NCERT Solutions - Chapter 11: Sound

Q1. How does the sound produced by a vibrating object in a medium reach your ear?

Answer - When an object vibrates, it causes the particles of the surrounding medium (like air) to vibrate as well. These vibrations create compressions and rarefactions, which travel as sound waves through the medium. When these sound waves reach our ear, they cause our eardrum to vibrate, allowing us to hear the sound.

Q2. Explain how sound is produced by your school bell.

Answer - The school bell produces sound when it is struck, causing it to vibrate. These vibrations disturb the air particles around the bell, creating sound waves. These waves travel through the air to our ears, where we hear the ringing sound.

Q3. Why are sound waves called mechanical waves?

Answer - Sound waves are called mechanical waves because they require a medium (like air, water, or solids) to travel. They cannot travel through a vacuum, as they need particles of a medium to transfer their energy from one place to another.

Q4. Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?

Answer - No, we will not be able to hear any sound on the moon because it has no atmosphere or air particles to carry sound waves. Since sound waves are mechanical waves that need a medium, the absence of a medium means no sound can be transmitted.

Q1. Which wave property determines (a) loudness, (b) pitch?

Answer - (a) Loudness is determined by the amplitude of the sound wave. Larger amplitude means louder sound.
(b) Pitch is determined by the frequency of the sound wave. Higher frequency means higher pitch.

Q2. Guess which sound has a higher pitch: guitar or car horn?

Answer - The guitar produces a higher pitch sound than a car horn because the strings of the guitar vibrate at a higher frequency compared to the lower frequency vibrations of a car horn.

Q1. What are wavelength, frequency, time period and amplitude of a sound wave?

Answer - Wavelength is the distance between two consecutive compressions or rarefactions in a sound wave. Frequency is the number of vibrations or oscillations per second. Time period is the time taken for one complete vibration or oscillation. Amplitude is the maximum displacement of particles from their mean position, which determines loudness.

Q2. How are the wavelength and frequency of a sound wave related to its speed?

Answer - The speed of a sound wave is the product of its wavelength and frequency. Mathematically, speed = wavelength × frequency.

Q3. Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/s in a given medium.

Answer - Given:
Frequency (f) = 220 Hz
Speed (v) = 440 m/s
We know, wavelength (λ) = speed / frequency = v / f
=> λ = 440 / 220 = 2 meters.

Q4. 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 - Frequency (f) = 500 Hz
Time period (T) = 1 / frequency = 1 / 500 = 0.002 seconds.
The time interval between successive compressions is the time period, so it is 0.002 seconds.

Q1. Distinguish between loudness and intensity of sound.

Answer -

	Loudness	Intensity
Definition	Loudness is the quality of sound by which we can judge whether a sound is loud or soft.	Intensity is the amount of energy flowing per second through unit area perpendicular to the direction of wave propagation.
Depends on	Depends on the amplitude of the sound wave and sensitivity of the ear.	Depends only on the amplitude of the sound wave.
Unit	Measured in decibels (dB) as perceived by the ear.	Measured in watts per square meter (W/m²).
Nature	Subjective — depends on the listener’s perception.	Objective — measurable physical quantity.

Q1. 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 at the same temperature. This is because sound travels faster in solids due to the closer packing of particles, which allows quicker vibration transfer.

Q1. 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?

Answer - Given:
Time taken for echo (total time) = 3 s
Speed of sound (v) = 342 m/s

The sound travels to the reflecting surface and back, so total distance = 2 × distance to the reflecting surface (d).
Using speed = distance / time,
=> 342 = 2d / 3
=> 2d = 342 × 3 = 1026 m
=> d = 1026 / 2 = 513 meters.

Therefore, the distance of the reflecting surface from the source is 513 meters.

Q1. Why are the ceilings of concert halls curved?

Answer - The ceilings of concert halls are curved to help in the proper reflection and spreading of sound waves throughout the hall. This ensures that sound reaches every part of the hall clearly, providing better acoustics and an enhanced listening experience.

Q1. What is the audible range of the average human ear?

Answer - The audible range of the average human ear is from 20 Hz to 20,000 Hz (20 kHz). Sounds below 20 Hz are called infrasound, and those above 20 kHz are called ultrasound.

Q2. What is the range of frequencies associated with (a) Infrasound? (b) Ultrasound?

Answer - (a) Infrasound refers to sound waves with frequencies below 20 Hz.
(b) Ultrasound refers to sound waves with frequencies above 20,000 Hz (20 kHz).

Q1. What is sound and how is it produced?

Answer - Sound is a form of energy that travels in the form of vibrations through a medium such as air, water, or solids. It is produced when an object vibrates and causes the particles around it to vibrate too, creating a wave that reaches our ears.

Q2. Describe with the help of a diagram, how compressions and rarefactions are produced in air near a source of sound.

Answer - When a sound-producing object vibrates, it creates regions of high pressure (compressions) and low pressure (rarefactions) in the air. These regions travel as longitudinal waves.

[Sorry, diagram is not available yet.]

Q3. Why is sound wave called a longitudinal wave?

Answer - A sound wave is called a longitudinal wave because the particles of the medium vibrate back and forth in the same direction as the wave travels. This results in compressions and rarefactions along the direction of wave propagation.

Q4. Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a dark room?

Answer - The quality or timbre of sound helps us identify our friend's voice. Even if the pitch and loudness are similar, the unique waveform or sound pattern of a person’s voice makes it recognizable.

Q5. 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 even though both flash and thunder are produced at the same time, light reaches our eyes almost instantly, while sound takes more time to travel through the air and reach our ears.

Q6. 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 - Given:
Speed of sound, v = 344 m/s

For 20 Hz:
λ = v / f = 344 / 20 = 17.2 m

For 20,000 Hz:
λ = 344 / 20000 = 0.0172 m

So, the wavelength ranges from 0.0172 m to 17.2 m.

Q7. 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

Let distance = d
Time in air = d / 344
Time in aluminium = d / 6420

Ratio = (d / 344) / (d / 6420) = 6420 / 344 ≈ 18.66

So, the sound reaches about 18.66 times faster through aluminium than air.

Q8. The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute?

Answer - Frequency = 100 vibrations/second
Time = 1 minute = 60 seconds

Total vibrations = 100 × 60 = 6000 times

Q9. 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 is equal to the angle of reflection.
The incident sound wave, the reflected wave, and the normal all lie in the same plane.

Q10. 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, we do. On a hotter day, the speed of sound increases because the air particles move faster. This can make echoes return sooner, but as long as the distance is enough (more than 17.2 m), we can still hear the echo clearly.

Q11. Give two practical applications of reflection of sound waves.

Answer -

Megaphones and loudhailers: Use reflection to direct sound in a specific direction.
Stethoscope: Reflects body sounds to the doctor's ears for better hearing.

Q12. 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 s–2 and speed of sound = 340 m s–1.

Answer - Time taken by stone to fall:
h = 500 m, g = 10 m/s²

Using the formula:
t₁ = √(2h/g) = √(2×500/10) = √100 = 10 s

Time taken by sound to travel up = t₂ = 500 / 340 ≈ 1.47 s

Total time to hear splash = t₁ + t₂ = 10 + 1.47 = 11.47 seconds

Q13. A sound wave travels at a speed of 339 m s–1. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible?

Answer - Speed = 339 m/s, Wavelength = 1.5 cm = 0.015 m

Frequency, f = v / λ = 339 / 0.015 = 22600 Hz

Since the human audible range is 20 Hz to 20,000 Hz,
22600 Hz is ultrasonic and not audible to humans.

Q14. What is reverberation? How can it be reduced?

Answer - Reverberation is the persistence of sound in a room due to multiple reflections from walls, ceiling, and floor.

It can be reduced by:

Using sound-absorbing materials like carpets, curtains, and acoustic panels.
Covering walls and ceilings with soft materials.

Q15. What is loudness of sound? What factors does it depend on?

Answer - Loudness is the sensation of how strong or weak a sound appears to our ears. It depends on:

Amplitude of the sound wave (greater amplitude means louder sound)
Sensitivity of the human ear

Q16. How is ultrasound used for cleaning?

Answer - Ultrasound is used to clean objects with small and complex parts. The object is placed in a cleaning solution, and ultrasound waves create vibrations that remove dirt and grease even from holes and tight corners.

Q17. Explain how defects in a metal block can be detected using ultrasound.

Answer - Ultrasound waves are passed into the metal block. If there is a defect, such as a crack or air bubble, the waves get reflected back from that spot. By analyzing the reflected waves, the location and size of the defect can be detected.