Parallax PING))) datasheet.
 muRata Application Manual.
 K. Yadav, I. Kymissis and P. Kinget, "A 4.4 uW Wake-up Receiver using Ultrasound Data".
 Ch. Papageorgiou, Th. Laopoulos, "Modification of Resonance Characteristics of Ultrasonic Transducers by the Driving Circuit".
 B. Vigraham and P. Kinget, "A Self-Duty-Cycled and Synchronized UWB Pulse-Radio Receiver SoC With Automatic Threshold-Recovery Based Demodulation".
 Ultrasonic Transducers Datasheet.