Willer Academy - Sound

Willer Academy

Nation Builders Through Education | Founded by Er. Rahul Kumar Dubey Sir

Chapter 14: Sound

Subject: Science (Physics)

Class: 7

Series: Base Builder for Moderate to Hard

Learning Objectives

By the end of this chapter, you, the future scientist and nation-builder, will be able to:

Understand how sound is produced and how it travels
Differentiate between longitudinal and transverse waves
Explain the characteristics of sound: amplitude, frequency, and timbre
Understand the structure and function of the human ear
Calculate the speed of sound in different media
Explain the concepts of echo and reverberation
Understand the applications of ultrasound in daily life

14.1 Introduction: Understanding Sound

Sound is a form of energy that produces the sensation of hearing. It is produced by vibrating objects and requires a medium (solid, liquid, or gas) to travel through.

Quick Check

List five different sources of sound in your environment. What is vibrating in each case to produce the sound?

14.2 Production and Propagation of Sound

Sound is produced when an object vibrates. These vibrations create disturbances in the surrounding medium that travel as sound waves.

Production of Sound

Vibrating objects produce sound
Example: Vocal cords, guitar strings, tuning fork
Vibration creates compression and rarefaction
Sound cannot travel in vacuum

Propagation of Sound

Sound travels as longitudinal waves
Requires a material medium
Travels fastest in solids, slowest in gases
Speed in air: ~340 m/s at room temperature

Sound Waves

Longitudinal waves: particles vibrate parallel to direction of wave
Transverse waves: particles vibrate perpendicular to direction of wave
Consist of compressions and rarefactions
Can be reflected, refracted, and absorbed

Numerical & Application 1: Speed of Sound

Scenario: A person claps hands near a cliff and hears the echo after 4 seconds.
Problem: How far away is the cliff? (Speed of sound in air = 340 m/s)
Solution:
- Distance = (Speed × Time) / 2
- Distance = (340 m/s × 4 s) / 2 = 680 meters

This calculation shows how echoes can be used to determine distances!

Experiment: Seeing Sound Vibrations

Stretch plastic wrap tightly over a bowl. Sprinkle some sugar or salt on the plastic. Hold a tuning fork above the bowl and strike it with a rubber mallet. Lower the vibrating tuning fork near the plastic (without touching) and observe what happens to the particles.

14.3 Characteristics of Sound

Sound has three fundamental characteristics: loudness, pitch, and quality (timbre).

Characteristic	Depends On	Unit	Example
Loudness	Amplitude of vibration	Decibel (dB)	Whisper (30 dB) vs. shout (80 dB)
Pitch	Frequency of vibration	Hertz (Hz)	Men's voice (low pitch) vs. women's voice (high pitch)
Quality	Waveform	-	Same note on piano vs. guitar

20 Hz

2,000 Hz

20,000 Hz

Human hearing range: 20 Hz to 20,000 Hz

Task 1: Sound Characteristics

Listen to different sounds around you (a bird chirping, a car horn, a friend's voice). For each sound, identify whether it has high or low pitch, and whether it's loud or soft.

14.4 Human Ear and Hearing

The human ear is a complex organ that detects and interprets sound waves. It has three main parts: outer ear, middle ear, and inner ear.

Outer Ear

Collects sound waves and directs them to the eardrum

Middle Ear

Amplifies vibrations through three tiny bones: hammer, anvil, and stirrup

Inner Ear

Cochlea converts vibrations to electrical signals sent to the brain

Numerical & Application 2: Hearing Range

Scenario: A dog whistle produces sound at 25,000 Hz.
Problem: Can humans hear this sound? Why or why not?
Solution:
- Human hearing range is 20 Hz to 20,000 Hz
- 25,000 Hz is above the upper limit of human hearing
- Therefore, humans cannot hear this dog whistle

This explains why some animals can hear sounds that humans cannot!

14.5 Reflection, Echo, and Ultrasound

Sound waves obey the laws of reflection similar to light waves. This property has many practical applications.

Echo

Reflection of sound that arrives after the original sound. Used to measure distances.

Ultrasound

Sound waves above 20,000 Hz. Used in medical imaging and cleaning.

SONAR

Sound Navigation and Ranging. Uses ultrasound to detect underwater objects.

Experiment: Sound Reflection

Take two cardboard tubes and place them at an angle against a hard surface. Have a friend speak softly into one tube while you listen through the other. Notice how the sound travels through the tubes and reflects off the surface.

14.6 Noise Pollution and Hearing Protection

Noise pollution is unwanted or excessive sound that can have negative effects on human health and the environment.

Sources of Noise

Traffic and transportation
Industrial activities
Construction work
Loud music and events

Effects

Hearing loss
Sleep disturbances
Cardiovascular issues
Reduced productivity

Prevention

Use of ear protection
Soundproofing buildings
Planting trees as sound barriers
Enforcing noise control regulations

Task 2: Noise Pollution Survey

Survey your school or neighborhood for sources of noise pollution. Create a list of the noisiest areas and suggest ways to reduce noise in these locations.

Section B: Multiple Choice Questions

Q1. Which part of the human ear amplifies sound vibrations?

A. Pinna
B. Eardrum
C. Cochlea
D. Ossicles (middle ear bones)

Q2. What is the approximate speed of sound in air at room temperature?

A. 140 m/s
B. 340 m/s
C. 1400 m/s
D. 5000 m/s

Q3. Which characteristic of sound determines its pitch?

A. Amplitude
B. Frequency
C. Timbre
D. Wavelength

Q4. Ultrasound refers to sound waves with frequency:

A. Below 20 Hz
B. Between 20 Hz and 20,000 Hz
C. Above 20,000 Hz
D. Between 100 Hz and 1000 Hz

Q5. Which of these is NOT a method to control noise pollution?

A. Planting trees
B. Using sound-absorbing materials
C. Increasing vehicle horns
D. Maintaining machinery properly

Q6. Sound travels fastest in which medium?

A. Air
B. Water
C. Steel
D. Vacuum

Q7. The unit of frequency of a sound wave is:

A. Decibel (dB)
B. Hertz (Hz)
C. Meter (m)
D. Newton (N)

Q8. What type of wave is a sound wave?

A. Transverse wave
B. Longitudinal wave
C. Surface wave
D. Electromagnetic wave

Q9. Which part of the ear converts vibrations into electrical signals?

A. Pinna
B. Eardrum
C. Cochlea
D. Ossicles

Q10. The loudness of a sound depends on:

A. Frequency
B. Amplitude
C. Wavelength
D. Speed of sound

Q11. Which of the following sounds cannot be heard by humans?

A. 15 Hz
B. 5,000 Hz
C. 10,000 Hz
D. 1,000 Hz

Q12. Echo is:

A. A type of noise
B. Reflection of sound
C. Production of sound by vocal cords
D. Vibration of air particles

Q13. Reverberation occurs due to:

A. Absorption of sound
B. Multiple reflections of sound
C. Interference of waves
D. Change in frequency

Q14. Sonar is used to:

A. Measure air pressure
B. Detect underwater objects using sound
C. Produce ultrasound images
D. Increase sound amplitude

Q15. Which of the following is a musical sound?

A. Clap of hands
B. Buzzing of a bee
C. Tuning fork note
D. Noise from traffic

Q16. Which of the following is an application of ultrasound in industry?

A. Medical imaging
B. Cleaning jewelry
C. Dog whistles
D. Musical instruments

Q17. Human hearing range is approximately:

A. 10 Hz to 10,000 Hz
B. 20 Hz to 20,000 Hz
C. 50 Hz to 50,000 Hz
D. 100 Hz to 15,000 Hz

Q18. Which of the following reduces noise pollution?

A. Soundproofing buildings
B. Playing loud music
C. Blasting horns frequently
D. Removing trees

Q19. Sound cannot travel in:

A. Air
B. Water
C. Steel
D. Vacuum

Q20. The time taken for sound to travel to a cliff and back is 6 s. If speed of sound is 340 m/s, the cliff is:

A. 510 m
B. 1020 m
C. 2040 m
D. 680 m

Answer Key: Section B

Q1: D
Q2: B
Q3: B
Q4: C
Q5: C
Q6: C
Q7: B
Q8: B
Q9: C
Q10: B
Q11: A
Q12: B
Q13: B
Q14: B
Q15: C
Q16: B
Q17: B
Q18: A
Q19: D
Q20: A

Section C: Answer in Detail (For Answer Writing Practice)

Q1. Explain how sound is produced and travels through different media. Why can't sound travel through vacuum?

Hint: Include vibration, compression, rarefaction, and the need for a medium.

Q2. Describe the journey of a sound wave from its source to the brain, explaining the role of each part of the ear.

Hint: Include outer ear, middle ear, inner ear, and auditory nerve.

Q3. What is ultrasound? Explain two practical applications of ultrasound in medicine and industry.

Hint: Include medical imaging and cleaning applications.

Section D: Tackle These! (Higher Order Thinking Skills - HOTS)

Q1. If a ship uses SONAR and detects an echo from a submarine after 4 seconds, how far away is the submarine? (Speed of sound in water = 1500 m/s) What would happen to the echo time if the submarine moved farther away?

Think about: Distance calculation and relationship between time and distance.

Q2. Design an experiment to measure the speed of sound in your classroom using only a stopwatch, a measuring tape, and two blocks of wood. Explain your procedure.

Think about: Echo method and necessary calculations.

Q3. Why do musicians often have difficulty tuning their instruments in a room with excessive echo? How would the design of a concert hall differ from that of a recording studio?

Think about: Sound reflection and acoustic design principles.