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.
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.
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.
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.
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
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.
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.
When the battery is switched ON the current is passedthrough the coils L1 and L2 of the primary circuit. Thebase circuit produces alternating voltage withfrequency is f = ½ π√L1C1 Due to transformer action, the current is transferredto the secondary circuit and fed to the electrodes Aand B.Now the crystal is under high frequency alternatingvoltage.
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
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.DemeritsCost is high.Cutting and shaping of crystal are complicated.
s.no Magnetostriction method Piezo – electric method1 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.
Kunds tube methodSensitive flame methodThermal methodPiezo electric detector
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.