Important Questions and Answers: Acoustic Phenomena

1. What is Doppler’s effect in sound?

Ans: The apparent change in frequency of sound due to the relative motion of sound of observer is called Doppler’s effect in sound. When the source of sound and the listener are in relative motion compared to each other, the listener will feel that the frequency of the sound being produced is different from the original. If the relative distance is increasing, the frequency is found to decrease and if they are approaching, the frequency increases. This phenomena was introduced by Christian Doppler.

2. What do you mean by Red Shift?

Ans: Doppler’s effect in light waves is the causative factor for Red shift. If the source of light moves away from the observer, the frequency of light he/she observes will be less than the actual frequency the source is emitting. This means the wavelength of the light will seem to increase which makes the spectral lines shift towards the red end of the visible spectrum. This is called ‘Red Shift’. It is an important discovery which has helped in explaining the expansion of universe from a strong explosion (Big Bang) and in determining the speed of which stars and other bodies are heading away from earth.

3. What are the differences between Doppler’s effect in sound and that in light?

 Ans: The basic principle of Doppler’s Effect is that when the source of energy is getting farther or nearer compared to the observer, the apparent frequency will be different, greater if the distance is decreasing and smaller if increasing.

Sound waves have lesser frequencies which are perceived by the ears and light has higher frequencies perceived by the eyes. So, Doppler’s effect in sound gives difference in pitch of the sound. Where as in Doppler effect of light, the main aspects involved here is colour.

Doppler’s Effect in sound is not applicable if the velocity of either the source or listener is more than that of sound. Doppler’s effect in light will also be not applicable if their velocities become more than that of light, which has not been observed yet.

 4. Why cannot we hear echo in a small room?

Ans: When sound wave gets reflected from a surface more than 17 meter, the reflected sound appears to be distinct from the original, the phenomenon called as “Echo”. If the distance distance of two bodies is smaller than 17 meter, the reflected sound follows the original sound so closely that they cannot be distinguished. The reflected sound simply appears to be a continuation (tail) of the first. Thus echo cannot exist within such distances. In a small room, the distance is definitely less than 17m. So echo cannot be heard in such rooms.

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

Ans: When two sound waves of nearly same frequency are travelling in the same direction, the intensity of the resultant sound rises and falls periodically and there is the similar changes in loudness of sound. This periodic variation is caused due to the superposition of two sound waves of slightly different frequencies and this phenomena is known as beats.  The number of beats occurring per unit time is called beat frequency.

 6. Define Reverberation.

Ans: If the reflecting surface is less than about 17m, the reflected sound follows the original sound so closely that the listener cannot distinguish them as separate sounds. It feels as if the original sound has been extended. Such phenomenon is called Reverberation.

7. Why are bells made of metal and not of wood?

Ans: Bells are meant to produce specific sounds for long durations of time. This requires that they should have low damping. Since wood produces very high clamping, it rapidly dampens the vibrations. On the other hand, metals have low damping. So, bells are made of metal and not of wood.

8. Why do we hear more clearly in a curtained room than in a non-curtained room?

Ans: This phenomenon is also based on the phenomenon of absorption of sound waves by certain substances. Curtains are soft materials and are able to absorb sound waves. When sound waves are produced in such rooms, there is less chance of reflection, which prevents unnecessary interference and gives clearer sound. If there are no curtains, there will be random reflections and they interfere with the original sound. So, hearing is clearer in curtained room than non-curtained one.

9. A beating drum with a large surface area produces a loud sound as compared to the beating of drums with small surface area. Why?

Ans: This phenomenon is based on the surface area of the vibrating substances. One of the factors on which the loudness of sound depends on is the surface area of the area available for vibration. When drums with large area are used, the vibration of its surface causes the vibration of a large area of air. This makes its sound appear loud. If drums with small area are used, small area of air vibrates and so sound will be feeble.

10. Why temple bells are made larger?

Ans: This phenomenon is based on the surface area of the vibrating substances. One of the factors on which the loudness of sound depends on is the surface area of the area available for vibration. When large bells are used, the vibration of its surface causes the vibration of a large area of air. This makes its sound appear loud. If small bells are used, small area of air vibrates and so smaller sound is produced.

11. A violin note and sitar note may have the same frequency, yet we can distinguish between the two notes. How?

Ans: This phenomenon is based on ‘Quality’ or ‘Timbre’ of sound which depends on the presence or absence of overtones in addition to the fundamental tone present in the note being produced. Any device can not produce totally pure note, it always has overtones. The relative number and intensities of these overtones make sound from the devices different.

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

Ans: This phenomenon is based on the similarity or difference in frequency. The sounds from each people have different frequencies. Even if the frequency is same (very rarely), the other harmonics present in their sound varies, which ultimately will vary the quality or timbre of the sound, making people distinguishable without looking.

13. When a certain person talks over the phone with the microphone strapped with a handkerchief, his/her voice become indistinguishable, why?

Ans: The voice of a human voice is characterized by the number and amplitudes of the harmonics is used, it also comes into vibration and its fundamental frequency also adds up to the sound. So, the overall quality of sound is slightly altered, possible to make is indistinguishable.

14. Can sound waves be polarized?

Ans:  No, polarization is not possible for sound waves ( because of their logitudinal nature).this is because of the very innate nature of sound waves. In a transverse wave, the particlesvibrate in a plane perpendicular to the direction of propagation of energy. So, if appropriate gaps (slits) are used, the vibrations in some planes can ebe limited or totally eliminated, which  makes polarization possible.

However, in longitudinal waves, the vibration occur in (or against) the direction of the propagation of the waves. So, the gaps used there will not limit or eliminate vibrations of any direction. So, polarization is not possible for them.

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

Ans: This phenomenon is related to the ability of air or liquids to vibrate around their mean positions. An empty vessel contains air column in it. When this vessel is struck then the air column comes into vibrations. Since air molecules are loosely held, they can vibrate around with large amplitude, which makes the sound large. This is also given by the relation, I ∝ A2, where I is the intensity and A is the area of the vessel. So, vessel with more area produces larger sound.

 16. Why is roaring of a lion different from the sound of a mosquito?

Ans: The main points of difference here are in the pitch and loudness. The sound of mosquito has a higher pitch (determined by frequency) than the roaring of a lion. In contrast, the lion roars loudly (with high amplitude) than a mosquito.

17. Why is a given sound louder in a hall (large closed room) than in the open?

 Ans: When a sound is produced in a hall, the waves are reflected repeatedly from the walls and ceiling. The reflected sounds produce constructive interference with the direct sound at different locations; as a result the intensity of sound increases. However, in open space, there are no such reflections and the intensity of sound will be low there.

18. What would have happened if we have had only one ear?

 Ans: When there are two ears, there exists a phase difference or path difference between the two waves reaching the two ears. When the same sound is perceived by the two ears, sound waves will travel larger distance to reach the farther ear compared to that needed for the nearer ear. This introduces certain path or phase difference between the two waves. Such difference helps in determining the direction of the source. Such phenomenon is called “Binaural effect”. If there is only one ear, that is not possible.

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