This is a very interesting, ambitious roundup that's unfortunately very flawed in acoustic SPL measurements.
65 fans on megahalems article -- said:
Sound pressure is measured in decibels, abbreviated as dB. The standard weighting is type A, abbreviated dBA. The SPL meter reports sound pressure levels (SPL) in tenths of a decibel. However, the meter specifications say it is accurate to 0.5 decibel above 30 dBA, so in the tables the SPL values were reported in 0.5 dB increments. When there is no extraneous noise, the basement where the fans were tested has a SPL of under 10 dBA, so background noise did not interfere with SPL measurements. The SPL measurements were made at 10 cm and converted to the standard 1 meter equivalent by subtracting 20 dB. Because a fan is loudest at its face, the reported SPL is 10 cm from the fan intake face. Note that your setup will have its own acoustics, so the noise at 10 cm may not drop off as rapidly as theory says; but these measurements are designed to allow us to compare fans, so they were converted to the standard measure: one meter (1 m).
Problems:
1) The Tenma 72-942 SLM used can only go down to 30 dB, so how did the author know the ambient in the room was 10 dBA?
2) Specs on this sound level meter that I found state accuracy to be ±1.4dB at 1KHz, 94dB; not the 0.5 dB claimed by the author. It may go up/down in 0.5 dB steps, but that is not its accuracy.
3) There's no mention of any calibration done on the meter, which means we don't know how closely the meter meets reference SPL standards.
4) The mic was placed 10cm or 4" away from the fan intake face, and the SPL reading converted to the standard 1 meter equivalent by subtracting 20 dB. How did the author come to this odd conversion formula? Never mind; these kinds of SPL conversions from "close mic" results rarely (if ever) work.
Examining the actual SPL reported in this review for fans I've also measured in the anechoic chamber, I can say that these results are all over the place. Mostly they appear to be too high, by up to 9 dBA compared to the anechoic chamber 1m SPL tests I've done on samples of some of the same fan models.
5) There's no mention of how many samples of each fan model were tested, so I presume just one -- which is really not good enough considering how much samples can vary acoustically, sometime due to subtle damage in shipping alone.
I appreciate the enormous time and effort that must have gone into this test project, but the resulting data is of dubious value.
The author's subjective comments are far more useful than the SPL results or any analysis based on SPL vs temperature. Obviously, the thermal results are worthwhile, possibly even invaluable... if you could assign accurate SPL values and subjective acoustic ratings to the fans.
SPCR is working on fan analysis using a similar approach, but in an anechoic chamber with a reference quality calibrated SLM, spectrum analyzer and audio recorder. The results there would be really interesting to compare -- at least for the cooling (temperature) results. (See prelim methodology article here:
http://www.silentpcreview.com/Fan_Test_System_2010
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Mike Chin,
Editor/Publisher,
www.silentpcreview.com
