Run Silent, Go Feep

Warning: The following blog entry provides way more than the
recommended daily allowance of geeking. If you don’t have a serious
propeller-head streak, surf outta here now before it’s too
late.

I’m mainly a software guy, but occasionally I build PCs for fun.
Design them, rather; the further away I stay from actual hardware the
happier it usually is for everybody. Last year, I designed an Ultimate
Linux Box
; the good folks at Los Alamos Computers built it and
will cheerfully sell you one. It was a successful design in most
respects, but unpleasantly noisy. This year, as we do the 2002
refresh, I’m going to be working hard at getting the most noise
reduction I can without sacrificing performance. I’m experimenting
now with ways and means.

So I spent a couple of hours today disassembling the case of my
wife Cathy’s machine (minx.thyrsus.com) and lining three sides of it
with Dynamat, a kind of stick-on
rubber acoustic insulation often used in car-stereo installations.
The malevolent god that normally attends me when I futz with hardware
must have been off tormenting some other hapless ex-mathematician; no
hardware was destroyed, no blood was shed, and I’m typing this on the
selfsame reassembled machine.

Minx is a pretty generic mid-tower system made with cheap Taiwanese
parts in mid-2002 by my local hole-in-the-wall computer shop: I
spent only $150 to have it built, recycling a few parts from an only
slightly older machine. It has a 300W power supply, Athlon 950 mobo
with stock CPU cooler fan, one 80mm case fan, 7200RPM ATA drive. I
succeeded in lining both 14″-square side panels and the case top; this
used up the 4′sq piece I bought so efficiently that there was only
about 10″sq in two small piece left over. I used those to cover the
only exposed solid section of the back panel.

If you want try this yourself, the tools I found useful were a
utility knife and a metal footrule, the latter useful both for
measuring to fit and as a cutting guide.

I took before and after measurements with the db meter. dbA scale,
measurements made with the probe one inch above the center-rear edge
of the case.

Machine off: 44dbA
Machine on, before: 63dbA
Machine on, after: 61dbA

In other words, only a 2dbA drop — marginal when you consider
that the meter is only rated 1.5dB accurate! but it’s worth bearing in
mind that the scale is logarithmic; 2dbA is more than it looks like.

I have studio-engineer ears and sensitive musician fingers. I took
before-and-after measurements with those, too, listening to the sound
tambre and feeling for case resonance.

My ears tell me that the box is only slightly quieter, but the noise
spectrum has changed. The proportion of high-frequency noise has
dropped; more of what I’m hearing is white noise due to turbulant
airflow, less is bearing noise. This is a good change even if total
emission hasn’t dropped much.

My fingers tell me that the amount of case resonance has dropped quite
dramatically, especially on the side panels.

Was it worth doing? I am not sure. There would probably be more
benefit on a system emitting more bearing noise from 10K or 15Krpm
drives. On this one, I think the power supply is emitting most of
the noise, and acoustic lining can’t do much against that.

In fact, my clearest take-away from this is that the big gains in
noise reduction on conventional PCs are likely to come from
obsessing about power-supply engineering — including details like
whether the fan blows through a slotted grille or a cutout with a
wire-basket finger guard (the latter will generate less turbulence
noise).

I’d like to retrofit minx with a Papst 12dbA muffin fan and see if
that makes a measurable difference. But the best change would
probably be one of the Enhance
300W PSUs that are supposed to only emit 26dbA. I’ll bet that would
win big.

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