==== Ultrasonic ====

life and vibration above 20 kHz

==== Bat listening devices...====
  * Wildlife Acoustics Echo Meter Touch 2 > https://www.wildlifeacoustics.com/products/echo-meter-touch-2-android
  * https://batdetecting.blogspot.com/2018/10/review-of-echo-meter-touch-2-pro-part-1_7.html

==== listening to surveillance capitalism ====
  * https://hackaday.com/2017/05/04/ultrasonic-tracking-beacons/