This is an Audio Project that i have been trying over the years that is not to difficult to do.
Using an op-amp with a high pass filter to filter out low frequency noise. Be cause i live right to a busy road and that can be a pain in the ass.
Also it needed some form of cut off when there was no signal deteted
so was working on a squealch device a few years ago and re kindled my intrest once again.
So as you have worked it out...is to listen to the birds when the sun goes down or rises..or other sounds in ones garden that the mic will pick up depending on the squealch setting.
The latest is a stereo one that uses only about 300 micro amp into an ear piece with no squealch so that it can be operated with a battery at about 9 volts and small caps value with a high quality op-amp the op group like Bur-Brown.
Also i have decided to do a transmit Audio listener with a small TX board for 88-108 MHz. Iam using this at this moment in time and the front end of ths transmitter need an preamp to it to make it more sentive to low level audio. Also ihave 3d printed a mounting board that will hold the Battery/Tx unit...