I have tried to get rid of this hum many times, both with ultrasonic mic or normal audio. It is the raspberry Pi and the cheap sound card that will make it impossible. There are two solutions.
1. Power the raspberry from batteries or from a power bank and the hum will be gone. ?Try it! It is easily done.
2. Buy a cheap XLR balanced sound card, single channel, usually ca 50 �. Use Phantom Power and simpleP48 for your capsule and the hum will be gone. In this case you can power the Pi with a power adapter from mains 220V.
All other things I tried, grounding, shielding, shorter cables, longer ones....it never worked.
