When I switch it on, it weights about 52.0 or so (dB ? the main number...), even if it is in a silent room where there is obvioulsy not so many dB.
I assume that you are using the built-in microphone on the iPhone 3G. In that case, the default mic sensitivity should get you within 1 or 2 dB of the actual sound level (unless the spectral content of the noise you are measuring has a lot of very low frequency energy, but in that case, the measured level would be biased low, not high.) If you have adjusted the mic sensitivity, you might want to try resetting it to the default value with the button on the Calibration screen.
"Silent" rooms often exhibit higher sound levels than you might expect, so the 52 dB value doesn't seem unrealistic. However, speaking normally should not produce a 90 dB reading unless you have your mouth very close to the microphone.
For both Lp and Leq measurements, the displayed value is given in dB referenced to 20 micropascals, by definition.
How can I calibrate that app ?
Calibration of the built-in mic will need to be performed relative to a separate calibrated sound level meter.
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