Following are the calibration results of ten Panasonic WM-61A microphone capsules.
I have no reason to assume otherwise, that the response is basically is flat to a few Hz.
The electronics and room interaction will affect any response calculations down there.
It would be interesting to see how low the actual 3 dB is. These things will respond to room
pressure, such as with floor movement and air ducting. If you move it back and forth, you
can see the voltage changing.
All Capsules
Averaged sum.
Worst Case
Phase differences