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 transmitter, an ac signal is superimposed over the dc bias. This oscillating voltage produces a corresponding change in charge between the two electrodes, and due to electrostatic attraction, the membrane displaces. The pockets of air trapped between the membrane and the backplate are compressed, and contribute to the restoring force of the membrane. This pushes the surrounding medium away from the transducer face and generates an ultrasonic wave, with a frequency corresponding to the frequency of the applied ac signal.
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.
Due to the small mass of the membrane, CUTs are inherently broadband and generate a wide range of ultrasonic frequencies very efficiently in air and other gases. The frequency response is determined by the membrane thickness, and the surface roughness or feature size of the backplate electrode. Due to the flexible nature of most membrane materials, curved devices can be manufactured to produced either tightly focused or divergent beams. Suitably sealed, the devices are also efficient broadband transducers in liquids, and, with an appropriate choice of membrane material, can operate at temperatures of over 500°C.
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