What is Ultrasonic waves || generation of ultrasonic ||uses of ultrasonic

The word 'sound' usually used to mean the stimulus that can produce the word However, in a broader sense the word sound is used to mean vibrational waves in matter having frequenicies in the audible range (20 Hz- 20 kl-Iz) as well as in the inaudible range. The whole spectrum of acoustic energy can be classified into the following four regions----

(i) Infrasonics: Sound waves having frequency below the audible range i.e., below 20 Hz.

(ii) Sonics : Sound waves of frequencies in the audible range 20 Hz-20 kHz.

(iii) Ultrasonics : Sound waves having frequencies beyond the audible limit in the range 20 kHz-1 GHz (1GHz- 10^9Hz).

(iv) Hypersonics: Sound of frequencies higher than 1 GHz. The upper frequency limit of sound waves is determined by the discrete nature of material medium. At very high frequencies when the wavelength of sound becomes smaller than the interatomic distance there is none to share the joy of vibration within a wavelength. Then the material medium cannot follow the input sound and the sound cannot propagate through the medium hearing in human intense ultrasonic waves has opened up wide fields of research Thus ultrasonics is the study of sound waves of frequencies in the rarn 20 kHz to 1 GHz. Ultrasonic vibrations cannot produce the sensation of ar. But many inferior animals can hear them. The rapid development of ultrasonic techniques and our ability to produce powerful and of technical applications in physics, chemistry, biology, medicine and in industry. Ultrasonic energy has been used for the investigation of properti of solids, liquids and gases. Thus it is important to study ultrasonic waves.

Generation of ultrasonic vibration :

There are different types of ultrasonic generators. They are classified according to the form of energy used to excite them into mechanical vibration, the transducer and the medium into which the wave is to be propagated. Here the term 'transducer' is used to mean a device which receives some form of energy from a source, vibrates in the ultrasonic range and supplies the related waves to a suitable medium. Thus we have (i) mechanical generators, (ii) thermal generators, (iii) piezo-electric generators, (iv) magnetostrictive generators, etc. The early methods of generation of ultrasonic vibration using mechanical and thermal generators are hardly used nowadays. Improved and efficient methods of generation of ultrasonic vibration can be obtained by using piezo-electric and magnetostrictive generators.

(1) piezo-electric generator:

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This method is based on piezo-electric effect. When certain non centro symmetric crystals such as quartz, tourmaline, rochelle salt, etc are abjected to mechanical stress in a specific direction an electric potential difference develops in a direction perpendicular to it. This phenomenon is known as piezo-electric effect. The p.d. developed is found to be proportional he mechanical stress and the sign of p.d. changes when compression of crystal changes into tension. The inverse of the effect is also true. Thus n electric field is applied in one specific direction, mechanical strain is uced in a direction perpendicular to it. This is called inverse piezo-electric effect. If the polarity of the applied electric field is reversed the direction of he corresponding mechanical stress is also reversed. This fact can be used for the generation of ultrasonic vibrations. Piezo-electric generators are commonly made from quartz, tourmaline, rochelle salt, ammonium thydrogen phosphate (ADP), barium titanate and ceramics having strong ferroelectric properties.

In a thin plate of piezo-electric crystal fitted with two conducting electrodes is subjected to an alternating vollage source the plate will vibrate mechanically. This vibration in turn produces an electrical signal which nteracts with the voltage source. If the frequency of the applied alternating tage coincides with one of the natural frequencies of the crystal the amplitude the resulting mechanical vibration of the crystal becomes large due to resonance. Then a strong beam of ultrasonic sound is generated.

Application of  Ultrasonics waves:

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(1) Use of directive property:

Ultrasonic waves have some special properties and this has opened e fields of research and of technical applications in physies, chemistry wology, medicine and in industry. Some of the important applications oned below.

Utrasonic waves because of their extremely short wavelength can be obtained in the form of a narrow directional beam. This fact makes it useful n echo-depth-sounding. It is used to map the ocean floor and to detect submerged objects such as submarines, mines and fish. The underlying principle is very simple. An ultrasonic sound pulse is transmitted and the time interval between the transmission and reception of sound distance of the object can thus be measured reflected back from a distant object is noted. As the velocity of sound is known, the distance of the object can thus be transmitted.

The echo-sounding principle can also be used for the detection of law in metal structures. Short ultrasonic pulse is propagated through the metal structure. The pulse gets reflected from the internal flaw, if any. From the time interval between the transmission and reception of the pulse the depth of the flaw can be known.

The echo-sounding principle is used by bats while flying in darkness. Bats emit ultrasonic waves, hear echoes from obstacles and change their course accordingly. In medicine ultrasonic waves are employed in ultrasonography for tumor detection, biological measurements and diagnostic work.

Radio communication under sea water is not possible due to high attenuation. However, Ultrasonic waves can be used for under-water communication. Sound navigation and ranging (SONAR) is such a system of under-sea signalling, primarily for the detection of submarines. The principle of ranging is similar to that used for echo-sounding. The time interval between the transmission of an ultrasonicpulse and the reception of the reflected pulse is measured. From this the distance of the target can be determined. To find the speed and direction of motion of the target a highly directional beam from a rotatable transmitter From knowledge of the distance of the target and the angular position of the transmitter for each received echo it is possible to find the speed and the direction of motion of the target.

(2) Use of Ultrasonic agitations:

The vibration associated with the propagation of high power ultrasonic waves can bring about a variety of physical, chemical and biological changes. For example, particles suspended in liquids and gases can dispersed or coagulated when agitated by powerful ultrasonic waves. Thus it is  possible to mix up immiscible liquids into a homogeneous emulsion and to remove smoke and dust particles from air and other gases.
Ultrasonic waves have germ killing ability. It has been used in sterilising equipments. Milk is made free from bacteria by such equipment.It is also used for fabric cleaning.
 Polymerized and long chain molecules can be disintegrated by using ultrasonic vibrations.

(3) Use of heating effect :

Ultrasonic waves have considerable heating effect. This effect has been used for diathermy. Using a concentrated beam of ultrasonic waves it is possible to destroy the diseased tissue inside the body. This is known as knifeless surgery.