Acoustic Phenomena - 17+ Short Questions With Answer | Physics Class 12

Physics | Wave Motion
Short Questions with Answer from Acoustic Phenomena
Class: 12

1. How are beats produced? What is beat frequency?

Ans. When two sound waves of nearly the same frequency are travelling in the same medium in the same direction, the intensity of the resultant sound rises and falls regularly a number of times a second. These periodic variations in the intensity of sound due to superposition of two sound waves of slightly different frequencies are called beats and hence in this way they are formed. The number of beats heard per second is called beat frequency.

2. What are ultrasonics and infrasonics?

Ans. Ultrasonics: Longitudinal oscillations and waves of frequency higher than 20 KHz are called ultrasonics. Ultrasonics are produced by suitable materials vibrating at these high frequencies.

Infrasonics: Longitudinal oscillations and waves of a frequency below than 20Hz are called infrasonics. Infrasonics waves can be generated by vibrating crystals i.e. quartz crystals.

3. Distinguish between ultrasonics and supersonics.

Ans. The waves whose frequency is greater than 20 KHz are called ultrasonics, while supersonics are those objects which travel faster than sound i.e. faster than 332 m/s. Jet planes or waves are supersonics.

4. How it is that one can recognize a friend from his voice without seeing him?

Ans. One can recognize a friend from his voice without seeing him. Human voice has a large number of frequencies which are simple multiple of a minimum frequency. It is called fundamental frequency. The frequencies which are simple multiple of the fundamental are called harmonics or overtones. In the voice of different people, different harmonics are present in different number. So by the help of the number and the type of harmonic present in the sound, one can distinguish voice of a friend from other.

5. Why is the roaring of a lion different than the sound of a mosquito?

Ans. The sound produced by a body differs from that produced by another body basically by its pitch. The sound produced by a lion is of low pitch (frequency) but the sound produced by a mosquito is of high pitch. So the roaring of a lion is different than the sound of a mosquito.

6. Why does an empty vessel produce more sound than a filled one?

Ans. We have the relation I α A^2 Where I is the intensity of sound produced by the particles of a medium and A is the amplitude of vibration of the particles. When we strike a vessel filled with water, the water molecules vibrate with small amplitude and hence the intensity of sound is small. But if we strike an empty vessel that contains air in it, the air molecules vibrate with great amplitude and thus the intensity of sound produced is much higher than that produced by water molecules. So, an empty vessel produces more sound than filled one.

7. What do you mean by the term threshold of hearing?

Ans. The lowest intensity of sound energy that can be perceived by our ears is called the threshold of hearing. A sound of given intensity produces different amount of hearing sensation in different ranges of frequency. That’s why; the threshold of hearing also varies with frequency. Moreover, the threshold of hearing at a frequency may vary from ear to ear as well. Hence, form a normal ear, the threshold of hearing has been defined at frequency of 1000 Hz. The threshold of hearing is taken as 10^(-12) watt m^(-2) for a pure ton of frequency 1000Hz. A normal ear can just detect the sound of frequency 1000 Hz if the intensity of the sound is 10^(-12) watt m^(-2).

8. Whistle of an approaching train is shriller. Why?

According to Doppler’s effect, the frequency received by a stationary observer (listener) when the sound source is approaching him/her, is given by 

F1 = {v/(v-ut)} × f

Where v is the velocity of sound, ut is the velocity of train and f is the frequency of the sound emitted by the whistle of the train. From this equation since v> (v-ut) and hence f1 > f. That is, the frequency of the sound received by the stationary listener is higher than the actual frequency of the whistle of the approaching train. Thus, whistle of an approaching train is shriller.

9. Why we can’t hear echo in small room?

Ans. For echo to be heard distinctly, the least distance between the source of sound and the obstacle should be about 17 m. In a small room, the distance between two opposite walls may not be 17 m. That’s why, incident wave and reflected wave interfere there and we can’t hear echo in a small room.

10. What is the good and bad effect of echo?

Ans. When a sharp sound is created and it’s reflected wave returns from a distant to the listener, we say an echo has been heard. Because of echo at reasonable degree, speech and orchestral music are heard clearly. In the absence of echo, the hall at which speech and orchestral music are played would be a ‘dead’ hall like the studio of a radio station or as if one is speaking in an open air.

11. A violin note and a guitar note may have the same frequency yet we can distinguish them just by hearing them, why?

Ans. The quality of a note emitted by a certain instrument depends on the presence or absence of overtones and not on the pitch (frequency) and the loudness (intensity). Thus, sounds from various instruments like violin and guitar may have same pitch and loudness but the quality of the notes emitted by them will be different depending on the number of overtones present in the note.

12. Bells are made of metal and not of wood, why?

Ans. Woods has much higher damping than a metal i.e. the amplitude of vibration goes on decreasing in damped oscillation and it is much higher in the case of wood than metal. The vibrating material for the source of sound must have low damping. Because of this, bells are made of metal and not of wood.

13. Why are all string instruments provided with hollow boxes?

Ans. The loudness of a musical sound is directly proportional to the surface area of the vibrating body. Greater is the area of the vibrating body, larger is the rate of energy transmitted into the medium and hence higher is the loudness. When the strings are set into vibration, the box begins to vibrate. As the box has much area in contact with air, more air is set in vibration. Consequently intensity of sound becomes higher and loud sound is heard.

14. Which has a more direct influence on the loudness of a sound wave: the displacement amplitude or the pressure amplitude? Explain your reasoning.

Ans. Loudness we sense is related to the intensity of sound though it is not directly proportional to it. It also depends upon the sensitivity of the individual ear. For a given frequency, greater the pressure amplitude of a sinusoidal wave, the greater the perceived loudness. Moreover, sinusoidal waves of the same intensity but different frequency have different displacement amplitude but the same pressure amplitude. Hence, the pressure amplitude has a more direct influence on the loudness of a sound wave than the displacement amplitude.

15. An empty vessel sounds more than a filled one. Why?

Ans. We have the relation I α A^2 where I is the intensity of sound produced by the particles of a medium and A is the amplitude of vibration of the particles. When we strike a vessel filled with water, the water molecules vibrate with small amplitude and hence the intensity of sound is small. But if we strike an empty vessel that contains air in it, the air molecules vibrate with great amplitude and thus the intensity of sound produced is much higher than that produced by water molecules. So, an empty vessel produces more sound than filled one.

16. Is there a physical difference between intensity and intensity level of a wave? How are these quantities related?

Ans. The rate of flow of sound (wave) energy through a unit area is called intensity of sound. It is measured in Wm^(-2) The difference in loudness between two sounds is called intensity level. It is measured in bel. The relation between these two quantities is given by 

Intensity level (β)=log(I/Io), here Io is the threshold of hearing.

17. Why is sounding board used in a string instrument?

Ans. The loudness of a musical sound depends upon the surface area of the vibrating body. If only the strings are vibrated then they will cause vibration of less amount of air outside resulting feeble sound. But if, a hollow box is provided then the string as well as air inside the box set into vibration. Due to this reason louder sound is heard.

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