Man so many messages in a short time.
I had to read back more messages that expected to see a bunch of questions from David.
The questions about the signal strength are quite difficult, how much Pa do you create being a bat shouting quite loud on 20mtrs distance..... I have no idea, translating this to millivolts and decibels will be hard since we do not even know where to start with.
The easy approach is looking at the opamp which has a 18:1 resistor ratio which means the voltage amplification should be 18 times. 2 millivolts of input should become 63 millivolts output.
With the caps in the circuit amplification is less on low frequencies and more on higher frequencies. It should translate to about 16dB on 40kHz and 20dB on 100kHz (-30db on 1kHz)
The ICS microphone is supposed to be the microphone with the best s/n ratio according to other builders, you can use it in a balanced configuration. The single ended output is a bit less than the SPU if I recall correctly but measured over the positive and negative output I guess it should give more signal.
I am not a big fan of the ICS element since it is less sensitive around 50kHz (a 15dB "gap" somewhere between 40 and 60) but you can see the species of bat well enough in the recordings. The simple test I did, did not show much better results on high frequencies, I think the SPU does perform better.
I also tried to use and FG element but these do seem to be quite insensitive compared to the ICS and SPU elements, so you need to amplify the signal more to get a similar signal strength.
The third question was about the LC filters used.
There has not been done any calculation on these filters. The LC filter kind off has a "knee" in the filter curve which you can calculate and it helps you to determine what values are high (or low) enough to filter out some unwanted noise.
The higher the capacity the lower the frequencies are that are attenuated, the higher the resistor is, the bigger the attenuation will be. You still do need to have enough voltage for the circuit to work so that kind of limits the value of the resistors used.
Since the microphone and amplifier use only very little current it is no problem to use a higher value resistor. The other parts use more current so it is better to use only just a few ohms so there is less voltage drop.
The first attempts of recreating Cor's ideas showed a whole lot of noise on the display and in the recordings. I just tried a lot of filtering in different ways. To be honest I don't know how much these LC filters actually help to reduce the unwanted noise because they cost little I never tried to remove them from the circuit.
Edwin