Sorry if I'm being stupid but I don't get the point of putting a stereo mic in a parabolic reflector. A parabolic reflector has a single focal point, incoming sound is concentrated at that point. The purpose of a parabolic mic is to record from a single distance point source there is no left/right image you are trying to record. Indeed if you are using two separate microphones next to each other or in a crossed pair there is a good chance that neither of them is actually at the focus of parabola.
I am not an sound expert..... all the contrary, a very beginner. I did this prototype to learn
You are right, the best is to search the focal point and place the mic there.
When I record with this setup, in stereo, I only use one channel in audacity because both are almost the same. So if I modify it, it will be mono for sure.
I did some experiments with high pass filters, all based in the information I found here:
Electret battery box
Summarising what I learned:
The circuit of the website works very well if you want to record audio. There is an option to use a high pass filter and an option to not use it, but all the time the mic is powered with around 8v.
What I did different is to use also a stronger high pass filter, change the the 100nF capacitor for 22nF/47nF so it emphasises its effect. It is not good for recording but it is good for listening in real time if you have a lot of low frecuency noise (ambient noise with cars, factories, etc. )
The option to remove the high pass filter (Eagle owls, tawny owls... Low frecuencies ), using around 800Hz high pass filter for overall recordings and using a stronger one for noisy areas only for listening is a very good setup
What I am using now is a completety different setup, ultralight and very portable that works very well. It has a permanent high pass filter, the mic is powered with around 8v and the mic can be moved on the axis to the desired position. Once you detect a bird, you can leave the dish tilted and aimed at the bird and walk away or hand held if it is not shy. You can also use a TRS output to record or an TRRS for birdnet, this one is my favourite so far