capacitance fluctuates, the plates alternately charge and discharge a little. This causesa weak ac signal to be produced, whose waveform is similar to that of the acoustic dis-turbance. The blocking capacitor allows this ac signal to pass to the transducer output,while keeping the dc confined to the plates.If an ac signal is applied to the terminals of the transducer, it passes through theblocking capacitor and onto the metal plates. The signal creates a fluctuating voltagebetween the plates. The dc source, combined with the ac signal, results in an electro-static field of constant polarity, but of rapidly varying intensity. This exerts a fluctuatingforce between the plates, causing the flexible plate to vibrate and generate an acousticwave in the air.The most well-known type of acoustic transducer has a coil surrounding a perma-nent magnet or electromagnet. A diaphragm or cone is attached to either the coil or themagnet. When the diaphragm vibrates as a result of acoustic waves striking it, ac is pro-duced in the coil. Conversely, if ac is applied to the coil, it produces electromagneticforce that causes the diaphragm to vibrate. This device is called a dynamic trans-ducer. Most microphones, headphones, and loudspeakers are of this type.Acoustic transducers are used in security systems. They are also used in roboticsto help mobile machines navigate in their surroundings. Acoustic transducers are em-ployed in depth-finding apparatus commonly found on boats.Radio-frequency transducersThe term radio-frequency (RF) transducer is a fancy name for an antenna. An-tennas are so common that you probably don’t think about them very often. Your carradio has one. Your portable headphone radio, which you might use while jogging ona track (but never in traffic), employs one. Cellular and cordless telephones,portable television receivers, and handheld radio transceivers use antennas. Hun-dreds of books have been written on the subject.There are two basic types of RF transducer: the receiving antenna and the trans-mitting antenna. A receiving antenna converts electromagnetic (EM) fields, in the RFRadio-frequency transducers613+_Signalinput/outputBlockingcapacitorRigidplateFlexibleplate32-5Simplified diagram of an electrostatic acoustic transducer.