Is the frequency response of the Audiomoth mic published somewhere?
Yes, bats tend to be so loud that they leak through most filters. If the calls you are looking occur in distinct frequency ranges it might be interesting to try out sampling at 384kHz and then use a frequency trigger set to the frequency band that you are interested in. If you have some regular 384kHz recording you can try out settings in the AudioMoth Filter Playground.
Yes migrating bird calls are around around 1-10 kHz range (maybe 0.5-8). So I will test out what you suggest. Right now there seems to be very little migratory movement, so I will try in a few weeks.
AudioMoth 1.2.0, AudioMoth Dev 1.0.0 and HydroMoth 1.0.0 all use the Knowles SPU0410LR5H microphone:
AudioMoth 1.0.0, AudioMoth 1.1.0, and uMoth 1.0.0 all use the Knowles SPM0408LE5H microphone:
The datasheets show the frequency response. However, the one for SPM0408LE5H is only shown up to 10kHz while it is still sensitive at higher frequencies.
See also the discussion here:
That's very helpful. So very briefly it looks to me very flat in audible range than a big peak at around 24 kHz before falling at higher frequencies.
I have been trying out for 'nocmig' recording and using 32 kHz (to save space / power) but I get of lot of 'bat noise' in the audible range - that is aliased from the higher frequencies and that big peak around 24 kHz helps to explain that. So maybe I should sample higher and filter out. I didnt really want to go much higher than 48 kHz just for space and power saving. So I will try that next and see how it compares.
I have previoulsy used a telinga Stereo mic with parabola and it is interesting to see how different it looks - I think the telinga amplifies low frequencies (complicated by the spatial aspects) and has no ultrasonic response. I only get a tiny bit of bat signal ( restricted to around 18-20 kHz - using 48 kHz sampling) that doesn't interfere with the birds.
I plan to do some side by side comparisons - but comparing these two barn owls (on different occasions) I think gives a bit of an idea of the frequency characteristics.