stress from the electronics' own waste heat
|
|
is almost certainly what will kill it. You want that fatal moment to
|
|
happen later rather than sooner. On the other hand, cooling makes acoustic
|
|
noise, which human beings don't tolerate well.</P
|
|
><P
|
|
>This tradeoff bites harder than you think; it's the fundamental
|
|
reason that, despite my money-is-no-object premise in the Ultimate Linux
|
|
Box artcles, I didn't go to relatively exotic technologies like
|
|
liquid-cooled overclocking or RAID disk arrays for a performance boost.
|
|
Sure, they may initially look attractive — but overclocked chips and
|
|
banks of disk drives require massive cooling with lots of moving parts, and
|
|
it's not good to be trying to do creative work like programming with
|
|
anything that sounds quite so much like an idling jet engine sitting beside
|
|
one's desk.</P
|
|
><P
|
|
>In 2001 we had already reached the point at which the thermal load
|
|
vs. cooling-noise tradeoff is the effective limiting factor in the
|
|
performance of personal machines. Ten years ago, even low-end and medium
|
|
"server" machines differed from personal-PC designs in fairly important
|
|
ways (different processor and bus types, different speed ranges, etc.)
|
|
Nowadays specialized server architectures are in retreat at the high end of
|
|
the market and everything else looks like a PC. And the difference between
|
|
a "PC" and a "server" is mainly that servers live in server rooms, and are
|
|
allowed to have monster cases with lots of noisy fans.</P
|
|
><P
|
|
>So how do we manage this tradeoff for a personal, desktop or
|
|
desk-side machine? Careful choice of components and being willing to pay
|
|
some price premium for cool-running and low-noise characteristics can help
|
|
a lot. Even exceptionally clueful system integrators can't generally
|
|
afford to do this, because they're under constant competitive pressure to
|
|
cut price and costs by using generic components.</P
|
|
><P
|
|
>Reducing expected noise and heat in a design call for different
|
|
strategies. It's relatively easy to find decibel figures for the
|
|
noisemaking parts in a PC design. And, once you know a little basic
|
|
audiometry and a few basic rules of thumb, it's not hard to form a fair
|
|
estimate of your design's noisiness. Estimating a design's heat
|
|
dissipation is harder, partly because the waste-heat emission of a PC's
|
|
subsystems tends to vary in a more complex way than the acoustic emissions
|
|
of the mechanical parts. This means that you can and should try to design
|
|
ahead for low noise, but on the other hand expect to have to monitor for
|
|
heat-dissipation problems in your prototype and solve them by building
|
|
in more cooling.</P
|
|
><P
|
|
>Here's the basic audiometry you need to know to control your
|
|
design's noise emissions:</P
|
|
><P
|
|
>Sound is measured in <I
|
|
CLASS="firstterm"
|
|
>decibels</I
|
|
>, abbreviated dB,
|
|
relative to the threshold of audibility, "A". (Thus, sound levels above
|
|
that threshold are written "dBA".) The scale is logarithmic, with every
|
|
3dB increment roughly doubling sound intensity.</P
|
|
><P
|
|
>For sounds that are not phase-related, decibel levels add as a
|
|
logarithmic sum. Thus if X and Y are uncorrelated sound sources,</P
|
|
><P
|
|
CLASS="literallayout"
|
|
><br>
|
|
dBA(X + Y) = 10 * log(10 ^ (dBA(X)/10) + 10 ^ (dBA(Y)/10))<br>
|
|
</P
|
|
><P
|
|
>A consequence of the above formula is that dBA(X + Y) cannot be
|
|
more than 3dB above the greater of dBA(X) and dBA(Y) for uncorrelated
|
|
sources (6dB for perfectly correlated ones).</P
|
|
><P
|
|
>Sound from a point source decays by an inverse-square law,
|
|
roughly 6dB for each doubling of distance.</P
|
|
><P
|
|
>Important thresholds on the decibel scale:</P
|
|
><P
|
|
></P
|
|
><DIV
|
|
CLASS="variablelist"
|
|
><DL
|
|
><DT
|
|
>0 dBA</DT
|
|
><DD
|
|
><P
|
|
>Threshold of hearing</P
|
|
></DD
|
|
><DT
|
|
>20 dBA</DT
|
|
><DD
|
|
><P
|
|
>Rustling leaves, quiet living room</P
|
|
></DD
|
|
><DT
|
|
>30 dBA</DT
|
|
><DD
|
|
><P
|
|
>Quiet office</P
|
|
></DD
|
|
><DT
|
|
>40 dBA</DT
|
|
><DD
|
|
><P
|
|
>Quiet conversation</P
|
|
></DD
|
|
><DT
|
|
>45 dBA</DT
|
|
><DD
|
|
><P
|
|
>Threshold of distraction, according to EPA</P
|
|
></DD
|
|
><DT
|
|
>50 dBA</DT
|
|
><DD
|
|
><P
|
|
>Quiet street, average office noise</P
|
|
></DD
|
|
><DT
|
|
>60 dBA</DT
|
|
><DD
|
|
><P
|
|
>Normal conversation (1 foot distance)</P
|
|
></DD
|
|
><DT
|
|
>70 dBA</DT
|
|
><DD
|
|
><P
|
|
>Inside car</P
|
|
></DD
|
|
><DT
|
|
>75 dBA</DT
|
|
><DD
|
|
><P
|
|
>Loud singing (3 feet)</P
|
|
></DD
|
|
><DT
|
|
>80 dBA</DT
|
|
><DD
|
|
><P
|
|
>Typical home-stereo listening level</P
|
|
></DD
|
|
></DL
|
|
></DIV
|
|
><P
|
|
>The acoustic noise emitted by PCs is normally a combination of white
|
|
noise produced by airflow, high-frequency noise produced by bearing
|
|
friction in drive spindles and fans, and the constant frequency "blade
|
|
passing" noise that all propellers emit (the latter is often more intense
|
|
than white noise and bearing whine).</P
|
|
><P
|
|
>The best low-noise ball-bearing case fans emit around 20dBA.
|
|
Typical sleeve-bearing fans emit 30-50dBA.</P
|
|
><P
|
|
>According to the indispensable <A
|
|
HREF="http://tomshardware.com/"
|
|
TARGET="_top"
|
|
>Tom's Hardware site</A
|
|
>, you can expect
|
|
to cut at least 5dB off the interior noise level of the computer with a
|
|
good choice of case. We'll improve on that by adding sound-absorbing
|
|
material to the interior.</P
|
|
></DIV
|
|
><DIV
|
|
CLASS="NAVFOOTER"
|
|
><HR
|
|
ALIGN="LEFT"
|
|
WIDTH="100%"><TABLE
|
|
SUMMARY="Footer navigation table"
|
|
WIDTH="100%"
|
|
BORDER="0"
|
|
CELLPADDING="0"
|
|
CELLSPACING="0"
|
|
><TR
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="left"
|
|
VALIGN="top"
|
|
><A
|
|
HREF="economizing.html"
|
|
ACCESSKEY="P"
|
|
>Prev</A
|
|
></TD
|
|
><TD
|
|
WIDTH="34%"
|
|
ALIGN="center"
|
|
VALIGN="top"
|
|
><A
|
|
HREF="index.html"
|
|
ACCESSKEY="H"
|
|
>Home</A
|
|
></TD
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="right"
|
|
VALIGN="top"
|
|
><A
|
|
HREF="laptops.html"
|
|
ACCESSKEY="N"
|
|
>Next</A
|
|
></TD
|
|
></TR
|
|
><TR
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="left"
|
|
VALIGN="top"
|
|
>But What If I'm Economizing?</TD
|
|
><TD
|
|
WIDTH="34%"
|
|
ALIGN="center"
|
|
VALIGN="top"
|
|
> </TD
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="right"
|
|
VALIGN="top"
|
|
>Special Considerations When Buying Laptops and Netbboks</TD
|
|
></TR
|
|
></TABLE
|
|
></DIV
|
|
></BODY
|
|
></HTML
|
|
> |