The human ear can hear the sound waves between 20 Hz to 20 kHz. This frequency range is known as “Audio Frequency Range”. The sound waves having frequencies above this audible range is known as “Ultrasonic Waves” or “Supersonic Waves”. Supersonic waves have the velocities higher than the velocity of sound i.e. more than 1200 km / hour. Ultrasonic waves can not be heard by a human being but a cat or dog may hear them. The wavelengths of ultrasonic waves are very small as compared to audible sound. The sound waves which have frequencies less than 20 Hz range are called “Infrasonic waves”
1.2 Production of Ultrasonic Wave:
The generator of ultrasonic wave is nothing but an oscillator, which can generate these high frequency waves. The ultrasonic waves can be produced by the following methods.
a) Magnetostriction Generator
b) Piezoelectric or Ultrasound Generator
a) Magnetostriction Effect
This effect is very useful in generating ultrasonic waves are known as Magnetostriction effect. This effect was first discovered by Joule in 1847 & subsequently used by G.W Pierre in an oscillator circuit. The Magnetostriction effect is that effect in which a rod of ferromagnetic material such as iron or nickel is placed in a magnetic field parallels to its length a small extension or contraction occurs is known as Magnetostriction effect. This change in length is found to be
i) Proportional to the applied field
ii) Is independent of the direction of the field applied
Fig.1.1 - Variation of length with the applied magnetic field.
Figure shows that the length of nickel & iron rod changes with the impressed magnetic field. This change of length is independent of the sign of the field & only depends on the magnitude of the field & nature of the material. If the rod is placed inside a coil carrying a high frequency D.C. then it suffers the same change in length for each half cycle of alternating current. This results in setting up vibrations in the rod whose frequencies is twice that of alternating current.
Generally amplitude of vibration of rod is small. But if the frequency of the A.C. is equal to the natural frequency of oscillator of the rod then resonance occurs and amplitude of vibration is considerably increased. Hence maximum energy is transferred at resonance of ultrasonic sound waves are emitted from the end of the rod.
An experimental arrangement for producing ultrasonic waves is shown in fig1.2. A steel or nickel rod is clamped at the centre which is permanently magnetized in the beginning by passing a direct current in the coil which is wrapped around the rod. There are two other coils L1 & L2 which are wrapped around the rod. The coil L2 & capacitor is connected in the plate circuit of a triode valve V. The frequency of the oscillating tuned circuit can be varied by the capacitor C. The coil L1 is connected in the grid circuit of the valve. Plate current is read by D.C. millimeter (mA).
The frequency of the plate circuit is same as that of the natural frequency of the rod then resonant vibrations are produced in the surrounding medium. The oscillations are maintained due to the coupling provided by coil L1. This emf acts on the grid & produces an amplified current change in the plate circuit i.e. in the coil L2.. In this way plate current build up to large amplitude with a frequency of the longitudinal vibration of the rod. Thus the vibrations of the rod are maintained.
When the frequency of the circuit is equal to the frequency of the rod then resonance occurs when sound waves of maximum amplitude are generated. By adjusting the length of the rod & the capacity of the condenser high frequency oscillations of different frequencies can be obtained.
b) Piezoelectric Effect
In 1880 two brothers J. Curie & P. Curie discovered that if mechanical...