Chapter 12 . Sound

Class 9 Science Chapter 12 Sound Internal / Example Questions and Answers :

Internal Questions :

1. How does the sound produced by a vibrating object in a medium reach your ear ?

Answer:  Sound is produced by vibrating objects, and the matter or substance through which sound is transmitted is called a medium. Sound travels through a medium from the point of generation to the listener. When an object vibrates, it causes the particles of the surrounding medium to vibrate as well. These vibrating particles do not travel all the way from the vibrating object to the ear. Instead, a particle of the medium in contact with the vibrating object is initially displaced from its equilibrium position, and it then exerts a force on the adjacent particle. Consequently, the adjacent particle is displaced from its position of rest. After displacing the adjacent particle, the first particle returns to its original position. This process continues within the medium until the sound reaches your ear.

Internal Questions :

1. Explain how sound is produced by your school bell.

Answer : The school bell makes sound when a clapper inside it hits the bell's surface. This impact causes the bell to vibrate, creating sound waves. These waves travel through the air to our ears, allowing us to hear the bell ringing. So, when the bell is struck, it vibrates and produces the sound we hear in school.

2. Why are sound waves called mechanical waves?

Answer : A wave is a disturbance that moves through a medium when the particles of the medium set neighbouring particles into motion. They in turn produce similar motion in others. The particles of the medium do not move forward themselves, but the disturbance is carried forward. This is what happens during propagation of sound in a medium, hence sound can be visualised as a wave. Sound waves are characterised by the motion of particles in the medium and are called mechanical waves .
3. Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend ?

Answer :  No, you will not be able to hear any sound produced by your friend on the moon. Sound waves require a medium, such as air or water, to travel through. Since the moon has no atmosphere and therefore no air, there is no medium to transmit sound waves. As a result, sound cannot propagate on the moon, and you will not be able to hear any sound produced by your friend.

Internal Questions :

1. Which wave property determines (a) loudness, (b) pitch?

Answer: (a) Loudness is determined by the amplitude of a sound wave. The greater the amplitude, the louder the sound.

(b) Pitch is determined by the frequency of a sound wave. The higher the frequency, the higher the pitch.

2. Guess which sound has a higher pitch: guitar or car horn?

Answer: The car horn has a higher pitch compared to the guitar .

Example 12.1 A sound wave has a frequency of 2 kHz and wave length 35 cm. How long will it take to travel 1.5 km?

Solution : Here, Frequency,

Wavelength,

We know that speed of the wave = wavelength × frequency

          

 

The time taken by the wave to travel a distance, d of 1.5 km is

 

Thus sound will take 2.14 s to travel a distance of 1.5 km.

Internal Questions :

1. What are wavelength, frequency, time period and amplitude of a sound wave?

Answer:  Wavelength : The distance between two consecutive compressions (C) or two consecutive rarefactions (R) is called the wavelength .

Frequency : The number of complete oscillations per unit time is called the frequency .

 Time period : The time taken by two consecutive compressions or rarefactions to cross a fixed point is called the time period of the wave .

 Amplitude : The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave.

2. How are the wavelength and frequency of a sound wave related to its speed?

Answer: The product of wavelength and frequency of a sound wave is equal to speed.

i.e.,   Speed = wavelength × frequency

 

3. Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/s in a given medium.

Solution:  Here, Frequency, , Speed  m/s and  Wavelength,

We know that ,

    

Therefore, the wavelength of a sound wave is 2 m .

4. A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source of the sound. What is the time interval between successive compressions from the source?

Solution : Here, Frequency, , Wavelength,

We know that ,

 

Therefore, the time interval between successive compressions from the source is 0.002 s .

Internal Questions :

1. Distinguish between loudness and intensity of sound.

Answer: The difference between loudness and intensity of sound are :

Characteristic	Loudness	Intensity
Definition	How our ears perceive the strength or volume of sound.	The actual amount of energy transmitted by sound waves per unit area.
Perception	Louder sounds are perceived as more intense.	Intensity is a physical quantity and is not influenced by perception.
Measured in	Decibels (dB)	Watts per square meter (W/m²) or Pascals (Pa)
Subjective	Subjective and can vary from person to person.	Objective and remains constant for a given sound wave.
Factors Affecting	Depends on both intensity and frequency of sound.	Depends only on the intensity of the sound wave.

Internal Questions :

1. In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature?

Answer: Sound travels the fastest in iron compared to air and water at a particular temperature.

Example 12.2 A person clapped his hands near a cliff and heard the echo after 2 s. What is the distance of the cliff from the person if the speed of the sound, v is taken as 346 m/s ?

Solution:  Here,  346 m/s ,  2 s

Distance travelled by the sound 

In 2 s sound has to travel twice the distance between the cliff and the person.

Hence, the distance between the cliff and the person

Internal Questions :

1. An echo is heard in 3 s. What is the distance of the reflecting surface from the source, given that the speed of sound is 342 m/s ?

Solution: Here,  342 m/s ,  3 s

Distance travelled by the sound

Therefore, the distance of the reflecting surface from the source

Internal Questions :

1. Why are the ceilings of concert halls curved ?

Answer : The ceilings of concert halls are curved so that sound, after reflecting off the curved surface, can reach all corners of the hall, making the music sound better for everyone in the audience.

Internal Questions :

1. What is the audible range of the average human ear?

Answer: The audible range of sound for human beings extends from about 20 Hz to 20000 Hz (one Hz = one cycle/s).

2. What is the range of frequencies associated with
(a) Infrasound?
(b) Ultrasound?

Answer:  (a) Sounds of frequencies below 20 Hz are called infrasonic sound or infrasound .

(b) Sounds of frequencies higher than 20 kHz are called ultrasonic sound or ultrasound.

Class 9 Science Chapter 12 Sound Exercise Questions and Answers :

1. What is sound and how is it produced ?

Answer:  Sound is produced due to vibration of different objects.

When an object vibrates, it sets the particles of the medium around it vibrating . A particle of the medium in contact with the vibrating object is first displaced from its equilibrium position. It then exerts a force on the adjacent particle. As a result of which the adjacent particle gets displaced from its position of rest. After displacing the adjacent particle the first particle comes back to its original position. This process continues in the medium till the sound reaches your ear.

2. Describe with the help of a diagram, how compressions and rarefactions are produced in air near a source of sound.

Answer:  When a vibrating object moves forward, it pushes and compresses the air in front of it creating a region of high pressure. This region is called a compression (C) . This compression starts to move away from the vibrating object.

When the vibrating object moves backwards, it creates a region of low pressure called rarefaction (R) . As the object moves back and forth rapidly, a series of compressions and rarefactions is created in the air.

       

Compression is the region of high pressure and rarefaction is the region of low pressure. Pressure is related to the number of particles of a medium in a given volume. More density of the particles in the medium gives more pressure and vice versa. Thus, propagation of sound can be visualised as propagation of density variations or pressure variations in the medium.

3. Why is sound wave called a longitudinal wave?

Answer: Sound waves are called longitudinal waves because the particles of the medium vibrate or move back and forth in the same direction as the wave travels. Also, the particles of the medium oscillate parallel to the direction of the wave propagation.

4. Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a dark room?

Answer : The characteristic of sound that helps you identify your friend by his voice in a dark room is pitch. Each person has a different pitch or tone to their voice, and this difference allows you to recognize your friend among others.

5. Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why ?

Answer : Flash and thunder are produced simultaneously, but we see the flash before hearing the thunder because light travels much faster than sound. Light moves almost instantly, while sound travels more slowly through the air. The speed of sound also depends on factors like air temperature and density, which can slightly affect the delay between the flash and thunder.
6. A person has a hearing range from 20 Hz to 20 kHz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies? Take the speed of sound in air as 344 m/s .

Solution:  We know that ,

For 20 Hz: Here,

Wavelength

For 20 kHz: Here,

Wavelength

Therefore, the typical wavelengths of sound waves in air corresponding to frequencies of 20 Hz and 20 kHz are approximately 17.2 meters and 17.2 millimeters, respectively.

7. Two children are at opposite ends of an aluminium rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound wave in air and in aluminium to reach the second child.

Solution:  Here, The speed of the sound in air = 346 m/s .

And the speed of the sound in Aluminium is = 6420 m/s .

Let  be the length of the aluminium rod .

We know that,  

 

For air  :  second

For aluminium :   second

 

  

The sound wave will reach the second child faster in aluminium compared to air.

The ratio of the time taken by the sound wave in air to the in aluminium is .

8. The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute?

Solution:  The frequency of a source of sound is the number of vibrations or cycles it completes in one second.

Frequency of the source = 100 Hz

Number of vibrations in a minute = Frequency × 60 = 100 Hz × 60 = 6000 vibrations

Therefore, the source of sound vibrates 6000 times in a minute.

9. Does sound follow the same laws of reflection as light does? Explain.

Answer:  Yes, the sound follows similar laws of reflection as light. When sound waves hit a surface, they bounce back just like light does in a mirror. The angle at which sound hits the surface is equal to the angle at which it bounces off. This is called the law of reflection and applies to both sound and light .

10. When a sound is reflected from a distant object, an echo is produced. Let the distance between the reflecting surface and the source of sound production remains the same. Do you hear echo sound on a hotter day?

Answer: Yes, you may hear an echo on a hotter day. Sound travels faster in warmer air than in colder air. So, on a hotter day, sound waves can travel more quickly to the reflecting surface and back to your ears, which may result in the perception of an echo, provided the distance remains the same.

11. Give two practical applications of reflection of sound waves.

Answer: Two practical applications of the reflection of sound waves are:

(i) Megaphones or loudhailers, horns, musical instruments such as trumpets and shehanais, are all designed to send sound in a particular direction without spreading it in all directions .

(ii) Stethoscope is a medical instrument used for listening to sounds produced within the body, mainly in the heart or lungs. In stethoscopes the sound of the patient’s heartbeat reaches the doctor’s ears by multiple reflection of sound .

12. A stone is dropped from the top of a tower 500 m high into a pond of water at the base of the tower. When is the splash heard at the top? Given, g = 10  and speed of sound = 340 m/s.

Solution: Here,  , and

Using the equation of motion:

  

 

 

So, it takes 10 seconds for the stone to fall from the top of the tower to the water surface.

We have, 

 

Therefore, it takes approximately 1.47 seconds for the sound to travel from the water surface to the top of the tower.

Total time = time for stone to fall + time for sound to travel

Total time = 10 seconds + 1.47 seconds Total time = 11.47 seconds

Thus, the splash is heard at the top of the tower approximately 11.47 seconds after the stone is dropped

13. A sound wave travels at a speed of 339 m/s . If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible?

Solution: Here, Wavelength  , Speed  and Frequency

We know that , Speed of sound = Frequency × Wavelength

    

The frequency of the sound wave is approximately 22,600 Hz.

The audible range for most humans is typically between 20 Hz and 20,000 Hz. Since the frequency of the given sound wave falls within this range, it should be audible to most people.

14. What is reverberation? How can it be reduced?

Answer: The persistence of sound in an auditorium is the result of repeated reflections of sound and is called reverberation.

To reduce reverberation, the roof and walls of the auditorium are generally covered with sound-absorbent materials like compressed fibreboard, rough plaster or draperies. The seat materials are also selected on the basis of their sound absorbing properties.

15. What is loudness of sound? What factors does it depend on?

Answer: Loudness of sound is a physiological response of the ear to the intensity of sound.

The loudness of sound depends on its amplitude, which represents the height of the sound waves, and its intensity, which measures the amount of energy carried by the waves. Greater amplitude and higher intensity produce louder sounds.

16. How is ultrasound used for cleaning?

Answer : Ultrasound is generally used to clean parts located in hard-to-reach places, for example, spiral tube, odd shaped parts, electronic components, etc. Objects to be cleaned are placed in a cleaning solution and ultrasonic waves are sent into the solution. Due to the high frequency, the particles of dust, grease and dirt get detached and drop out. The objects thus get thoroughly cleaned .

17. Explain how defects in a metal block can be detected using ultrasound.

Answer :  Ultrasounds can be used to detect cracks and flaws in metal blocks. The cracks or holes inside the metal blocks, which are invisible from outside reduces the strength of the structure. Ultrasonic waves are allowed to pass through the metal block and detectors are used to detect the transmitted waves. If there is even a small defect, the ultrasound gets reflected back indicating the presence of the flaw or defect .