Ultrasonics: Important part-A 2 marks Short Questions and Answers

ANNA UNIVERSITY 'PART‒A' QUESTIONS & ANSWERS

1. How are sound waves classified.

Sound waves are classified into three categories on the basis of frequency.

i. Infrasonics (below 20 Hz)

ii. Audible sound (between 20Hz to 20,000 Hz)

iii. Ultrasound (above 20,000 Hz)

Audible sound is further classified as

i. Musical sound which produces pleasing effect on the ear.

ii. Noises which produces unpleasant effect on the ear.

2. Are the ultrasonic waves electromagnetic waves? Give proper reasons to your answer.

Ultrasonic waves are not electromagnetic waves because they are sound waves, which does not consist electric and magnetic vectors an in electromagnetic waves.

3. Name methods by which ultrasonic waves are produced.

In general, there are three methods by which the ultrasonic waves can be produced viz.,

1. Mechanical generators (Galton's Whistle).

2. Magnetostriction generator (or) Oscillator.

3. Piezo‒electric generator (or) Osciallator.

4. Why not ultrasonics be produced by passing high frequency alternating current through a loud speaker ?

Ultrasonics cannot be produced by passing high frequency alternating current through loudspeaker due to following reasons.

1. Loud speaker coil cannot vibrate at such high frequency.

2. Inductance of the speaker coil becomes so high and practically no current flows through it.

5. Mention the properties of ultrasonics.

(i) They are highly energetic.

(ii) They travel through longer distances.

(iii) They are reflected, refracted and absorbed similar to ordinary sound waves.

(iv) When ultrasonics are passed through liquid, it produces stationary wave pattern and makes the liquid to behave as an acoustical grating element.

(v) It produces heating effect.

6. State any two demerits of the piezoelectric oscillator.

(i) Cutting and shaping the crystal is difficult.

(ii) It is too expensive.

7. Mention some of the engineering / industrial applications of ultrasonics.

1. By oscillating a glass rod with ultrasonic frequency, holes can be drilled in hard steel plates.

2. They are used in welding and soldering.

3. They are used for cleaning cloths and tiny parts of watches.

4. They are used to form alloys of uniform compositions.

8. What is meant by SONAR? Mention two applications of it.

SONAR is a device which stands for SOund NAvigation and Ranging. The principle of SONAR is based on the echo sounding technique of ultrasonics. It is the acoustical technique for locating the objects like submarine or icebergs in sea, by transmitting a high frequency sound pulse and receiving it after reflection from that object.

Applications:

i. To find the depth of sea.

ii. To detect the submarines.

9. Mention the any three applications of ultrasonics. (or) Explain in detail, any four applications of ultrasonics.

The following are some of the applications of ultrasonics

1. Sound signalling.

2. Measurement of the depth of the sea

3. Detection of flaws in materials.

4. Drilling, welding and grinding

5. Speeding up the chemical reaction (coagulation)

6. Relieving body pains.

7. Cleaning and drying the clothes.

10. How are Ulrasonic waves used to measure the depth of sea?

Depth sounding: Echo sounding is the principle used to find the depth of the sea. A beam of ultrasonics is directed towards the bottom of the sea and the reflected signal is received as shown in Fig. .

The time interval between the transmitted and received signal is noted and let it be 't'. If 'v' is the velocity of the ultrasonics, then

Velocity = Distance travelled / Time taken

From Fig. 5(a),, we can write

 v  = [PR+ RQ]  / t = 2RO (approx) / t

RO = Depth of sea = vt / 2

The depth of the sea can be directly calibrated using the instrument called as Fathometer (or) Echometer.

11. How is acoustic grating formed?

When ultrasonic waves are passed through a liquid, the density of the liquid varies layer by layer due to the variation in pressure and hence the liquid will act as a diffraction grating, so called acoustical grating. Under this condition when a monochromatic source of light is passed through the acoustical grating, the light gets diffracted. Then by using the condition for diffraction, the velocity of ultrasonic waves can be determined.

12. What is meant by cavitation? What is its use.

Cavitation is the process of creation and collapse of bubbles, due to the principle of negative local pressure created inside the bubble.

Uses:

i) It is used to produce shock waves.

ii) It is used to increase the temperature in liquids.

13. What is meant by piezo‒electric effect?

When potential difference (or) e.m.f. is applied along certain axis (electrical axis) perpendicular to optic axis of the piezo electric crystals then, the crystal starts vibrating along the optic axis (mechanical axis) of the crystal. This effect is known as inverse piezo‒electric effect.

14. What is meant by inverse piezo‒electric effect?

When potential difference (or) e.m.f. is applied along certain axis (electrical axis) perpendicular to optic axis of the piezo electric crystals then, the crystal starts vibrating along the optic axis (mechanical axis) of the crystal. This effect is known as inverse piezo‒electric effect.

15. Why are ultrasonic waves not audible to humans?

The audible range of frequencies for human beings is between 20Hz to 20,000 Hz. Since the frequency of ultrasonic wave is having above 20,000 Hz. it is not audible to humans.

ADDITIONAL PART‒A QUESTIONS AND ANSWERS

1. What is meant by Ultrasonics?

Ultrasonics are the sound waves of frequency above audible range i.e., above 20,000 Hz. This sound wave cannot be heard by human ear, but it has many useful applications in engineering and medical fields.

2. What are the methods used to produce ultrasonics?

There are three methods used to produce ultrasonics. viz.,

(i) Mechanical generator (or) Galton's whistle.

(ii) Magnetostriction oscillator method.

(iii) Piezo‒electric oscillator method.

3. Mention some of the chemical applications of ultrasonics.

i. They are used to increase the sensitivity of colour in photographs by dispersion of dye in the emulsion.

ii. They are used to remove air bubbles in the liquid metals and convert them into fused metals.

iii. Low frequency ultrasonics are used in sorting paper fibers from the paper pulp.

4. Explain Doppler effect?

There is an apparent change in frequency of the sound waves emitted from the source, when there is a relative motion between the source and observer. This effect is called Doppler effect and the shift in frequency is called as doppler shift.

5. What is principle used for finding the velocity of ultrasonics using acoustical grating.

Principle: When ultrasonics are passed through a liquid like kerosene contained in a tank, due to variation in pressure, the liquid act as acoustical grating. Now when monochromatic source of light is passed through the acoustical grating, it produces different orders of spectrum due to diffraction. Using diffraction condition we can find the velocity of ultrasonics i.e. v = v_uλ_u. where v_u → frequency of ultrasonics and λ_u → wavelength of ultrasonics.

6. What are the applications of ultrasonics in industries?

(i) Ultrasonics are used in cutting, drilling, welding, soldering etc.

(ii) They are used to increase the sensitivity of colour in photographs by dispersion of dye in the emulsion.

(iii) They are also used to remove air bubbles in the liquid metals and convert them into fused metals.

(iv) Low frequency ultrasonics are used in sorting paper fibers from the paper pulp.