Quantity   Component name
1 × A USB microphone or a 3 MHz low cost ultrasound probe It costs only $50, but indeed it does not offer beam forming!
1 × A Raspberry Pi 3B or similar This is done with transistors and other discrete components!
1 × ML classifier To detect features in the training set, I used a modified subset of jAudio. The training set comes from Physionet, but there are other freely accessible on the net. The classifier is a HMM derived from a speech processing project. I want to use a HMM because it is able to predit states and offer an explanation of the model (the probability of transitions between states)