Capacitive Micromachined Ultrasound Transducer (CMUT)

CMUT is a relatively recent transducer technology and is likely to soon replace piezoelectric transducer technology.

As its name indicates, a CMUT is a micromachined electronic capacitor that has the capability of converting electrical energy into ultrasound and vice versa.  A CMUT is tiny with a diameter of only 0.1 mm.  It is a microcapacitor that consists of a thin flexible membrane at the front connected to a fixed plate at the back. 

To transmit ultrasound, a high-frequency alternating AC current is applied causing the membrane to vibrate at the frequency of the AC voltage.  On reception, when an echo strikes the membrane, the membrane vibrates which generates a tiny electrical voltage proportional to the strength of the echo.  

Due to their small size and the fact that they are micromachined and can be produced on an electronic chip, CMUTS are easier and less expensive to produce than piezoelectric crystals.  Additionally, they have a very broad bandwidth which allows them to operate at a wide range of frequencies. This can enable a single transducer to be used for both superficial and deep imaging.  Philips, Butterfly, Clarius, and others are all currently using CMUT technology.