The 400SR120 is a standard open type Transducer Receiver consist of a piezoelectric ceramic disc mounted at the node of its fundamental resonant frequency, a conical metal resonator bonded at the center of disc acting as a rigid piston. This special construction provides high sensitivity, wider bandwidth and excellent durability in wide temperature and humidity ranges and high stability in electrical and mechanical characteristics. Due to its free vibration character the open type transducers are very suitable for continual wave driving, such as Doppler motion detector.