2. Infrasonics
The frequency of Sound waves below 20 hertz is called
infrasonic range. These waves are called as infrasonic
waves.
Ultrasonics
The frequency of Sound waves above 20,000 hertz is
called ultrasonic range. These are supersonic waves and
generally called as ultrasonic waves.
4. It is the principle of producing ultrasonic
waves. (i.e.,) when an alternating magnetic
field is applied to a rod of ferromagnetic
material such as nickel, iron, cobalt etc., alloys
of it, then the rod is thrown into longitutional
vibrations thereby producing ultrasonic waves
at resonance.
5.
6. The rod is permanently magnetised in the beginning
by passing direct current.
The battery is switched ON.
The current is passed through the transistor.
This current is passed through the coil L1, which
causes a corresponding change in the magnetisation of
the rod.
The rod starts vibrating due to magnetostriction effect.
When a coil is wounded over a vibrating rod then e.m.f
will be induced in the coil,(i.e.,) called as converse
magnetostriction effect.
7. The induced e.m.f is fed to the base of the transistor,
which act as a fedback continuously.
The current in the transistor is built up and the
vibrations of the rod is maintained.
The frequency of the oscillatory circuit is adjusted by
the condenser (C1).
when this frequency is equal to the vibrating rod,
resonance occurs.
At resonance the rod vibrates longitudinally with
larger amplitude producing ultrasonic waves of high
frequency along the ends of the rod.
8. Frequency of the ocillatory circuit = frequency of the
vibrating rod.
½ π√L1C1 = P/2l . √E/ρ
Where,
“l” length of the rod
“E” youngs modulus of the rod
“ρ” density of material of the rod
9. Merits
It is mechanically versatile
Cost is low.
It can produce large acoustical power with high efficiency.
Demerits
It can produce frequencies upto 3 MHz only
It is noy possible to get a constant single frequency,
because it depends on the temp and the degree of
magnetisation.
As the frequency is inversely proposional to the length of
the vibrating rod, to increase the frequency, the length of
the rod should be decreased which is practically
impossible.
10. When one pair of opposite faces of
crystals like quartz, rochelle salt,
tourmaline etc., are strectched (or)
compressed by mechanical pressure,
an electric potential difference is
developed on the other pair of
opposite faces of the crystal . This
effect is called piezo- electric effect.
11.
12. When the battery is switched ON the current is passed
through the coils L1 and L2 of the primary circuit. The
base circuit produces alternating voltage with
frequency is
f = ½ π√L1C1
Due to transformer action, the current is transferred
to the secondary circuit and fed to the electrodes A
and B.
Now the crystal is under high frequency alternating
voltage.
13. Due to inverse piezo – electric effect, the crystal starts
vibrating along the mechanical axis of the crystal.
The frequency of the base circuit is adjusted by the
variable capacitor C1.
If the frequency of alternating voltage is equal to the
frequency of the vibrating crystal, resonance occur. At
resonance the crystal vibrates and ultrasonic waves are
produced along both the ends of the crystal.
n = P/2l . √E/ρ
14. Merits
It can generate frequency upto 500 MHz.
Generation of frequency of ultrasonic waves does not
depand on temperature and humanity.
It produces constant frequency output.
It is more efficient than magnetostriction oscillator.
Demerits
Cost is high.
Cutting and shaping of crystal are complicated.
15. s.no Magnetostriction method Piezo – electric method
1 We cannot obtain constant frequency We can obtain constant frequency of
of ultrasonic waves. ultrasonic waves.
2 It generates low frequency of It generates very high frequency of
ultrasonic waves( 3 MHz) ultrasonic waves( 500 MHz)
3 The peak of resonance curve is broad. The peak of resonance curve is
narrow.
4 Frequency of oscillation depends on Frequency of oscillation independent
temperature. of temperature.
17. 1. They are highly energetic.
2. They travel through longer distances.
3. They are reflected, refracted and absorbed similar to
ordinary sound waves.
4. When ultrasonics are passed through the liquid, it
produces ststionary wave pattern and makes the liquid to
behave as acoustical grating element.
5. When an object is exposed to ultrasonics for a longer
time it produces heating effect.
