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Detection of Ultrasonic Waves

1. Piezoelectric Detector

Piezoelectric effect can also be used to detect ultrasonics. If ultrasonics comprising of compressions and rarefactions are allowed to fall upon a quartz crystal a certain potential difference is developed across the faces which after amplification by a value amplifier can be used to detect ultrasonics.


2. Kundt’s Tube Method 

Kundt’s tube is a long glass tube supported horizontally with a air column in it when the ultrasonic waves are passed the Kundt’s tube, the lycopodium powder sprinkled in the tube collects in the form of heaps at the nodal points and is blown off at the antinodal points. This method is used provided that the wavelength is not very small.


3. Thermal Detector

Thermal detectors can also be used to detect ultrasonics. A thermal detector is nothing but a fine platinum wire fixed at two ends as shown in Figure 3.5.

Figure 3.5.



It is placed in the region to be tested for waves. The wire is set into rapid vibrations. At a node  compressions and rarefactions occurs very quickly and cause adiabatic changes. The platinum wire is, therefore, alternately heated and  cooled and its resistance changes accordingly. These changes in resistance can be detected by suitable means. No such changes occur at an antinode.


4. flame  Method

A narrow sensitive flame is moved along the medium. At the position of the antinode, the flame is steady. At the position of the node the flame flickers because there is change in pressure. In this way the positions of nodes and antinodes can be found out in a medium. The average distance between two adjacent nodes is equal to half the wavelength. If the value of frequency of ultrasonic wave is known, the velocity of the ultrasonic wave through the  medium can  be calculated.


5. Acoustic Diffraction  Method

This method is based on the fact that ultrasonic waves which consist of alternate compressions and rarefactions changes the density of the medium through which they  pass.

This leads to a periodic variation of refractive index of the liquid, such a liquid column is subjected to ultrasonic waves constitutes an acoustical grating. If monochromatic light is passed through the waves the liquid causes the diffraction of light.

Figure 3.6 shows the experimental arrangement, standing ultrasonic waves are produced in  a liquid  contained in a glass tube. The density and so the refractive index of the  liquid is maximum at the  nodal point and minimum at antinodal points. Hence the  nodal area acts  as opaque region, while antinodal area acts  as  transparent region for  light. The  liquid column thus resembles the  rules grating.

The  grating period d equal to /λ/2 and is given by


d sine θ=mλ


λ= wavelength of monochromatic light beam

m = order of minima.

An  acoustic diffraction grating produced by  a liquid column subjected to  ultrasonic waves.

FIGURE3.6  Experimental set up of acoustic grating                  


  1. 1.     Depth sounding

The depth of sea or depth  of water below a ship can be found out by using the echo sounding

principle. Ultrasonic waves generated by a crystal transducer in the range of 40-50 kHz are directed towards the bottom of the sea. The time lag between the sending of ultrasonic signals and return back as an echo to the ship is recorded. Knowing the velocity and with measurement of the time lag, the depth of sea water below the ship can be calculated.


depth of sea= velocity of sound in sea * time /2


The velocity v of sound in sea water at t°C is given by


v = v0 + 1.14 S + 4.21 t – 0.037 t2


where v0 = velocity of sound in sea water at 0°C = 1510 m/ sec

S = The salinity (gm/litre)

t = temperature of sea water in oc.


2. Signalling

Since the frequency of the ultrasonics can be made very high so that wavelength is reduced to a small value. Therefore ultrasonics can be transmitted in the form of a narrow beam of small amplitude and large energy. The ship captain uses ultrasonics to steer its path in mist when light fails within few metres.


3. Sonar Exploration

Sonar stands for sound navigation and ranging.


SO stands for sound


N stands for Navigation


A stands for And


R stands for Ranging


Sonar is similar to RADAR in working. In radar we use microwaves, which are electromagnetic in nature, whereas in SONAR, we make use of ultrasonics. Ultrasonic waves are sent by the ultrasonic transmitter towards the target through water and are received back by the receiver lowered into the sea water from a ship as shown in Figure 3.7. Knowing the time interval between sending and receiving of ultrasonics  e can find the position of target or sub-marine. We can also find the direction and the velocity of the          moving sub-marine with this apparatus. So this sonar technique is used for the detection of  nergy sub-marine inside water during war time in a similar manner as RADAR is used to trace the enemy aeroplanes in the atmosphere. During piece time, sonar is used for detecting the glaciers, icebergs, searocks and other submerged objects.



4. Drilling and Etching


Drilling and etching, which is very difficult or impossible with the usual machine process, can be conveniently done by ultrasonics. This is particularly useful for square drills and when the dimensions of the required drill are of tight tolerance. A transducer producing ultrasonics having a core of soft material is fixed with a bit of hard material and set over the plate to be processed. Slurry of water and carborandum powder are passed over the plate to be drilled just below the bit and ultrasonics applied. Cavities when break send carborandum particles with very high velocity drilling a hole (circular or square) in the plate depending upon the section of the hard material bit.



5. Cold Welding

In ordinary process, the temperature of the plates to be welded is raised which might bring certain undesirable change in the physical properties, like brittleness, magnetic etc. So by . ultrasonics, two pieces to be welded are placed together and ultrasonic source is applied. The particles of two pieces vibrate violently resulting in the diffusion and binding forces are developed so that welding is completed. It is clear from Figure 3.9. (Figure 3.8)

6. Flew Detection


Ultrasonic waves can be used to detect cracks (flaws) or discontinuity in metal structures. In this case, an emitter and detector of ultrasonic waves are used: Ultrasonic waves from the emitter are directed towards the metal. The reflected beam is detected by the detector. If there is a flaw or discontinuity there will be rise in energy received by the detector, if the emitter and detector are on the same side. If the emitter and the detector are on the opposite side of the metal, there will be fall in energy at the regions of flaws or discontinuity.


7. Formation of Alloys


The alloys of different compositions are prepared by using ultrasonic waves. These alloys are homogenous in nature and well mixed.