I use an Audiomoth 1.2.0 with an external mic (2 Primo EM272 enclosed in a soundtrap with a big windshield) for recording nocturnal migration calls of birds. I often have problems with the wind noise producing very low sounds which make the recordings saturate (vertical white bands). Although the Audiomoth's high-pass filter option works a bit, I find it lacking, especially compared to the same option found in the Wildlife Acoustics SM4s I use professionally. For the same wind intensity, the SM4 microphones with a small windscreen + the 200Hz high-pass filter on an SM4 hardly saturate, whereas the EM272 with a significant windscreen system (homemade Baby Ball Gag + good windscreen) + the 1000Hz high-pass filter Audiomoth saturates a lot. I don't know if it would be possible to manage the intensity of the filter to make it more powerful? Or should I set it lower to concentrate the filter on very low frequencies? (I really don't know how a live filter works sorry).
Thanks a lot !
Thank you both for your answers. I will look into hardware filters.
The big problem with digital filters is that distortion due to overloading or to intermodulation has already happened and can't be undone. Indeed if the signals are to be analysed songraphically the FFT process provides good visual separation of different frequency bands making filtration somewhat redundant except for aesthetic purposes. If you are using an external microphone it should be possible to add some hardware filtering in line between the microphone and the Audiomoth. Passive Pi or T filters are not difficult to implement if you can find the right tables of coefficients, and if you know the input impedance of the Audiomoth. Active Filter Design by Arthur Williams (publ Artech, 1975) is a good reference for filter design in general. I also have some photocopied design tables for passive LC designs from 'Simplified Modern Filter Design, but I am afraid I do not have the full reference for that work.
Hi Paul, If you have an unfiltered recording, you can try out the effect of the filter settings in the Filter Playground - https://playground.openacousticdevices.info. The filter on the AudioMoth is a 1st order Butterworth filter so it doesn't have a particularly sharp cut-off and it is in software after the analog stage so it won't prevent wind from saturating the input but might reveal sounds that were masked by the wind. Alex