Background on Ultrasonic Transducers
The purpose of most ultrasonic transducers is to convert electrical energy to acoustic energy, do something useful with the acoustic energy, and convert the acoustic energy back into electrical energy. The conversion of electrical energy to acoustic (and vice versa) is a function of the piezoelectric element. Manipulating the acoustic energy and interfacing with the rest of the world is a function of the remainder of the transducer.
The electronics of the instrument generate a pulse, usually a sine wave, square wave, or spike, by applying the appropriate voltage to the transducer. Applying this alternating electrical field to a piezoelectric element causes it to expand and contract. This creates pressure waves, also known as sound, emanating outward from the element into the surrounding medium. Typically we want the sound to go in a specific direction, so the surrounding transducer parts are engineered to efficiently pass sound in that direction and absorb or reflect the sound in all other directions.
To keep the sound from going out the back of a transducer a backing layer is usually used. This consists of a material which will allow the sound in, absorb most of it, and allow very little back out. It is usually made of a soft material loaded with very heavy particles (like Tungsten) which can vibrate freely and dissipate the sound energy. Other materials are sometimes added to scatter the sound waves and sometimes the backing is shaped to create multiple reflections within the backing to allow more opportunity to absorb the sound.
To efficiently propagate the sound out into the world a matching layer is used. This is an intermediate layer (or layers) between the ceramic element and the medium into which the sound is propagating. It is often a quarter wavelength thick and made of a material which has an acoustic impedance midway between that of the Piezo and the medium. It acts much like an anti-reflection coating on glass, allowing the sound to pass freely in both directions.
Pulse-Echo devices are used by sending out a pulse of sound and then listening for the return echo. Two types of information can be obtained from such a measurement. First, if the speed of sound in the medium is known, the time between the creation of the pulse and the return can be measured and used to calculate the distance the sound wave traveled. If the distance is known, then the time can be used to calculate the acoustic velocity of the material the sound is traveling through. This can be related to density or composition. Second, the amplitude of the returned signal can be used to infer the amount of attenuation which has occurred. If the distance is known, then the acoustic density can be inferred. Again, this can be related to composition or even temperature in some cases. The limitation of both these methods is typically the resolution of the timing circuitry. With modern electronics this can be very good. Microsecond time resolution typically allows millimeter spatial resolutions and nanosecond time scales equate to micron distances.
Pitch-Catch devices use a separate transmitter and receiver. The transmitter emits a sound pulse or continuous wave and the receiver captures the wave after it has traversed the medium. By measuring the frequency or phase shift of the signal the flow rate of the medium can be calculated. By measuring the attenuation of the signal the acoustic density can be calculated. The pitch-catch method is good for looking at changes in the intervening medium over time.
Power devices are used to project energy into a medium. The energy can be used to create cavitation, to create heat or pressure, or to move things. Power transducers differ from contact or immersion transducers in that they are designed to project power rather than a signal, like the difference between an electric power line and a phone line.