Ultrasonic waves are sound waves above 20,000 Hz that can be generated using a magnetostriction oscillator. The oscillator works by applying an alternating magnetic field to a ferromagnetic rod, causing it to vibrate longitudinally above 20,000 Hz. This produces ultrasonic waves that are emitted from the ends of the rod. The rod is clamped between coils connected to an oscillator circuit containing a transistor, capacitor and coils. Feedback between the coils drives oscillations in the rod at the resonant frequency set by the capacitor, generating high frequency ultrasonic sound waves.

1. 1
ULTRA SOUND
The Sound waves greater than 20,000 Hz are called Ultrasonics or Ultrasound
MAGNETOSTRICTION OSCILLATOR
PRINCIPLE :
When an alternating magnetic field is applied to a ferromagnetic rod (nickel, iron, cobalt)
and then the rod is set into longitudinal vibrations at a frequency of greater than 20,000 Hz called
ultrasonic waves. This effect is known as magnetostriction effect. ie in this effect small change in
length of the ferromagnetic rod.
CONSTRUCTION :
The magnetostriction generator consists of a the NPN Transistor is collector circuit is
connected to oscillating circuit (Tank circuit). The oscillating circuit is consists of the inductance
coil L1 and capacitor C. The Base circuit is connected to coils L2, ferromagnetic rod clamped at
the centre of the rod. The two ends of the rod are wound by the coils L1 and L2.
mA
Clamp
A
C
L2L1
npn transistor
D.C. source
Ultrasonics
Ultrasonics
Battery
B
N
Ni
S

2. 2
Working
Initially the rod is permanently magnetized by passing direct current. The battery is
switched on and hence current is produced by the transistor. This collector current is passed
through the coil L2. The e.m.f will be induced in the coil. Due to this effect, an emf is induced in
the coil L1.
The induced e.m.f is fed to the base of the transistor, which act as a feed back
continuously. In this way the current in the transistor is built up and the vibration of the rod is
maintained. The frequency of the oscillatory circuit is adjusted by the condenser (C) and when
this frequency is equal to the frequency of the vibrating rod, resonance occurs. At resonance, the
rod vibrates longitudinally with larger amplitude producing ultrasonic waves of high frequency
along both the ends of the rod.
Condition for Resonance
1 1
1 1
22
E
lL C 

Where, l → length of the rod
E → young’s modulus of the rod
ρ → density of the material of the rod
Advantages
 Construction is simple.
 Cost is low.
Disadvantages
 It cannot produce frequencies above 3000 KHz.
 It is not possible to get a constant single frequency.
 The frequency of oscillation depends on temperature.
PROPERTIES
 They are highly energetic.
 Non audible wave
 They travel through longer distance.
 They are reflected, refracted and absorbed similar to ordinary sound waves.
 Its continuously passing thro’ the metal, the temperature is increased.
 Its passing same medium, there is no change in velocity
Frequency of the oscillatory circuit = Frequency of the vibrating rod