I am extremely new to AudioMoth, this is my third recording. A large part of the recordings from my most recent attempt at recording bats have these crazy inverted harmonics, like mirrored reflections that appear at least twice for each pulse. Never seen anything quite like it (I'm used to recording with Pettersson D500X and handheld detectors). Have I messed up the configuration somehow or is this normal? I have attached the sound file. Also, is there a specific reason why the recording is cut off at 124 kHz? Thanks!
Hi, You've recorded at 250kHz so the maximum frequency of signal that can be discerned in the recording is half that at 125kHz. This is known as the Nyquist frequency and a spectrogram will always have a frequency scale from 0 to the Nyquist frequency. That is 0 to 125kHz in this case. The inverted signals result from harmonics of the original call occurring at frequencies above the Nyquist frequency. These get aliased to frequencies below the Nyquist frequency and appear as the 'flipped' signal. If you record at 192kHz you will still have a high enough Nyquist frequency to record these bats, but will get a slightly better recording as the AudioMoth will sample at 384kHz and downsample. This reduces the aliased features as it imposes a low-pass filter on the recording.