Sound

Activity 11.1 Questions: “• Take a tuning fork and set it vibrating by striking its prong on a rubber pad. Bring it near your ear.
• Do you hear any sound?
• Touch one of the prongs of the vibrating tuning fork with your finger and share your experience with your friends.
• Now, suspend a table tennis ball or a small plastic ball by a thread from a support. Touch the ball gently with the prong of a vibrating tuning fork. Observe what happens and discuss with your friends.”

Answer:

  • Yes, when you strike the tuning fork and bring it near your ear, you hear a sound.
  • When you touch a vibrating prong of the tuning fork with your finger, you feel it shaking or vibrating.
  • When you bring the vibrating tuning fork close to the suspended ball, the ball also moves. This shows that the fork’s vibrations make the ball move, proving that sound is produced by a vibrating object.

Activity 11.2 Questions: “• Fill water in a beaker or a glass up to the brim. Gently touch the water surface with one of the prongs of the vibrating tuning fork. Next, dip the prongs of the vibrating tuning fork in water. Observe what happens in both cases. Discuss with your friends why this happens.”

Answer:

  • When you gently touch the water surface with a vibrating tuning fork, small ripples (waves) form on the water.
  • When you dip the vibrating prongs into the water, you see water splashing or droplets jumping up.
  • This happens because the fork’s vibrations pass to the water, causing it to move. Thus, sound is caused by vibrations, and these vibrations can pass into other substances, like water.

Activity 11.3 Question: “• Make a list of different types of musical instruments and discuss with your friends which part of the instrument vibrates to produce sound.”

Answer:

  • Guitar: Its strings vibrate.
  • Tabla: Its stretched membrane (skin) vibrates.
  • Flute: The air column inside the flute vibrates.
  • Drum: Its stretched membrane vibrates.
  • Harmonium: The metal reeds inside vibrate.

From these examples, we see that whether it is a string, a membrane, an air column, or a reed, something must vibrate to produce sound.


(In-text Questions after Section 11.2)

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

Answer:
When an object vibrates, it makes the nearby particles of the medium (like air) start to vibrate. These vibrations move from one particle to another, traveling through the medium. Finally, these vibrations reach our ears, making our eardrums vibrate and allowing us to hear the sound.

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

Answer:
When the school bell is struck, the metal bell starts vibrating. These vibrations make the nearby air particles vibrate, and this vibration travels through the air as sound. When it reaches our ears, we hear the bell’s ring.

  1. “Why are sound waves called mechanical waves?”

Answer:
Sound waves need a material to travel through, like air, water, or solid objects. They move by causing the particles of the material to vibrate. Because they need a medium and involve the motion of particles, they are called mechanical waves.

  1. “Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?”

Answer:
No, because there is no air or atmosphere on the moon. Sound needs a medium (like air) to travel. Without a medium, sound cannot travel, so you cannot hear your friend’s voice on the moon.


Activity 11.4 Questions: “• Take a slinky. Ask your friend to hold one end. You hold the other end. Stretch the slinky. Give it a sharp push towards your friend. What do you notice? If you move your hand pushing and pulling the slinky alternatively, what will you observe? If you mark a dot on the slinky, you will observe that the dot moves back and forth parallel to the direction of the wave. Discuss this with your friends.”

Answer:

  • When you push or pull the slinky, you see the coils move back and forth along the length of the slinky.
  • A marked dot on the slinky just moves forward and backward but doesn’t move along the entire slinky.
  • This shows that in such waves (longitudinal waves), the particles of the medium move back and forth parallel to the direction of the wave.

(In-text Questions after Section 11.2.2)

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

Answer:
(a) Loudness is determined by the amplitude of the sound wave.
(b) Pitch is determined by the frequency of the sound wave.

  1. “Guess which sound has a higher pitch: guitar or car horn?”

Answer:
A guitar’s sound usually has a higher pitch compared to a car horn. A car horn often has a lower pitch sound.


(In-text Questions after Example 11.1)

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

Answer:

  • Wavelength: The distance between two similar parts of a wave (like two compressions) is the wavelength.
  • Frequency: The number of complete waves (or vibrations) passing a point each second is the frequency.
  • Time period: The time it takes for one full wave (one complete vibration) to pass a point is the time period.
  • Amplitude: The maximum height of a wave from its middle (rest) position is the amplitude.
  1. “How are the wavelength and frequency of a sound wave related to its speed?”

Answer:
Speed = Wavelength × Frequency. If you know the wavelength and frequency, you can find the speed.

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

Answer:
Wavelength = Speed / Frequency = 440 m/s ÷ 220 Hz = 2 m.

  1. “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?”

Answer:
Time period = 1 / Frequency = 1/500 seconds = 0.002 seconds.
So, successive compressions reach 0.002 seconds apart.


(In-text Question after section on Loudness and Intensity)

  1. “Distinguish between loudness and intensity of sound.”

Answer:

  • Loudness: How strong or weak a sound seems to a listener. It depends on the amplitude and how our ears respond.
  • Intensity: The amount of sound energy passing per second through a certain area. It is a physical measure, not just what we feel.

(In-text Question after Speed of Sound in Different Media)

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

Answer:
Sound travels fastest in iron, slower in water, and slowest in air.


(In-text Question after Reflection of Sound)

  1. “Why are the ceilings of concert halls curved?”

Answer:
Ceilings of concert halls are curved so that when sound hits them, it reflects and spreads evenly throughout the hall. This helps everyone in the audience hear the sound clearly.


(In-text Questions after discussing Audible, Infrasonic and Ultrasonic Ranges)

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

Answer:
The human ear can usually hear sounds from about 20 Hz to 20,000 Hz.

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

Answer:
(a) Infrasound: Less than 20 Hz.
(b) Ultrasound: Greater than 20,000 Hz.


(Exercise Questions)

  1. “What is sound and how is it produced?”

Answer:
Sound is a form of energy that we hear. It is produced when something vibrates, causing the particles around it to move back and forth.

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

Answer:
When an object vibrates, it pushes nearby air particles. This creates a region where particles are close together (compression). When it moves back, it leaves a region with fewer particles (rarefaction). As the object keeps vibrating, a pattern of compressions (C) and rarefactions (R) forms and moves through the air.

  1. “Why is sound wave called a longitudinal wave?”

Answer:
Sound waves are called longitudinal waves because the particles of the medium (like air) move back and forth in the same direction as the wave travels.

  1. “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 called the quality or timbre of the sound helps us identify a friend’s voice because every person’s voice is unique.

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

Answer:
Light travels much faster than sound. So, we see the flash immediately. The sound travels slower, so it reaches us later.

  1. “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.”

Answer:
For 20 Hz: Wavelength = 344/20 = 17.2 m
For 20 kHz: Wavelength = 344/20000 = 0.0172 m

  1. “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:
Speed of sound in air ≈ 344 m/s
Speed of sound in aluminium ≈ 6420 m/s (from text)
Ratio of time = (time in air) / (time in aluminium) = (distance/344) ÷ (distance/6420) = 6420/344 ≈ 18.66.
So, sound takes about 18.66 times longer in air than in aluminium.

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

Answer:
100 vibrations per second means in one minute (60 seconds): 100 × 60 = 6000 vibrations.

  1. “Does sound follow the same laws of reflection as light does? Explain.”

Answer:
Yes, sound follows the same laws of reflection. The angle of incidence equals the angle of reflection, and the incident sound, the reflected sound, and the normal to the surface all lie in the same plane.

  1. “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 remain the same. Do you hear echo sound on a hotter day?”

Answer:
On a hotter day, the speed of sound is higher, so the sound returns faster. But since the distance is the same, the echo is still heard as long as it returns after at least 0.1 second. The distance needed does not change. So yes, you will still hear the echo.

  1. “Give two practical applications of reflection of sound waves.”

Answer:

  • Megaphones, trumpets, and horns use reflection of sound to direct sound in one direction.
  • In stethoscopes, sound from the heart and lungs is reflected inside the tube to the doctor’s ears.
  1. “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 m/s² and speed of sound = 340 m/s.”

Answer (in simple steps):
Time to fall: Distance = 500 m.
Time of fall (t1) = √(2h/g) = √(2×500/10) = √100 = 10 s.
Time for sound to travel up (t2) = Distance/speed = 500/340 ≈ 1.47 s.
Total time = t1 + t2 ≈ 10 + 1.47 = 11.47 s (approximately).

  1. “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?”

Answer:
Wavelength = 1.5 cm = 0.015 m
Frequency = Speed/Wavelength = 339/0.015 = 22600 Hz (approximately)
Since 22600 Hz is above 20000 Hz, it will not be audible to humans (it’s ultrasound).

  1. “What is reverberation? How can it be reduced?”

Answer:
Reverberation is when sound continues to be heard due to repeated reflections in a hall.
It can be reduced by covering walls with sound-absorbing materials like curtains, carpets, and using soft seats.

  1. “What is loudness of sound? What factors does it depend on?”

Answer:
Loudness is how strong or weak we hear a sound. It depends on the amplitude of the sound wave and how our ear perceives it. Larger amplitude means louder sound.

  1. “How is ultrasound used for cleaning?”

Answer:
Objects are placed in a cleaning solution. Ultrasound makes the solution vibrate very fast. This loosens and removes dirt from the objects.

  1. “Explain how defects in a metal block can be detected using ultrasound.”

Answer:
Ultrasound waves are passed through the metal block. If there is a crack or hole, some of the ultrasound reflects back early. By detecting these early reflections, we can find defects inside the metal.

Leave a Comment

error: Content is protected !!