1.How are sound wave classified?
Sound waves are classified into three categories on the basis of frequency.
i. Ultrasonic waves (above 20,000 Hz )
ii. Audible waves (20 Hz to 20,000 Hz )
iii. Infrasonic waves (below 20 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. What are the characteristics of musical sound?
Musical sound is characterized by
- Pitch or frequency
- Quality or timbre
- Loudness or intensity
3. What is loudness?
The characteristic of sound due to which a sound appears to be feeble or loud is called Loudness. Degree of sensation produced in our ears.
It’s unit is Phon and Sone.
4. Define intensity of sound.
It is defined as the amount of sound energy flowing per unit area per second held normally to the direction of propagation of sound wave. The unit of Intensity is Watt / m2 .
5. Distinguish between loudness and intensity of sound.
| S.no | Loudness | Intensity |
| 1. | It is the amount of sensation produced in the ears. | It is the amount of sound energy flowing per unit area per second held normally to the direction of propagation of a sound. |
| 2. | It is a psychological quantity. | It is a physical quantity. |
| 3. | It is a very difficult to measure. | It can be measured easily and accurately. |
| 4. | Unit : Bel, decibel | Unit : Watt / m2 |
6. What is a decibel?
The logarithmic ratio of intensity of a sound to the standard intensity is called as intensity level or relative intensity (IL ).
\text { Sound intensity level }\left(I_L\right)=\frac{\text { Intensity of sound }(I)}{\text { Standard intensity of sound }\left(I_0\right)}7. State Weber-Fechner law in sound.
According to Weber-Fechner law, the loudness of a sound wave is proportional to the logarithm of its intensity.
(i. e) L α log I
L = K log I
where K is the proportional constant.
8. What is reverberation?
“Reverberation is defined as the persistence or prolongation of sound even after the source is cut off”.Due to these multiple reflections, the intensity of the sound wave decreases and finally the sound becomes inaudible. In this process the listener has a tendency of existence of sound for some time even after the source has stopped its sounding. This is called Reverberation.
9. What is reverberation time?
It is defined as the time taken by the sound wave to fall below the minimum audibility measured from the instant (i.e., from the maximum) when the source stopped sounding.
10. What is standard reverberation time?
It is defined as the time taken by the sound wave to fall to one millionth of its original intensity even after the source is cut off.
11. Define absorption co-efficient of a material?
The absorption coefficient of a material is defined as the ratio of the sound energy absorbed by some area of the surface to that of the same amount of sound energy absorbed by the same area of an open window.
\alpha=\frac{\text { sound energy absorbed by } 1 \mathrm{~m}^2 \text { of the surface }}{\text { sound energy absorbed by } 1 \mathrm{~m}^2 \text { open window }}12. What are the acoustical factors to be considered while we construct any buildings.
- Reverberation and Optimum Reverberation time
- Loudness
- Focussing due to walls and ceilings
- Echo
- Echelon effect
- Resonance
- Extraneous noises.
13. What are echoes? How is it avoided?
When the reflecting surfaces is far away from the source (more than 17 metres) then the sound is reflected back as a distinct repetition of direct sound, this reflected sound is called echo.
Remedy
Echoes can be avoided by covering walls and ceiling with suitable sound absorbing materials.
14. What is echelon effect? Mention the remedy.
“The echoes are formed when the time interval between direct and reflected waves are at about 1/ 15th of a second. The regular repletion of echoes of the original sound to the observer is called as Echelon effect. This should be avoided.
Remedies
a. Avoiding such structures
b. Covering the staircase with carpets.
15. What is noise? How is it classified?
The unwanted sound waves to produce adverse effect to our health is called noise. There are 3 types of noises they are,
a. Inside noise
b. air borne
c. Structure borne
16. What is meant by Ultrasonics?
Ultrasonics is the science and application of sound waves above the limits of human audibility. Sound waves having frequencies greater than 20KHz called ultrasounds or supersonic waves.
17. Mention the properties of ultrasonics.
a. They are highly energetic.
b. They travel through longer distances.
c. They are reflected, refracted and absorbed similar to ordinary sound waves.
d. When ultrasonics are passed through liquid, it produces stationary wave pattern and makes the liquid to behave as an acoustical grating element.
e. It produces heating effect.
18. What are the methods used to produce ultrasonics?
There are three methods used to produce ultrasonics.
- Mechanical oscillator (Galton’s whistle).
- Magnetostriction oscillator method.
- Piezo – electric oscillator method.
19. 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 as in electromagnetic waves.
- Ultrasonic waves travel with velocity of sound (i.e.,330 m/s ). But Electromagnetic waves travel with the velocity of light (3 x 108 m /s).
20. Why not ultrasonics be produced by passing high frequency alternating current through a loud speaker?
a. Loud speaker coil cannot vibrate at such high frequency.
b. Inductance of the speaker coil becomes so high and practically no current flows through it.
21. What is the principle behind the magnetostriction oscillator?
22. What is meant by piezo-electric effect?
When pressure or mechanical force is applied along the optic axis (mechanical axis) of the crystals like quartz, tourmaline, rochelle y salt etc., then equal and opposite charges are produced along the axis (electrical axis) perpendicular to the optic axis of the crystal. This effect is called piezo-electric effect.
23. What are the differences in the quality of ultrasonics waves produced by piezo electric and magnetostriction methods?
| S.NO | Piezo electric method | Magnetostriction method |
| 1. | It generates very high frequency ultrasonic waves (500 MHz) | It generates low frequency ultrasonic waves( 3 MHz) |
| 2. | We can obtain constant frequency of ultrasonic waves. | We cannot obtain constant frequency of ultrasonic waves. |
| 3. | The peak of resonance curve is narrow. | The peak of resonance curve is broad. |
| 4. | Frequency of oscillation is independent of temperature. | Frequency of oscillation is depends on temperature. |
24. What is meant by inverse piezo-electric effect?
When potential difference 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-elecric effect.
25. What is the principle used for finding the velocity of ultrasonics using acoustical grating.
When ultrasonics are passed through a liquid like kerosene contained in a tank, due to variation in pressure, the liquid act as acoustic grating. Now when monochromatic source of light is passed through the acoustic grating, it produces different orders of spectrum due to diffraction. Using diffraction condition we can find the velocity of ultrasonics.
i.e. ν = νu λu
where νu→ frequency of ultrasonics and λu → wavelength of ultrasonics.
26. Compare destructive and non-destructive testing.
| S.NO | Destructive testing | Non – destructive testing |
| 1. | Tested parts are damaged during testing. | Tested parts are not damaged during testing. |
| 2. | This method is applied only to a sample. | This method is applied directly on production items. |
| 3. | Time consumption is high. | Time consumption is low. |
| 4. | Equipment and labour costs are high. | Labour costs are low. |
27. What is NDT method?
NDT is the method of testing the specimen without impairing or changing it usefulness for future service. It is used to examine the material and to detect the flaws present in the material without damaging it.
| Read More Topics |
| Crystal Physics – Questions and Answers |
| Acoustics – Questions and Answers |
| Properties of Matter – Questions and Answers |
