But it would make a big difference depending on the case, from which direction measurements were taken and so on. So it would still not be the end of it.
YUP! One solution is a test jig designed specifically to test for fan vibrations using an accelerometer. Standards for such exist, of course, but no DC fan makers (for PCs) use them, it seems. The UBC fan project will built one.
What I learned from these measurements is how difficult it is to measure sound level in an accurate way.
It's so nice to hear someone else say this!
It is true. And you have to be meticulous to get repeatable results.
with a higher noise level the value was 11 dB lower at 1 meter than at 0.125 meters. Lowering the speed of the fan resulted in a 15 db decrease in sound level at 1 meter compared to that of 0.125 meters.
This sounds a bit odd. Implies SLM non-linearity. Was this repeatable with more than one fan? Several? How about dfferent mic positions? (not distance)