I've always wanted to try this, so I built this kit a while ago: https://www.eio.com/products/velleman-k8118-stereo-ultrasonic-sound-and-bat-detector https://w

I've always wanted to try this, so I built this kit a while ago: https://www.eio.com/products/velleman-k8118-stereo-ultrasonic-sound-and-bat-detector https://www.eio.com/products/velleman-k8118-stereo-ultrasoni... It actually turned out even cooler than I expected. A lot of things emit ultrasound that I didn't realise, like jingling keys, walking on dry grass, and all kinds of things in urban environments. And of course bats sound really neat also!