I know this has been discussed before. I recorded over several nights at 192 kHz in supplied case attached to tree branch - particularly interested in bats. On nearly all recording have a rapid on/off pulsing noise at 53 kHz. On previous discussion it was concluded this was to do with writing to SD card (SanDisk ultra). Then I repeated with same card in a different location on a tripod in an open position without the case - same batteries, no further charging. This time I get a completely clean recording - not a hint of any noise. I have obtained new cards anyway, but puzzled as to why I had noise on the first occasion and not the second.
Automatic algorithms mistake the noise for soprano pupistrelle (albeit low probability)
Many thanks
Jim
The SD card noise often manifests as actual physical noise caused by the rapid expansion and compression of capacitors inside the SD card (and to a lesser degree on the PCB). When the AudioMoth is inside the case, these sounds seem to propagate to the microphone more easily. The SanDisk Ultra card has quite a high current drain, and we find that SanDisk Extreme (gold and red) are much quieter. Version 1.2.0 of AudioMoth is also quieter than version 1.1.0. All tend to be quieter outside the case compared to inside the case.
Ps I have just remembered that the recording schedule in the 2 instances was different. In the first instance I was recording hour long files. In the second the device cycled recording for 55 seconds then slept for 5 seconds. Maybe this explains the difference?