What are Ultrasonic Waves?
The ultrasonic waves are the ultrasound waves. The band of the frequency of ultrasonic waves lies above 20 Khz, which is above the band of frequency that human beings can hear. Thus though the ultrasonic waves exist in atmosphere, they cannot be heard. The speed of transmission of ultrasonic waves depends on the media through which they are passing. In most of the cases where ultrasonic waves are used in the instruments, the waves have to travel through the air, where their speed is affected by environmental factors like temperature of the air, the existing humidity, and the turbulence of air. Of all these, the temperature of the air has major impact on the speed of the ultrasonic waves.
What are Ultrasonic Transducers?
The devices that use ultrasonic waves for the measurement of certain parameters are called as the ultrasonic transducers. The measurement devices using the ultrasonic waves comprise of the two major parts. One part of the device transmits the ultrasonic waves and the other part of the devices receives the ultrasonic waves.
The devices transmitting and the receiving the ultrasonic waves may be located at the two opposite ends. In some cases the transmitting and the receiving devices are located at the same end and on the same side. In such cases the sending device emits the ultrasonic waves, which strike some obstacle, get reflected from there and come back to the sending end where they are received by the receiving device.
No matter what the arrangement of the transmitting and the receiving device is, there is a timer that measures the time difference between the sending of the ultrasonic waves and receipt of the ultrasonic waves. This time is calibrated against the parameter to be measured. Thus the speed of the ultrasonic waves is an important property based on which the transducers using the these waves work. In some ultrasonic transducers, instead of the speed, the change in the phase or frequency of the transmitting and the receiving ultrasonic waves is used as the basis for the measurement of the parameter.
The ultrasonic transducers are used commonly for the measurement of flow rates of the fluids, the level of the liquid, displacement of the object etc.
Piezoelectric Crystal: An Important Ultrasonic Transducer Element
The piezoelectric crystal is one of the most commonly used elements in the ultrasonic transducers. It can be used as the both, transmitting as well as the receiving device, in the transducers. It is enclosed within the casing so that it can work efficiently and securely.
The piezoelectric crystals can work in the frequencies ranging from 20 KHz to 15 MHz. The voltage passed through these devices generates the ultrasonic waves.
Capacitive Ultrasonic Elements
The capacitive ultrasonic transducers comprise of two conducting layers separated by a backplate. When the varying voltage is applied across the two layers, ultrasonic waves are generated and transmitted by them. These devices can work as the transmitting device as well as the receiving device. The capacitive ultrasonic elements can work in the frequency range of 30 KHz to 3 MHz.
Reference and Images Courtesy
1) Book: Measurement and Instrumentation Principles by Alan S. Morris, third edition.