Since shock waves formed by cavitation are strong enough to damage the moving parts, cavitation is an undesirable phenomenon in many applications of industry. It is specifically avoided in the design of machines such as turbines and propellers and eliminating cavitation is a major field in the study of fluid dynamics.

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(a)Using radiometer: Ultrasonic waves can be detected using Radiometer. In this method ultrasonic beam is made to fall on a thin mica fan suspended by a thin wire carrying a small mirror from one end of a light rod. Due to pressure exerted by ultrasonic waves the fan gets detected along with the mirror. The deflection can be noted by a lamp and scale arrangement. A beam of light is made incident on the mirror and reflected beam falls back on the origin of scale attached to lamp. When mirror shows deflection by angle 0, then reflected beam on the scale defects by angle 20. Since 20 can be noted from scale, hence deflection of mirror can be found. The deflection is directly proportional to the intensity of ultrasonic waves. Hence we can calculate the intensity of ultrasonic waves with this method.

(b) Kundt’s tube method: Kundt’s tube filled with lycopodium power can also be used for detecting ultrasonic waves whose wavelength is of the order of a few millimeters. When ultrasonic waves pass through tube then stationary waves are formed due to super position of incident and reflected waves. Heaps are formed at the position of nodes. The distance between adjacent nodes is calculated, which is equal to half the wavelength of ultrasonic waves. Hence with this method wavelength of ultrasonic waves can be calculated.

(c) Thermal method for detection of ultrasonic waves: When ultrasonic waves pass through a medium, then alternative compressions and rarefactors are formed. At compression, particles of medium are brought closer and collisions between them increases. As a result of this the temperature of medium increases at compressions. On the other hand, the temperature of medium decreases at rarefaction due to the fact that particles of medium go move away from each other and frequency of collisions is decreased. Thus if we introduce a platinum resistance thermometer in the path of ultrasonic waves in a medium, and move the thermometer along the direction of propagation of waves then temperature reading of the thermometer changes alternatively confirming the presence of ultrasonic waves in the medium. Some times stationary waves are formed in the medium due to superposition of incident and reflected ultrasonic waves. In such a case nodes and antinodes are formed in the medium. At nodes the pressure varies alternatively resulting in cooling and heating effect. Thus a platinum resistance introduced at nodes will store in temperature, while no change in temperature is recorded at antinodes. Thus if we move thermometer in the medium, then resistance of platinum wire of thermometer will change alternatively confirming the ultrasonic waves in medium.

These are the methods to detect ultrasonic waves. If you know another, you can discuss in the comments section.

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      Piezoelectric Generator. It is found that when pressure or compression  is applied on two opposite faces of a quartz crystal, then charges are produced on a set of opposite faces which are perpendicular to the faces at which pressure is applied. The magnitude of charge developed is proportional to the amount of pressure applied. Charge produced on one face is positive and on other face is negative. Further more, if instead of compression, the faces of crystal are subjected to some tension, then nature of charges developed also gets reversed. “The process of appearance of charges on transverse faces of certain crystals when subjected to external stress is called piezoelectric effect.”

         But if an electric field is applied across two opposite faces of a Quartz crystal, then extension or compression is produced in the crystal in a direction transverse to the direction of electric field. This effect is called Antipiezoelectric effect. The extent of compression or extension is proportional to the strength of electric field.

         If applied electric field is alternating in nature then quartz crystal starts vibrating at the frequency of electric field and hence produces acoustic waves in air. If the frequency of electric field is same as natural frequency of the crystal, then amplitude of crystal oscillations is quite large. This property of Quartz crystal is used for producing ultrasonic waves. Apart from Quartz, other materials showing this property are Rochella salt (Sodium potassium tartrate), tourmaline etc. The effect is best in Quartz cut in a particular manner.

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When magnetic field is applied across the length of a ferromagnetic  rod such as Nickel, then change in the length of rod is observed. This is called Magnetostriction. The charge in length is propotional to the strength of megnetic field.  The increase in length is very small in practice. Amongst all ferromagnetic substances, increase in length is maximum for Nickel.

Production of ultrasonic waves:  

Experimental arrangement to produce Ultrasonic Waves by magnetostriction method is shown in figure. Here AB is a rod of nickel, which is clamped in the centre. Here T is a valve oscillator. On coil L₁ in the grid circuit of value oscillator is wound on one side of the rod and another L₂(also shown by thin line) in the anode circuit of valve is wound on the other side of the rod. The coil L₂ and variable capacitor C from a tank circuit, which produces electronic oscillations of frequency

                                                f = 1/ 2π√L₂C                          (1)

Due to this a longitudinal alteraning magnetic field is produced around the rod. The frequency of oscillation of magnetic field is also v as given in (1). Since capacitor C is variable capacitor. Hence by adjusting value of C, we can adjust the frequency of alternating magnetic field produced around the nickel rod. However the oscillations of magnetic field should be sustained(i.e. amplitude of magnetic field should not chand=ge with time).

For this the feedback is provided by voltage induced on L₁ connected in the grid circuit of valve oscillator. Whenever the magnitude of altering magnetic field increases, the length of nickel rod also increases independent of direction of magnetic field, provided it should be parallel to length of rod). Thus alternating magnetic field sets up viberations in the nickel rod such that frequency of vibration of rod is double of frequency of oscillation of magnetic field. This sitution is unwanted as amplitude of vibrations of rod will be very small in this case because the two kinds of vibrations are non resonant. To avoid this another coil L3 is wound over the rod and connected to a Polarizing d.c. supply. This produces a permanent and steady magnetic field alone one direction parallel to the length of rod. By changing current through D.C. supply, strength of steady magnetic field can be adjusted to a suitable value.When these two magnetic fields (one alternating and one steady) are super imposed, then half part of alternating magnetic field is cancelled by steady magnetic field. In this case the frequency of vibration of nickel rod will be same as that of frequency of alternating field. Thus whatever is frequency of a.c. field, same is frequency of vibration of rod.

            By changing the capacity of capacitor and by taking suitable length of nickel rod, we can produce alternating magnetic field, whose frequency matches with natural frequency of vibration of nickel rod. Due to this nickel rod shows resonant vibrations of maximum amplitude and produces ultrasonic waves of frequency at which it is vibrating. In the fundamental mode, clamped end of nickel rod acts as nod and the ends of rod act as antinodes. We know that distance between a node and antinode is ¼ of wavelength. Hence if length of rod is L, then wavelength of ultrasonic waves is given by

                                                L/2 = ¼ λ

                                                λ = 2L

The speed of ultrasonic waves in the rod is given by

                                                V = √Y/p

Where Y = Young’s modulus, p = density of nickel

The frequency of ultrasonic waves produced is given by

                        V = v/ג = 1/2L √Y/p

By varying the length of rod high frequency oscillations can be obtained. A nickle rod 10 cm long gives out ultrasonic wave of frequency = 24kHz.

This is how the ultrasonic waves are produced or generated by magnetostriction method.

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Galton’s Whistle. It contains of essentially a short cylindrical pipe blown in the form of an annular nozzle. The distance of nozzle from the edge of pipe can be varied by turning a micrometer screw. By suitable adjustment of this distance and the pressure of air blast, the pipe is set into resonant viberation at a frequency depending on length and diameter of pipe. The Galton’s Whistle method can be used to produce ultrasonic waves of low frequency upto 100kHz.

I will discuss the other methods in next articles.

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1. The ultrasonic waves cannot travel through vacuum.
2. These waves travel with speed of sound in a given medium.
3. Their velocity remains constant in homogeneous media.
4. These waves can weld certain plastics, metals etc.
5. These can produce vibrations in low viscosity liquids.
6. The ultrasonic waves are reflected and refracted just like light waves. i.e.

(a) Angle of incidence is equal to angle of reflection.

(b)   Incident ray, reflected ray and normal lie in same plane.

(c)    If i is angle of incidence, r is angle of refraction, V₁ is velocity of ultrasonic waves in incident medium and  V₂ in refracted medium then

                             Sin i/ sin r = V₁ / V₂

This is called Snell’s law of refraction.

7.    The speed of ultrasonic waves/acoustic waves is more in more dense media                    i.e. V_g < V_l < V_s. If ultrasonic waves enter from rarer medium to dense medium, then V₁ < V₂  so equation (5) gives sin i / sin r < 1 => i<r. Thus ultrasonic waves will bend away from normal. Similarly when ultrasonic waves enter from denser to rarer medium, then they bend toward normal. This property is just opposite to that of light.

8. As ultrasonic waves cannot travel through vacuum, therefore if these waves travel through a non- homogeneous medium, then at each discontinuity like crack or change in density or presence of impurity etc., the amplitude and thus intensity of ultrasonic waves decreases by some amount. This decrease in intensity of ultrasonic waves as these travel through a medium is called Attenuation. The vacuum in the material causes strong reflection of ultrasonic waves while impurities or discontinuity cause the scattering of ultrasonic waves leading to net decrease in intensity. The attenuation is increased with increase in frequency of ultrasonic waves for a given medium. The intensity of ultrasonic waves decreases exponentially according to the relation

                                             I = I₀ e^-ax

where                                   I₀ = Intensity at surface

                                             I = Intensity at depth x inside the sample.

α is called Monochromatic Attenuation coefficient. Its value is different for different media and for a given medium, its value is different for different frequencies/wavelengths.
With increase in frequency of ultrasonic waves, value of α also increases.

The above are the major 9 properties of ultrasonic waves.

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Ultrasonic waves are acoustic waves whose frequency is more than 20kHz .They travel with the speed of sound. Hence their wave length is smaller than 333200cms-1/ 20000Hz = 1.66 cm

 (ג = v/υ) . These waves possess a number of properties of sound waves and exhibit some new phenomena also.

Types or modes of ultrasonics waves:

Ultrasonic waves can propagate through a medium as stress or strain waves depending upon the elastic properties of medium. Based on particle displacement of the media, ultrasonic waves are are classified into four types or modes:

(i)                 Longitudinal or Compressional or Pressure ultrasonic Waves. In the longitudinal waves particles of medium vibrate back and forth parallel to the direction of propagation of wave. These waves propagate through the medium as a series of alternate compression and rarefaction. These waves are most widely used in the ultrasonic inspection of materials. This mode is exhibited when medium of propagation has no boundaries i.e. it has infinite span. Due to propagation of these waves both pressure and density of medium fluctuate periodically.

(ii)               Transverse or Shear ultrasonic Waves. In the transeverse waves particles of the medium vibrate perpendicular to the direction of waves propagation. In this case the medium undergoes shear deformations periodically. These waves can propagate through this rods.

(iii)             Surface or Rayleigh Waves. The surface waves travel along the flat or curved surface of thick solids without influencing the bulk of medium below the surface. The depth to which these waves propagate below the surface with considerable intensity is approximately equal to wavelength of the wave.Practically all of its energy is attenuated within this depth. These waves are used to detect cracks or flaws on or near the surface of test objects. During the propagation of surface waves, the particles of medium describe elliptical orbits.

(iv)             Lamb or Flexural or Plate Waves. The lamb waves are produced in thin metal, whose thickness  is comparable to the wavelength of ultrasonic wave.

These are the four types or modes of ultrasonic waves.

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