From Wisegeek.com “What is an Acoustic Transducer?” (16 January 2013)
An acoustic transducer is an electrical device that coverts sound wave vibrations into mechanical or electrical energy. They have various practical applications, including sound recording and sound playback. A specialized model, called an ultrasonic acoustic transducer, can be used to measure distance to, as well as the mass of, an object.
Common types of acoustic transducers used in sound recording include microphones, earphones, and guitar pickups. These create electrical energy when moving parts inside the transducer, such as electrical plates or ribbons, are exposed to sound vibrations. The electrical energy produced inside the transducer is sent first to an amplifier.
The amplifier then sends this energy to its final destination, usually a loudspeaker or recording device. The loudspeaker reproduces the sound at a level that the human ear can hear. A recording device will retain the electrical signal information. The recorder will send the stored signal to a loudspeaker during playback.
An ultrasonic acoustic transducer can be used to measure distance or the mass of an object. The most common type is the piezoelectric acoustic transducer. These include a piezoelectric ceramic element that creates and distributes ultrasonic sound waves.
Sound waves travel to an object from a piezoelectric transducer through material called a couplant. The couplant is usually water. Sound waves bounce off the object and return to the transducer in the form of an echo. The time it takes for these echoes to return to the transducer is used to calculate the distance to the object.
Underwater sound navigation and ranging (SONAR) is a common use of an ultrasonic acoustic transducer. SONAR uses directional beams of sound waves. This enables the SONAR operator to determine the direction and distance to an object.
SONAR systems can be active or passive. An active system sends out sound waves and listens for echoes. A passive system listens for noises made by ships, fish, and landmasses.
An electromagnetic acoustic transducer (EMAT) is another form of ultrasonic transducer. Instead of a ceramic element, an electro magnet is the main component of an EMAT. This is a type of non-contact, or non-destructive transducer. Unlike piezoelectric transducers, EMATs do not need a couplant to carry sound waves. Instead, two electromagnetic fields are generated to disburse ultrasonic waves.
EMATs can easily be used almost anywhere since no liquid is needed. For example, EMATs can be used to check for flaws in underground pipes. A downside to EMATs, compared to piezoelectric transducers, is that EMATs create weaker sound fields.