Several MacNN readers report receiving replacement power supply/fan units from Apple in recent days. Replacements were sent to owners of Apple's latest Power Mac series (MDD), to address noise output issues.

A number of readers report significant decreases in noise output. Reader Jeff Swayze offers the following analysis:

Just an FYI, I received my replacement power supply and fan from Apple
today. I had an SPL (sound pressure level) meter handy and measured the
"before" sound level to be about 56 dB at 1 foot away from the intake
vents. After about an hour's work (and some sore fingers) I powered the
machine back up with the new power supply and fan and was very pleased
with the results. The machine is noticeably quieter, and the SPL went
below the range of my meter, which is 50 dB. As I'm running SETI, the
fan kicked up a notch from its standard speed and I measured the new
sound level at about 54 dB. To make it bump up once more, I opened the
case which moved the heat sink away from the fan. This third level of
cooling registered about 62 dB. I didn't measure the "before" levels
for anything but the "resting" fan level, so I can't compare the higher
speeds, but since the highest level of the new fan is only 2 dB louder
than the resting level of the old one, I think we're doing pretty well.

To refresh your physics knowledge, decibels are exponential. A sound 10
dB louder than another is 10 times as loud; a sound 20 dB louder is 100
times as loud. Divide the difference by 10, then raise 10 to that
power, and you have your "X times louder" number.

In this case, at least in the lowest speed setting, the new fan is
about 10 ^ ((56-50)/10), or about 4 times as quiet.

by MacNN Staff