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The values in here - https://github.com/OpenAcousticDevices/White-Papers/blob/master/Using_AudioMoth_with_Filtering_and_Amplitude_Threshold_Recording.pdf - 50kHz high pass filter with an amplitude threshold of 512 should work well. Increase the threshold to 1024 if there is significant background noise.
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It will attenuate calls below this frequency so you might lose some calls depending on how close the bat passes to the AudioMot but the roll-off of the filter is quite slow. The real aim is to ensure that low-frequency background noise is attenuated so that you can set a sufficiently low amplitude threshold to not trigger on this noise. This will depend a bit on the environment - e.g. wind noise, running water, etc. You could probably achieve the same effect with a much lower frequency around the call frequencies that you are interested in.
Yes - been trying it out. 512 is still relatively aggressive if you have quiet bat calls and want to catch everything that comes by but weould be ideal in busy areas where losing some quiet calls is acceptable. The new config app and firmware apepars to work well though there are some artefacts when using the highpass filter. I'm looking forward to the promised app to apply the filteringpost hoc so that the optimal values can be determined. It would also be great for running over old data to elimiate unnecessary silence.