Ultrasonics are generated by means of following:
1. Galton Whistle
2. Magnetostriction Generator
3. Piezoelectric Generator.
1. Galton Whistle
Galton whistle works on the principle of organ pipe. It consists of a closed end air Column A whose length can be adjusted with the help of a movable piston. The piston P can be moved to the desired position with the help of a screw 51. The open end of the pipe A is fitted with a lip L. The position of the pipe C can be adjusted with the help of the screw 52. The gap between the ends of A and C can be adjusted with the help of the screw 52 (Figure 3.1)
An air blast is blown through the nozzle N at the top. The blast of air coming out of C strikes against the lip L and the column of air in the pipe is set into vibration. By adjusting the
length of the air column in A, it is brought to the resonant position. The resonant frequency
will depend on the length and diameter of the pipe A. If l is the length of the air column in A,
x the end correction, then the wavelength
λ= 4 ( L + x)
The frequency of sound is
v = v /λ = V / 4( L+x)
with the help of this whistle, frequencies of the order of 30,000 Hz can be produced. The micrometer screw 51 can also be calibrated to give directly the frequency the sound.
2. Magnetostriction Generator
It is found that the length of a bar of a ferromagnetic material material such as iron or nickel changes when the bar is subjected to strong magnetic field parallel to its length. This phenomenon is known as magnetostriction. However, if the bar is subjected to an alternating magnetic field, it expands and ontracts allternately. Due to this linear contraction and expansion, longitudinal waves are produced in the medium surrounding the bar. If the rod is clamped in the center, the frequency of vibration n is given by
where L is the length of rod, Y is its Young’s modulus and r is the density of the material of the rod.
Figure 3.2 shows the electric circuit used for the generation of ultrasonic waves using magnetostriction. The coils L1 and L2 are wraped round the ferromagnetic rod AB; One is connected in the grid circuit and the other to the plate circuit of a triode valve. The rod is clamped in the middle. It is magnetised by the plate current flowing in the coil L1. A change in tum changes its length due to the magnetostriction effect. The change in the length of the rod alters the magnetic field across the coil L2 due to converse magnetostriction effect. The varying field, so produced across L2 changes its flux causing an induced emf across this coil, which changes the potential difference across the grid circuit. These vibrations are amplified by the triode valve and passed on the plate circuit. The system thus provides a feedback for the triode valve as an oscillator.
FIGURE 3.2 Generation of Ultrasonic waves using the effect of magnetostriction
The frequency of the oscillator can be adjusted by changing the capacitance of the condenser C. A magnetostriction generator produces ultrasonic waves of comparatively low frequency, upto 200 kHz.
3. Piezoelectric Generator
For generating ultrasonic waves of high frequency (about 50 MHz) a generator using the piezoelectric effect is employed. It is found that when crystals of some materials such as quartz, tourmaline, rocksalt etc. are subjected to a mechanical pressure in a certain direction, each charges of opposite sign develop as their faces, normal to the direction of the direction of the applied pressure. This phenomenon is known as the piezoelectric effect.
Referring to figure 3.3 which is a cross sectional view of a quartz crystal, if pressure is applied along the axis x2-x2 electrical charges appear on the faces ab and a’b’ conversely if two opposite faces of a crystal are subjected to a potential difference (in order to provide charge on them), a tensile pressure appears on the crystal. This pressure alters the length of the pressure alters the crystal. This pressure alters the length of crystal in that direction. If the applied potential is alternating the crystal, the crystal, the crystal begins to oscillate with a frequency which lies in the ultrasonic range.
FIGURE3.3 Cross-sectional view of a quartz crystal. If pressure is applied along the axis X2-X2, electric charge appears on the faces ab and a’b’
Figure 3.4 shows circuit arrangements that can be used to generate ultrasonic waves by using the piezoelectric effect. A thin slice of quartz crystal R is placed between two metal plates A and B to form a parallel plate capacitor, with the quartz crystal as dielectric. The plates are connected to the terminals of a coil which is inductively coupled to the oscillating circuit of a triode valve. An alternating potential difference developed across the condenser plates due to the electrical circuits. The quartz slab is thus subjected to an alternating electric field which produces alternate contraction and expansion of the slab in the perpendicular direction leading to the oscillation of the crystal.
FIGURE 3.4 Circuit arrangement used to generate ultrasonic waves using piezoelectric effect
The variable condenser C is adjusted so that the frequency of the oscillatory circuit is equal to the natural frequency of one of the modes of vibration of the crystal. This produces resonant mechanical vibrations in the crystal due to the linear expansion and contraction. If one of the faces of the crystal is placed in contact with some medium in which elastic waves can be propagated, ultrasonic waves are generated.