Hello @fujiapple ,
The electret elements are the microphones. These can detect the ultrasonic sound of bats.
info about electret microphone: Electret microphone - Wikipedia
Product code of the electret mic. is M63 available in our webshop M63: ELECTRET MICROCEL Ø6mm – Velleman – Wholesaler and developer of electronics or at our local shops: https://www.velleman.eu/shops/
specs:
Best Regards,
Velleman Support