📝 Sound Physics - 80 Mark Question Paper

Class 9 | Science | Comprehensive Assessment

Total Marks

80

Time Duration

3 Hours

Total Questions

28

Question Sections

4 Types

📌 General Instructions:

• This question paper contains 28 questions divided into 4 sections
• Section A: 1 mark questions (MCQ) - 5 questions
• Section B: 2 mark questions (Short Answer) - 8 questions
• Section C: 3 mark questions (Long Answer) - 10 questions
• Section D: 5 mark questions (Numerical & Descriptive) - 5 questions
• All questions are compulsory
• Use of calculator is not permitted

Section A: Multiple Choice Questions (1 Mark Each) 5 Marks

1

1 Mark

Sound is produced by:

(A) Heat energy

(B) Vibrating objects

(C) Light energy

(D) Magnetic field

2

1 Mark

Which of the following is NOT a longitudinal wave? Hard

(A) Sound wave in air

(B) Sound wave in water

(C) Light wave

(D) Sound wave in metal

3

1 Mark

The SI unit of frequency is:

(A) Meter (m)

(B) Hertz (Hz)

(C) Second (s)

(D) Pascal (Pa)

4

1 Mark

Speed of sound is maximum in:

(A) Air

(B) Water

(C) Steel

(D) Vacuum

5

1 Mark

Human ear can hear frequencies in the range:

(A) 10 Hz to 10 kHz

(B) 20 Hz to 20 kHz

(C) 50 Hz to 50 kHz

(D) 5 Hz to 5 kHz

Section B: Short Answer Questions (2 Marks Each) 16 Marks

6

2 Marks

Define vibration. Give two examples of objects that vibrate to produce sound.

7

2 Marks

Distinguish between compression and rarefaction in a sound wave.

8

2 Marks

What do you understand by wavelength? How is it related to frequency and speed of sound?

9

2 Marks

Why can sound not travel in vacuum? Medium

10

2 Marks

Define an echo. What is the minimum distance required to hear a distinct echo? (Take speed of sound = 344 m/s)

11

2 Marks

What is reverberation? How can excessive reverberation be reduced in a concert hall?

12

2 Marks

Distinguish between infrasound and ultrasound. Give one example each.

13

2 Marks

List two applications of ultrasound in medicine.

Section C: Long Answer Questions (3 Marks Each) 30 Marks

14

3 Marks

Explain with a diagram how sound propagates as compressions and rarefactions through a medium. Why is it called a mechanical wave?

15

3 Marks

Explain why sound waves are called longitudinal waves. Draw a diagram showing a longitudinal wave in a slinky. Medium

16

3 Marks

What is the relationship between frequency, wavelength, and speed of sound? Explain with an example.

17

3 Marks

How does pitch differ from frequency? Explain with examples how different musical instruments produce different pitches.

18

3 Marks

Distinguish between loudness and intensity of sound. What factors affect the loudness of a sound?

19

3 Marks

Explain how sound gets reflected. State the laws of reflection for sound. Medium

20

3 Marks

How is a stethoscope designed to use the principle of sound reflection? Explain its working.

21

3 Marks

Why is the audible range of human ear limited? What animals can produce infrasound? Name animals that produce ultrasound.

22

3 Marks

Explain how ultrasound is used to detect cracks or flaws in metal blocks. Why can't ordinary sound be used for this purpose?

23

3 Marks

Describe the process of echocardiography. How is it different from ultrasonography? Hard

Section D: Numerical & Descriptive Questions (5 Marks Each) 25 Marks

24

5 Marks

A sound wave has a frequency of 2 kHz and a wavelength of 35 cm. Calculate:
(a) The speed of sound in the medium
(b) The time taken for the sound to travel 1.5 km
(c) How many compressions and rarefactions will pass a fixed point in 2 seconds? Hard

25

5 Marks

A person claps near a cliff and hears an echo after 3 seconds. If the speed of sound is 342 m/s:
(a) What is the distance of the cliff from the person?
(b) How far will sound travel in one minute?
(c) Would this be a distinct echo? (Assume time for sound perception = 0.1 s) Medium

26

5 Marks

Explain the following with proper reasoning:
(a) Why do we see lightning before hearing thunder?
(b) Sound travels faster in solids than in gases. Explain why.
(c) Why are the ceilings of concert halls curved? Medium

27

5 Marks

A stone is dropped from the top of a 500 m high tower into a pond. When is the splash heard at the top? (Given: g = 10 m/s², speed of sound = 340 m/s)
(Hint: Find time for stone to fall + time for sound to travel back)

28

5 Marks

Write a comprehensive note on applications of ultrasound. Include at least three applications in medicine/industry and explain how they work. Hard

📊 Answer Key & Detailed Solutions

Complete answers with explanations for all 28 questions

Section A: Answer Key (1 Mark Each) 5 Marks

Question 1 Answer

Correct Answer: (B) Vibrating objects

Why this is correct:

Sound is produced only when an object vibrates (moves rapidly back and forth). The vibration creates disturbances in the medium (usually air) which travel as sound waves.

Question 2 Answer

Correct Answer: (C) Light wave

Why this is correct:

Light is a transverse wave. Sound waves in air, water, and metals are all longitudinal waves where particles vibrate parallel to wave direction.

Question 3 Answer

Correct Answer: (B) Hertz (Hz)

Why this is correct:

Frequency measures oscillations per unit time. The SI unit is Hertz (Hz). 1 Hz = 1 oscillation per second.

Question 4 Answer

Correct Answer: (C) Steel

Why this is correct:

Sound travels fastest in solids because particles are closely packed. Speed in steel ≈ 5960 m/s, in air ≈ 346 m/s. Sound cannot travel in vacuum.

Question 5 Answer

Correct Answer: (B) 20 Hz to 20 kHz

Why this is correct:

The audible range for humans is 20 Hz to 20,000 Hz (20 kHz). Below 20 Hz is infrasound; above 20 kHz is ultrasound—both inaudible to humans.

Section B: Answer Key (2 Marks Each) 16 Marks

Question 6 Answer

Definition: Vibration is a rapid to-and-fro motion of an object about its equilibrium position.

Examples:

• Vibrating tuning fork
• Vibrating vocal cords
• Vibrating guitar string
• Vibrating drum membrane

Question 7 Answer

Distinction:

Key Differences:

• Compression (C): Region of high pressure where particles are crowded together
• Rarefaction (R): Region of low pressure where particles are spread apart
• In a graph: Compression appears as peaks (crests), rarefaction as valleys (troughs)

Question 8 Answer

Wavelength (λ): Distance between two consecutive compressions or rarefactions. Unit: meter (m).

Relationship:

v = λ × ν

If frequency increases, wavelength decreases (for same speed), and vice versa. Their product always equals the speed of sound.

Question 9 Answer

Reason: Sound cannot travel in vacuum because there are no particles to vibrate.

Detailed Explanation:

Sound is a mechanical wave requiring a medium. In vacuum, there are no particles to oscillate or transmit energy. Therefore, sound waves cannot propagate.

Question 10 Answer

Echo: Same sound heard again after being reflected from a large obstacle.

Minimum Distance Calculation:

Brain retention: 0.1 s
Total distance = 344 × 0.1 = 34.4 m
Distance to obstacle = 34.4 ÷ 2 = 17.2 m

Sound must travel to the obstacle and back, covering 34.4 m in 0.1 s minimum for a distinct echo.

Question 11 Answer

Reverberation: Persistence of sound due to repeated reflections until inaudible.

How to Reduce It:

• Using sound-absorbing materials (compressed fibreboard)
• Installing draperies and curtains
• Using rough plaster instead of smooth surfaces
• Selecting sound-absorbing seat materials

Question 12 Answer

Distinction:

Infrasound vs Ultrasound:

• Infrasound: Frequencies below 20 Hz. Example: Whale communication
• Ultrasound: Frequencies above 20 kHz. Example: Bat echolocation

Question 13 Answer

Medical Applications:

Applications:

1. Echocardiography: Ultrasonic waves reflect from heart to form images
2. Ultrasonography: Images internal organs like liver, kidneys, gallbladder
3. Fetal Imaging: Examines developing fetus during pregnancy
4. Kidney Stone Treatment: Breaks down stones into fine grains

Section C: Answer Key (3 Marks Each) 30 Marks

Question 14 Answer

Sound propagates as compressions and rarefactions through a medium.

Process:

When a vibrating object moves forward, it compresses the medium creating high-pressure regions (compressions). When it moves backward, it creates low-pressure regions (rarefactions). These alternating regions travel outward as sound waves.

Why It's Called a Mechanical Wave:

Sound requires a material medium and depends on particle motion, making it a mechanical wave (not electromagnetic like light).

Question 15 Answer

Particles oscillate parallel to the direction of wave propagation.

Explanation:

In a slinky, when pushed and pulled repeatedly, coils compress together (C) and spread apart (R). If you mark a dot, it moves back-and-forth along the same direction the wave travels—parallel motion defines longitudinal waves.

Question 16 Answer

Relationship:

v = λ × ν

Example:

If ν = 500 Hz and λ = 0.68 m, then v = 0.68 × 500 = 340 m/s (speed of sound in air)

Question 17 Answer

Pitch: How high or low a sound seems. Frequency: Number of oscillations per unit time.

Examples:

• Piccolo: High frequency = high pitch
• Tuba: Low frequency = low pitch
• Female voice: Higher pitch than male voice

Question 18 Answer

Loudness: Subjective perception. Intensity: Measurable sound energy.

Factors Affecting Loudness:

• Amplitude of sound wave
• Distance from source
• Nature of medium
• Frequency sensitivity of human ear

Question 19 Answer

Sound bounces off solid or liquid surfaces following reflection laws.

Laws of Reflection:

• Incident ray, reflected ray, and normal are in same plane
• Angle of incidence = Angle of reflection
• Both angles measured from the normal (perpendicular) to surface

Question 20 Answer

Stethoscope uses multiple reflection of sound to transmit internal body sounds.

Working:

Diaphragm collects sound from body. Sound vibrates air in tube. Curved metal cavity creates multiple reflections. Sound is amplified and reaches earpieces clearly.

Question 21 Answer

Human audible range limited due to ear's biological design limitations.

Animals:

Infrasound: Whales, elephants, rhinoceroses
Ultrasound: Bats, dolphins, porpoises, rats, moths

Question 22 Answer

Ultrasound has short wavelength, travels in well-defined paths, reflects precisely from defects.

Why ordinary sound fails:

Longer wavelengths bend around small defects (diffraction), entering detector without reflecting—giving false results. Ultrasound's short wavelength reflects accurately from tiny flaws.

Question 23 Answer

Echocardiography: Images the heart using ultrasonic wave reflections.

Difference from Ultrasonography:

Echocardiography specifically images the heart and produces real-time moving images. Ultrasonography images various organs (liver, kidney, fetus) as static 2D/3D images.

Section D: Answer Key (5 Marks Each) 25 Marks

Question 24 Answer

Given: ν = 2 kHz = 2000 Hz, λ = 35 cm = 0.35 m

(a) Speed of Sound:

v = λ × ν = 0.35 × 2000 = 700 m/s

(b) Time for 1.5 km:

t = d/v = 1500/700 = 2.14 seconds

(c) Compressions and Rarefactions in 2 seconds:

In 2 seconds = 2 × 2000 = 4000 compressions/rarefactions

Question 25 Answer

Given: Echo time = 3 s, v = 342 m/s

(a) Distance of Cliff:

Total distance = 342 × 3 = 1026 m
Distance to cliff = 1026/2 = 513 m

(b) Distance in One Minute:

Distance = 342 × 60 = 20,520 m = 20.52 km

(c) Is It Distinct?

YES. Required time = 0.1 s; Given time = 3 s. Since 3 s >> 0.1 s, this will be a very distinct echo.

Question 26 Answer

(a) Lightning vs Thunder: Light travels at 3×10⁸ m/s (almost instantaneous); Sound travels at ~340 m/s (much slower). We see lightning immediately but hear thunder seconds later.

(b) Speed in Solids vs Gases:

In solids, particles are closely packed with strong forces. Vibrations transmit quickly (v ≈ 5960 m/s in steel). In gases, particles are far apart; vibrations transmit slowly (v ≈ 346 m/s in air).

(c) Curved Concert Hall Ceilings:

Curved ceilings act as reflectors, scattering sound evenly across the auditorium. Flat ceilings cause uneven reflection, creating loud and quiet zones.

Question 27 Answer

Given: h = 500 m, g = 10 m/s², v_sound = 340 m/s

Step 1: Time for Stone to Fall (h = ½gt²):

500 = ½ × 10 × t₁²
t₁² = 100
t₁ = 10 seconds

Step 2: Time for Sound to Return (t = d/v):

t₂ = 500/340 = 1.47 seconds

Total Time:

Total = 10 + 1.47 = 11.47 seconds

Question 28 Answer

Ultrasound Applications: Comprehensive note covering medicine and industry.

Medical Applications:

• Echocardiography: Real-time heart imaging for valve and rhythm diseases
• Ultrasonography: Organ imaging (liver, kidney, fetus) to detect stones and tumors
• Lithotripsy: Breaking kidney stones for natural elimination

Industrial Applications:

• Flaw Detection: Detecting cracks in metal structures (bridges, aircraft)
• Cleaning: Removing dirt from electronic components and complex shapes
• Welding: Joining plastic and metal sheets efficiently

Key Advantages:

Short wavelength enables precision detection and imaging. Non-invasive, radiation-free, safe for vulnerable populations, cost-effective, and portable.

📊 Detailed Marking Scheme

Complete breakdown of marks distribution and scoring guidelines

Marks Distribution

Section A

MCQ

5 × 1 mark

5 Marks

Section B

Short Answer

8 × 2 marks

16 Marks

Section C

Long Answer

10 × 3 marks

30 Marks

Section D

Numerical

5 × 5 marks

25 Marks

Total Marks: 5 + 16 + 30 + 25 = 80 Marks

Marking Guidelines

📝 Section A & B Guidelines

Section A (MCQ - 1 Mark): ✓ Correct = 1 Mark | ✗ Wrong = 0 Mark

Section B (2 Marks): Full = 2 | Partial = 1 | Wrong = 0

📝 Section C & D Guidelines

Section C (3 Marks): Full = 3 | Good = 2 | Partial = 1 | Wrong = 0

Section D (5 Marks): Numerical (1+2+1.5+0.5) or Descriptive (5=5|4=4|3=3|2=2|1=1|0=0)

© 2025 Sound Physics Question Paper | Class 9 Science | CBSE Pattern

Comprehensive 80-Mark Assessment with Complete Answer Key