Saw some interesting research on using infrasound to locate elephants. they managed to come up with something that was about $9 in parts including microphone. (Plus a SBC for analysis).

A bit of searching lead to some DIY stuff using subwoofers (speakers) as the microphone input then some low-pass filtering and then fed to an op-amp.

Here's the link to the research paper on the elephant detector https://www.researchgate.net/publication/320596116_Eloc_Loca...