I recently purchased some Audiomoths with their waterproof cases for terrestrial recordings. I did run some test in the lab recording my simple voice without and with the case. I payed attention to have the microphone in front of the acoustic vent of the case. I can see a high difference in the recordings with a huge attenuation of the sound when the recorder is in its case. What do I do wrong ? Here a spectrogram of the three scenarios: 1) without protection, 2) in a plastic bag 3) in its protection box.
To adress @Selene's comment, here is the result of my (quick and dirty) experiment. I have broadcasted white noise to an Audiomoth in the case and one outside of the case for 40 minutes from the moment I closed the case (thanks @Amandine Gasc for the info). If you wait a little bit, the amplitude of the recording comes back to normal (it was already fine 10 minutes after I closed the case). Here is the graph of the difference between the amplitude at the end of the experiment (40 minutes after closing the case) and the amplitude at each time point: