Chapter 12 . Sound

Internal Questions :

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

Answer: When an object vibrates and produces sound, the sound waves travel through the air and enter your ear. The sound waves make your eardrum vibrate, which passes the vibrations to tiny bones in your ear. These bones amplify the vibrations and send them to your inner ear. Your inner ear converts the vibrations into signals that your brain understands as sound.

Internal Questions :

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

Answer: The school bell produces sound through mechanical vibrations. When an external force, such as a clapper or electromagnet, strikes the bell, it sets the metal in motion. This motion creates compressions and rarefactions in the surrounding air, forming sound waves. These sound waves travel through the air, reaching our ears, where they are detected and interpreted by our brain as the sound of the school bell ringing.
2. Why are sound waves called mechanical waves?

Answer: Sound waves are called mechanical waves because they require a material substance (medium) to travel through, such as air or water. They need particles to physically vibrate and transfer energy from one particle to another, unlike waves like light that can travel through empty space.
3. Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend ?

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

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 :

The loudness is a physiological response of the ear to the intensity of sound.

 The intensity of sound is the amount of sound energy passing each second through unit area .

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 to help improve the acoustics or sound quality in the hall. The curved shape helps to distribute sound waves more evenly and allows them to bounce off the ceiling and walls in a controlled manner. This helps to create a more immersive and balanced sound experience for 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.

Example 12.3 A ship sends out ultrasound that returns from the seabed and is detected after 3.42 s. If the speed of ultrasound through seawater is 1531 m/s, what is the distance of the seabed from the ship ?

Solution: let d is the depth of the sea.

Here,   3.42 s ,   1531 m/s

Distance travelled by the ultrasound

 

Thus, the distance of the seabed from the ship is 2618 m or 2.62 km.

Internal Questions :

1. A submarine emits a sonar pulse, which returns from an underwater cliff in 1.02 s. If the speed of sound in salt water is 1531 m/s, how far away is the cliff ?

Solution:  let d be the distance between the cliff and submarine .

Here, 1531 m/s

The distance travelled by sonar pulse  speed of sound  time

A/Q,   

 

Therefore, the distance between the cliff and submarine is 780.81 m .

Exercises :

1. What is sound and how is it produced?

Answer:  Sound is a form of energy that we can hear. It is produced when an object vibrates or moves back and forth rapidly. When an object vibrates, it causes the particles in the surrounding medium, like air, to also vibrate. These vibrations create waves of compressed and rarefied air particles, known as sound waves. These sound waves travel through the air or other mediums and reach our ears, allowing us to perceive the sound.

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

Answer: Photo
3. Cite an experiment to show that sound needs a material medium for its propagation.

Answer:  One simple experiment to demonstrate that sound needs a material medium for its propagation is the following:

Take a ringing alarm clock or a musical instrument, such as a guitar, and place it in a glass jar with an airtight lid. Make sure the alarm clock or instrument is audible when placed inside the jar.

Close the lid tightly and try to listen to the sound. You will observe that the sound becomes fainter or almost inaudible. This is because the sound waves produced by the alarm clock or instrument require a material medium, like air, to travel through and reach our ears. When the jar is sealed, it prevents the sound waves from propagating effectively through the air inside the jar, resulting in a decrease in sound intensity or complete silence.

4. 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. In other words, the particles of the medium oscillate parallel to the direction of the wave propagation.

5. 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 the unique quality or tone of his voice. Each person has a distinct vocal tone, pitch, and timbre that distinguishes them from others.
6. Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why?

Answer: Thunder is heard a few seconds after the flash is seen because sound travels slower than light. Light travels very fast, so we see the flash instantly. However, sound waves take more time to reach our ears, so we hear the thunder a few seconds later.
7. 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.
8. 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.

Answer: 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 time taken in aluminium will be less than 1.
9. 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.

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

Answer: Yes, sound follows the same laws of reflection as light. The angle of incidence of a sound wave is equal to the angle of reflection when it encounters a reflecting surface. The reflected sound wave bounces off the surface, obeying the principles of reflection. These laws allow us to hear echoes and determine the direction of sound sources through reflection.

11. 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. The temperature does not significantly affect the production of echoes. However, the speed of sound can be slightly higher on hotter days due to the increase in temperature. This may cause a slight difference in the time it takes for the echo to reach your ears, but it would not eliminate the occurrence of an echo.
12. Give two practical applications of reflection of sound waves.

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

Echoes: Echoes are created when sound waves reflect off surfaces and reach our ears after a short delay. This phenomenon is utilized in various applications such as determining distances using echolocation, creating special sound effects in music or movies, and in architectural designs for enhancing acoustics in concert halls or theaters.

Sonar Systems: Sonar (Sound Navigation and Ranging) systems use the reflection of sound waves to detect and locate objects underwater. A sound wave is emitted into the water, and when it encounters an object, it reflects back to the source. By analyzing the time it takes for the reflected sound wave to return and its characteristics, sonar systems can determine the presence, distance, and position of objects underwater, making it valuable for navigation, mapping the ocean floor, and locating underwater structures.

13. 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: , 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

14. 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.

15. 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.

Reverberation can be reduced by using sound-absorbing materials in the room. These materials, such as acoustic panels or curtains, can absorb the sound waves and prevent excessive reflections. Additionally, adding carpets, drapes, or furniture to a room can help reduce reverberation by absorbing sound energy. Another method is to design the room with curved or irregular surfaces that scatter sound waves rather than reflecting them directly back.

16. 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 a few factors are :

Amplitude: The amplitude of a sound wave determines its loudness. A larger amplitude corresponds to a louder sound, while a smaller amplitude corresponds to a softer sound.

Distance: The distance between the sound source and the listener affects the loudness. Sound waves spread out as they travel, so the farther away you are from the source, the softer the sound will be.

Environment: The environment in which the sound is heard can also impact its loudness. Factors like the presence of obstacles or the reflective properties of surfaces can influence how the sound waves propagate and reach the listener.

17. Explain how bats use ultrasound to catch a prey.

Answer: Bats use ultrasound to catch prey through a process called echolocation. They emit high-frequency sound waves, which bounce off objects and return as echoes. By listening to these echoes, bats can determine the distance, direction, and size of objects, including potential prey. They interpret the time it takes for the echoes to return and make adjustments in flight to accurately capture their prey. This unique ability allows bats to navigate and hunt successfully, even in the dark.

18. How is ultrasound used for cleaning?

Answer: Ultrasound is used for cleaning through a process called ultrasonic cleaning. It involves using high-frequency sound waves, typically above the range of human hearing, to create microscopic bubbles in a liquid cleaning solution. These bubbles collapse rapidly near the objects being cleaned, producing a scrubbing action that removes dirt, grease, and contaminants from the surfaces. Ultrasound is commonly used for cleaning delicate items, jewelry, medical instruments, and industrial parts.
19. Explain the working and application of a sonar.

Answer: Sonar, an acronym for "Sound Navigation and Ranging," is a technology that uses sound waves for underwater navigation and object detection. A sonar system emits sound waves into water, and when these waves encounter objects, they bounce back as echoes. By analyzing the time it takes for echoes to return, the distance to objects can be determined. Sonar finds applications in various fields, including maritime navigation, submarine detection, fish finding, underwater mapping, and scientific research in marine environments.
20. A sonar device on a submarine sends out a signal and receives an echo 5 s later. Calculate the speed of sound in water if the distance of the object from the submarine is 3625 m.

Solution:  Here,  and

We have,

Speed of sound

Therefore, the speed of sound in water, based on the given information, is 725 m/s.

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

Answer: Ultrasound can be used to detect defects in a metal block through a technique called ultrasonic testing. In this method, high-frequency sound waves are directed into the metal block using a probe or transducer. These sound waves travel through the material and bounce back when they encounter any internal defects or boundaries. By analyzing the echoes and their characteristics, such as amplitude and time of flight, technicians can identify and locate defects such as cracks, voids, or inclusions within the metal block.
22. Explain how the human ear works.

Answer: The human ear is an organ that allows us to hear sounds. It consists of three main parts: the outer ear, middle ear, and inner ear.

Outer Ear: The outer ear consists of the visible part called the pinna and the ear canal. The pinna collects sound waves from the environment and directs them into the ear canal.

Middle Ear: The ear canal leads to the middle ear, which contains three small bones called the ossicles: the hammer (malleus), anvil (incus), and stirrup (stapes). These bones amplify and transmit sound vibrations from the eardrum to the inner ear.

Inner Ear: The inner ear contains the cochlea, a snail-shaped structure filled with fluid. Sound vibrations cause the fluid in the cochlea to move, stimulating tiny hair cells. These hair cells convert the movement into electrical signals that are sent to the brain through the auditory nerve.

Auditory Nerve and Brain: The electrical signals from the hair cells travel through the auditory nerve to the brain. The brain processes these signals and interprets them as sounds, allowing us to perceive and understand what we hear.