In this lecture, we are going to learn about the Ultrasonic Wave, properties, Production, Detection, and application of Ultrasonic waves. So let’s start with the basic definition of Ultrasonic Waves.
What is Ultrasonic Waves
- The sound waves of frequency greater than 20Khz are called Ultrasonic Waves.
- These sound waves are inaudible to the human ear.
- The ultrasonic waves due to their shorter wavelength have a greater penetrating power.
- Ultrasonic waves are widely used in medical diagnosis, marine applications, NDT, etc.
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Properties of Ultrasonic Waves
The top 10 Important Properties of Ultrasonic Waves are:
- Ultrasonic waves vibrate at a frequency greater than the audible range for humans (20 kilohertz).
- They have smaller wavelengths. As a result, their penetrating power is high.
- They cannot travel through a vacuum.
- Ultrasonic waves travel at the speed of sound in the medium. They have maximum velocity in a denser medium.
- In a homogeneous medium, they travel at a constant velocity.
- In low-viscosity liquids, ultrasonic waves produce vibrations.
- They undergo reflection, refraction, and absorption.
- They have high energy content. They can be transmitted over a large distance without much loss of energy.
- They produce intense heat when they are passed through objects.
- Like sound waves, ultrasonic waves are longitudinal waves that produce alternate compressions and rarefactions.
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Production of Ultrasonic Waves
Based on frequency range and power output, the ultrasonic wave generators are divided into two groups.
- Mechanical Generator
- Electrical Generator
In this section, the electrical generator methods of producing ultrasonic waves are explained.
The electrical generators are subdivided into two categories.
| 1. | Magnetostriction generator |
| 2. | Piezoelectric generator |
1. Magnetostriction method to produce ultrasonic waves
Principle of Magnetostriction effect
- When a ferromagnetic material in the form of a rod is subjected to an alternating magnetic field parallel to its length, the rod undergoes alternate contractions and expansions at a frequency equal to the frequency of the applied magnetic field. This phenomenon is known as the magnetostriction effect.
- Due to resonance, the rod is thrown into longitudinal vibrations, thereby producing ultrasonic waves in the surrounding medium. Such ferromagnetic materials which are used for the production of ultrasonic waves are called magnetostriction materials.
Merits of the Magnetostriction method
- The design of this generator is very simple and the production cost is low.
- At low ultrasonic frequencies, large power output is possible without the risk of damage to the circuit.
- Frequencies ranging from 100Hz to 3000KHz can be produced.
Demerits of the Magnetostriction method
- It cannot generate ultrasonic frequencies above 3000KHz.
- The frequency of oscillations depends greatly on temperature.
- There will be losses of energy due to hysteresis and eddy current.
2. Production of Ultrasonic Waves by Piezoelectric method
The piezoelectric method of producing ultrasonic waves is based on the principle of inverse piezoelectric effect.
Principle of Inverse piezoelectric effect
- If an alternating voltage is applied to one pair of opposite faces of the crystal, alternatively mechanical contractions and expansions are produced in the crystal and the crystal starts vibrating. This phenomenon is known as the inverse piezoelectric effect or electrostrictive effect.
- If the frequency of the applied alternating voltage is equal to the vibrating frequency of the crystal, then the crystal will be thrown into resonant vibration producing ultrasonic waves.
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Detection of Ultrasonic Waves
The presence of ultrasonic waves can be detected by using any one of the following methods:
| Sr. No. | Detection of Ultrasonic Waves Methods |
|---|---|
| 1. | Quartz crystal method |
| 2. | Thermal detection method |
| 3. | Sensitive flame method |
| 4. | Kundt’s tube method |
1. Quartz Crystal Method
- This method of detecting ultrasonic waves is based on the principle of the piezoelectric effect.
- When one pair of opposite faces of the quartz crystal is exposed to ultrasonic waves, in the other pair of opposite faces, charges develop.
- These charges are amplified and detected using suitable electronic circuits.
2. Thermal Detection Method
- When the ultrasonic waves are propagated through a medium the temperature of the medium changes due to alternate compressions and rarefactions.
- In the case of stationary waves, there is a change in temperature at nodes and no changes in temperature at antinodes.
- based on this principle, by sensing the changes in temperature using suitable components, ultrasonic waves can be detected.
3. Sensitive Flame Method
- This is a qualitative method to detect ultrasonic waves. When a narrow sensitive flame is moved in a medium of ultrasonic waves, the flame remains steady at the antinodes but flickers at nodes due to maximum changes in pressure.
- Thus, by observing the behaviors of the flame, the presence of ultrasonic waves can be detected.
4. Kundt’s Tube Method
- This method is used to detect ultrasonic waves of low frequency.
- A Kundt’s tube apparatus, shown below Figure, consists of a long glass tube of more than 1m in length and 5 cm in diameter kept horizontally with two supports on a wooden baseboard. One end of the tube is fitted with an adjustable piston rod with cork. A quartz crystal placed in between the two metal plates is placed at the mouth of the other end of the tube.
- The glass tube is thoroughly dried by passing through in a hot blast of air. A thin layer of dry lycopodium or cork powder is spread along the entire length of the glass tube. When stationary ultrasonic sound waves generated by using the piezoelectric method are passed in the air contained in the long glass tube, the lycopodium powder gets collected in the form of heaps at the nodes and is blown off at the antinodes. Thus, the formation of heaps at nodes confirms the nature of the transmitted sound wave to be ultrasonic waves.
- The average distance between two consecutive heaps gives the value of half the wavelength. Thus using the relation v = fλ=2fd, the velocity of the ultrasonic waves in the medium can also be calculated.
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Application of Ultrasonic Waves
SONAR:
- One of the major applications of ultrasonic waves is used in SONAR. SONAR is a device that stands for Sound Navigation and Ranging.
- It is based on the principle of echo sounding. In this acoustical technique, high-frequency ultrasonic waves are used.
- Using SONAR, the distance and direction of submarines, depth of the sea, depth of rocks in the sea, the shoal of fish in the sea, etc, can be determined.
Cavitation:
- One of the major applications of high-power and low-frequency ultrasonic sound waves is an ultrasonic processor and cleaner. In a processor and cleaners, the principle of cavitation is used.
- Ultrasonic Cleaning
- Ultrasonic Drilling and Cutting
- Ultrasonic Welding
- Ultrasonic Soldering
Other Applications of Ultrasonic Waves
Science and Engineering
- It is used to detect flaws or cracks in metals.
- It is used to detect ships, submarines, icebergs, etc., in the ocean.
- It is used for soldering aluminum coil capacitors, aluminum wires, and plates without
- It is used to weld some metals which cannot be welded by electric or gas welding.
- It is used for cutting and drilling holes in metals.
- It is used to form stable emulsions of even immiscible liquids like water and oil or water and mercury which finds application in the preparation of photographic films, face creams, etc.
- It acts like a catalytic agent and accelerates chemical reactions.
Application of Ultrasonic Waves in the Medical field
- It is used to remove kidney stones and brain tumors without shedding any blood.
- It is used to remove broken teeth.
- It is used for sterilizing milk and for killing bacteria.
- It is used to study the blood flow velocities in the blood vessels of our body.
- It is used as a diagnostic tool to detect tumors, breast cancer, and also the growth of the fetus.
Ultrasound scans work by exposing a body part to a high sound wave, which sends dynamic visual pictures of your body’s interior structures without the use of radiation.