Wave Transducers

In electronics, wave transducers convert ac or dc into acoustic or electromagnetic (EM) waves. They can also convert these waves into ac or dc signals.

Dynamic Transducer for Sound

A dynamic transducer is a coil-and-magnet device that translates mechanical vibration into varying electrical current, and can also do the reverse. The most common examples are the dynamic microphone and the dynamic speaker.
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Functional diagram of a dynamic sound transducer.
Above figure shows a functional diagram of a dynamic transducer. A diaphragm is attached to a coil that is mounted so it can move back and forth rapidly along its axis. A permanent magnet is placed inside the coil. Sound waves cause the diaphragm to move; this moves the coil, which causes fluctuations in the magnetic field within the coil. The result is ac output from the coil, having the same waveform as the sound waves that strike the diaphragm.
 
If an audio signal is applied to the coil, it generates a magnetic field that produces forces on the coil. This causes the coil to move, pushing the diaphragm back and forth, creating acoustic waves in the surrounding medium.

Electrostatic Transducer for Sound

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Functional diagram of an electrostatic sound transducer.
An electrostatic transducer takes advantage of the forces produced by electric fields. Two metal plates, one flexible and the other rigid, are placed parallel to each other and close together (above figure). In an electrostatic pickup, incoming sound waves vibrate the flexible plate. This produces small, rapid changes in the spacing, and therefore the capacitance, between the two plates. A dc voltage is applied between the plates. As the interplate capacitance varies, the electric field intensity between them fluctuates. This produces variations in the current through the primary winding of the transformer. Audio signals appear across the secondary.
 
In an electrostatic emitter, fluctuating currents in the transformer produce changes in the voltage between the plates. This results in electrostatic field variations, pulling and pushing the flexible plate in and out. The motion of the flexible plate produces sound waves.
 
Electrostatic transducers can be used in most applications where dynamic transducers are employed. Advantages of electrostatic transducers include light weight and good sensitivity. The relative absence of magnetic fields can also be an asset in certain situations.

Piezoelectric Transducer for Sound and Ultrasound

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Functional diagram of a piezoelectric transducer for sound and ultrasound.
Above figure shows a piezoelectric transducer. This device consists of a crystal of quartz or ceramic material, sandwiched between two metal plates. When sound waves strike one or both of the plates, the metal vibrates. This vibration is transferred to the crystal. The crystal generates weak electric currents when subjected to this mechanical stress. Therefore, an ac voltage develops between the two metal plates, with a waveform similar to that of the sound.
 
If an ac signal is applied to the plates, it causes the crystal to vibrate in sync with the current. The metal plates vibrate also, producing an acoustic disturbance. Piezoelectric transducers can function at higher frequencies than can dynamic or electrostatic transducers. For this reason, they are favored in ultrasonic applications, such as intrusion detectors and alarms.

Transducers for RF Energy

The term radio-frequency (RF) transducer is a fancy name for an antenna. There are two basic types: the receiving antenna and the transmitting antenna. You learned about antennas topics.

Transducers for IR and Visible Light

Many wireless devices transmit and receive energy at IR wavelengths. Infrared energy has a frequency higher than that of radio waves, but lower than that of visible light. Some wireless devices transmit and receive their signals in the visible range, although these are encountered much less often than IR devices.
 
The most common IR transmitting transducer is the infrared-emitting diode (IRED). Fluctuating dc is applied to the device, causing it to emit IR rays. The fluctuations in the current constitute modulation, and this produces rapid variations in the intensity of the rays emitted by the semiconductor P-N junction. The modulation contains information, such as which channel your television set should seek, or whether the volume is to be raised or lowered. Infrared energy can be focused by optical lenses and reflected by optical mirrors. This makes it possible to collimate IR rays (make them parallel) so they can be transmitted for distances up to several hundred meters.
 
Infrared receiving transducers resemble photodiodes or photovoltaic cells. The fluctuating IR energy from the transmitter strikes the P-N junction of the receiving diode. If the receiving device is a photodiode, a current is applied to it, and this current varies rapidly in accordance with the signal waveform on the IR beam from the transmitter. If the receiving device is a photovoltaic cell, it produces the fluctuating current all by itself, without the need for an external power supply. In either case, the current fluctuations are weak, and must be amplified before they are delivered to whatever equipment (television set, garage door, oven, security system, etc.) is controlled by the wireless system. Infrared wireless devices work best on a line of sight.