Capacitive Ultrasonic Transducers (CUTs)
Capacitive ultrasonic transducers (CUTs) are efficient devices for generating and detecting ultrasound in air. Sometimes referred to as electrostatic transducers, the devices are essentially a parallel-plate capacitor with one rigid immovable electrode (the "backplate") and one flexible movable electrode (the "membrane" or "film"). The membrane is typically a dielectric polymer, such as polyethylene terephthalate (PET, or Mylar), electroded on one side with aluminium. A dc biasing voltage is usually applied, such that opposing charges build up on the two electrodes and the membrane is attracted to the backplate. Small pockets of air become trapped between the membrane and backplate, and the response of the transducer is largely determined by the membrane thickness and mass, and the surface profile of the backplate, as this dictates the size and number of the air pockets.
When operating as a receiver, an ultrasonic wave striking the membrane causes it to displace, and as the separation between the two electrodes changes, a corresponding proportional change in charge is also produced, which may be detected and amplified using suitable electronics. Capacitive ultrasonic transducers are highly efficient as both transmitters and receivers; a typical signal across a 10mm air gap is shown on the left.
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