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Originally Posted by Cathar
*snort*
...Secondly, and as a bit of an eye opener for those who know their physics, the sound pressure level drops off at an inverse square to distance.
If sound is being properly measured at 1x distance (12"), then if properly measured at 3x distance (36"), then the sound level would be 1/9th the amount at 12" distance.
The dBA scale is a log-base-10 scale. This would mean that at 36" distance, the dBA level would be 9.5dBA lower than at 12" distance.
Now take a look at the Eheim 1048 lines again, and we see a 2dBA separation between 12" and 36".
Primarily the results don't match any form of understood physical theory.
Secondarily for there to be such a small difference, one would have to assume that the Eheim 1048 was vibrating against a larger surface and at 36" vs 12" distance the actual distance of the sound-meter from the thing making the noise (whatever the 1048 was sitting on) did not really change much at all.
As we all know, it is possible to construct tests to suit our needs. Its the issue of making it believable that's the problem.
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Actually they are running into a problem known as the noise floor. Or the sound level of the empty room without the pump running.
Since I'm not awake yet I won't try and go into exact numbers but this background noise will compress all readings as you get closer to it. My math escapes me at the moment but I'm sure someone else can help out.
You just couldn't measure 9dB below 35dB. As the background noise of most quiet rooms is around 30dB.
Companies use an ancheoic chamber to lower the ambiant noise so that one can measure sound levels lower than the normal background noise of a room.