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PHYSICS
UNIT - 1
Chapter 2 : ULTRASONICS
Sound waves having frequency above the audible range, i.e. greater than 20,000 Hz are called
ultrasonics or ultrasonic waves (USW)
Production of Ultrasonic Waves
a) Magnetostriction Method
When a rod of ferromagnetic material such as iron or nickel is kept in a magnetic field parallel to
its length, the rod suffers a change in its length. This phenomenon is called magnetostriction.
The change in length of the rod is independent of the direction of the applied magnetic field
(whether parallel of anti-parallel) and depends only on the magnitude of the field and the material of the rod.
Use of magnetostriction to generate USW
If a ferromagnetic rod is placed in an alternating magnetic field of frequency ๐, the rod will
change in length twice in each cycle resulting in vibrations of frequency 2๐. If ๐ is greater than
10,000Hz, the rod will vibrate at a frequency greater than 20,000Hz, thus producing USW.
Further, if the amplitude of the USW produced is to be large, the rod must be made to vibrate at
is natural frequency so that resonance will occur and the amplitude will be large. The natural frequency ๐๐ of a rod of length ๐ฟ made of material having density ๐ and Youngโs
modulus ๐is given by the formula
๐๐ =
1 2 ๐ฟ โ
๐ ๐
Magnetostriction Oscillator
The circuit is
basically a Colpittโs
oscillator which
generates a.c.
signals of any
desired frequency. When the circuit is
switched on, the
tank circuit,
comprising of
inductor L and
variable capacitors C 3 and C 4 , generates
small a.c. signals. These are amplified by the transistor T which is biased to work as a positive
feedback amplifier. The current level of the signal is also amplified and it is then applied to a
solenoid inside which a ferromagnetic rod is placed. The frequency of the circuit is adjusted using
variable capacitors C 3 and C 4 to be exactly equal to half the natural frequency of the rod (๐๐/2).
The rod will then vibrate at its natural frequency ๐๐thereby producing USW of large amplitude.
Using this method USW of frequency upto about 100 KHz can be generated. (The length of the
rod puts a limit on the maximum frequency of USW that can be generated by this method.)
b) Piezoelectric Method When certain crystals (called piezoelectric crystals) such as quartz, tourmaline, Rock salt,
Rochelleโs salt, etc. are subjected to mechanical pressure along a certain direction called the
mechanical axis, an electric potential is developed across the crystal along a perpendicular
direction called the electrical axis. This phenomenon is called direct piezoelectric effect.
The polarity of the electric potential developed depends on whether the applied pressure is
compressive or elongative.
Conversely, if an electric potential is applied to a piezoelectric crystal along its electrical axis, the crystal will either expand or contract along the mechanical axis, depending on the polarity of the
applied potential. This phenomenon is called inverse piezoelectric effect.
Use of Inverse piezoelectric effect to generate USW
If an alternating electric potential of frequency ๐ is applied to a piezoelectric crystal along its
electrical axis, the crystal will expand and contract once in each cycle thus producing vibrations
of frequency ๐. If ๐ is greater than 20,000Hz, the crystal will also vibrate at the same frequency
and thus produce USW. Further, if the frequency ๐ matches with the natural frequency ๐๐ of the crystal, the crystal will vibrate at resonance and produce USW of large amplitude.
The natural frequency ๐๐ of a crystal of thickness ๐ก made of material having density ๐ and
Youngโs modulus ๐is given by the formula
๐๐ = (^21) ๐ก โ๐ ๐
Piezoelectric Oscillator
The circuit is basically a
Hartleyโs oscillator
which generates a.c.
signals of any desired
frequency. When the circuit is switched on,
the tank circuit,
comprising of inductors
L 1 and L 2 and variable
capacitor C 3 , generates
small a.c. signals. These
are amplified by the transistor T which is
biased to work as a
positive feedback
amplifier. The amplified a.c. signal is then applied to a piezoelectric crystal C which is kept
between two metal plates A and B. The frequency of the circuit is adjusted using variable
capacitor C 3 to be exactly equal to the natural frequency of the crystal (๐๐). The crystal will then
vibrate at its natural frequency ๐๐thereby producing USW of large amplitude. Using this method USW of frequency upto about 500MHz can be generated.
SONAR (Sound Navigation and Ranging) This is a technique used to detect the presence of hidden underwater objects like submarines, icebergs, shipwrecks, etc. An ultrasonic generator mounted at the bottom of the ship sends a pulse of USW in all directions. A reflected pulse received from any particular direction indicates the presence of an underwater object in that direction. By measuring the time between sending and receiving of the pulse, the distance of the object can be determined. Also by using the phenomenon of Doppler effect, the speed and direction of motion of the object can be determined.
Echo-Sounding This is a technique used to determine the depth of the ocean floor. An ultrasonic generator mounted at the bottom of the ship send a pulse of USW towards the ocean floor. The waves reflected from the ocean floor are received. The time (๐) between sending and receiving of the waves is noted. By knowing the velocity (๐ฃ) of USW in sea water, the depth of the ocean floor can be determined using the formula:
๐ฃ.๐ 2
Ultrasonic soldering To form a proper electrical joint the surfaces to be joined have to be cleaned to make them free from dust, grease, oxide films, etc. For this purpose flux is used. However, aluminium cannot be soldered using normal soldering due to the formation of aluminium oxide film. In ultrasonic soldering, an ultrasonic generator mounted on the soldering rod generates intense high frequency vibrations. This breaks down the oxide film and helps in the formation of a good joint. Thus in ultrasonic soldering flux is not required since the cleaning is done by the USW.
Ultrasonic Cleaning The phenomenon of cavitation in liquids is used in ultrasonic cleaning (also called ultrasonic bath). The object to be cleaned is immersed in a detergent solution and intense high frequency USW are passed through the solution. Due to cavitation, high pressure points are created in the liquid which help in removing any contaminants present on the surface of the object, thus giving a thoroughly clean object. Ultrasonic bath is used in cleaning sensitive machine parts and jewellery.
Medical Uses a) Ultrasound Ultrasound is a technique involving USW and is used for imaging internal organs like kidney, liver, pancreas, etc. and to monitor the growth of the fetus in the womb. It works on the principle that USW get reflected from the boundaries of two different tissues. Ultrasound is better that X-ray because it is not harmful and also organs like kidney, liver, etc. cannot be seen on an x-ray. b) Deep heat therapy An ultrasonic generator is pressed against the affected area. Due to the high frequency vibrations in the muscle tissue, heat is generated. Heat relieves pain. Since USW can penetrate deep inside the body, pain in muscles even deep inside the body can be relieved using deep heat therapy.
Acoustic diffraction grating (determination of wavelength and velocity of USW in a liquid)
The experimental setup consists of a rectangular glass tank which is filled with the liquid in which velocity of USW is to be found. A piezoelectric generator mounted at the bottom of the tank send USW through the liquid (upwards). A reflector mounted at the top of the tank reflects the USW downwards thus forming a standing wave. The nodal points are more denser and hence behave like opaque regions for light, while antinodal points behave like transparent. Therefore, alternate periodic opaque and transparent regions are created in the liquid column. It thus behaves like a grating. (Such a grating is called acoustic grating since it is produced using sound waves.) When a parallel beam of monochromatic light is incident normally on such a grating, a diffraction pattern consisting of a central bright spot and less brighter spots on either side of the central spot will be produced as shown in the figure. Analysis Applying the theory of diffraction we have for maxima, ๐ ๐ ๐๐๐ = ๐ ๐ โฆโฆ.eqn. 1 where ๐ --- grating element ๐ --- angle of diffraction maxima ๐ --- order if diffraction maxima ๐ --- wavelength of monochromatic light used In the acoustic grating, the grating element, ๐ = ๐๐ข/2 (as shown in the figure where ๐๐ข --- wavelength of USW Substituting the value of ๐ in eqn. 1 we get, ๐๐ข
- ๐ ๐๐๐ = ๐ ๐ โ ๐๐ข = (^) ๐ ๐๐๐^2 ๐๐
Thus, by measuring the diffraction angle ๐ and by knowing the values of ๐ and ๐, the wavelength of USW ๐๐ข can be determined. Further, the velocity of USW in the liquid can be determined using the formula, ๐ฃ = ๐. ๐๐ข where ๐ is the frequency of the piezoelectric generator.
