CBSE Class 8 Science — Sound

Chapter Overview & Weightage

Sound is Chapter 13 of CBSE Class 8 Science. It covers the nature of sound as a wave, its properties, and practical applications like musical instruments and noise pollution. The concepts here directly prepare students for deeper study of waves and acoustics in Class 9 and Class 11 physics.

Exam Year	Marks Allocated	Question Types
2024	8–10 marks	1 MCQ + 2 short + 1 long
2023	8 marks	1 fill-in + 2 short + 1 long
2022	8 marks	2 short + 1 long
2021	6 marks	2 short + 1 long

Sound is a consistent source of 6–10 marks in Class 8 exams. Questions cover properties of sound (amplitude, frequency, pitch, loudness), how sound travels, and noise pollution. Diagram questions asking you to sketch a sound wave are common — know how to label amplitude, wavelength, and crest/trough.

Key Concepts You Must Know

Sound as a mechanical wave:

Sound is produced by vibration of objects
Sound needs a medium to travel (solid, liquid, or gas) — it cannot travel through vacuum
Sound travels faster in solids than liquids than gases (at 20°C: ~343 m/s in air, ~1480 m/s in water, ~5100 m/s in steel)

Properties of sound waves:

Amplitude: Maximum displacement of the medium particles from equilibrium. Determines loudness — greater amplitude = louder sound.
Frequency: Number of vibrations per second. Unit: Hertz (Hz). Determines pitch — higher frequency = higher pitch (shriller sound).
Time period: Time for one complete vibration. $T = 1/f$ .
Wavelength: Distance between two consecutive crests (or troughs). Symbol: $\lambda$ (lambda)

Sound wave equation: $v = f \times \lambda$ (wave speed = frequency × wavelength)

Audible range:

Humans: 20 Hz to 20,000 Hz (20 kHz)
Infrasound: below 20 Hz (elephants, whales produce/detect this)
Ultrasound: above 20 kHz (bats, dolphins; used in SONAR, medical imaging, cleaning instruments)

Amplitude and loudness: Sound is measured in decibels (dB). Prolonged exposure to sounds above 85 dB causes hearing damage.

Noise pollution:

Unwanted sound that is unpleasant and harmful
Sources: traffic, construction, loudspeakers, factories
Effects: hearing damage, stress, disturbed sleep, reduced productivity
Control: plantation of trees (absorb sound), sound barriers, noise-proof buildings

Echo: Reflection of sound. Minimum distance from reflecting surface for distinct echo = 17.2 m (based on persistence of sound in human ear = 1/10 second, and sound travels 34.4 m in 0.1 s → needs 17.2 m each way).

Important Formulas

Wave speed: $v = f \times \lambda$

Frequency and time period: $f = 1/T$

For echo: Minimum distance $= \frac{v \times 0.1}{2} = \frac{343 \times 0.1}{2} \approx 17.2$ m

Speed of sound in air at 0°C ≈ 332 m/s Speed of sound in air at 20°C ≈ 343 m/s

Higher frequency → Higher pitch (shriller) Lower frequency → Lower pitch (deeper)

Greater amplitude → Louder sound Smaller amplitude → Softer sound

Note: Pitch and loudness are independent properties — a sound can be loud with low pitch (bass) or soft with high pitch.

Solved Previous Year Questions

PYQ 1: (CBSE 2023, 3 marks)

Q: Define amplitude of a sound wave. How is it related to the loudness of sound?

Solution: The amplitude of a sound wave is the maximum displacement of the particles of the medium from their equilibrium (undisturbed) position. It is measured in metres.

Amplitude determines the loudness of sound. A sound wave with greater amplitude carries more energy and produces a louder sound. A sound wave with smaller amplitude carries less energy and sounds softer. For example, striking a drum harder increases the amplitude of vibration → louder sound.

PYQ 2: (CBSE 2024, 4 marks)

Q: Explain noise pollution. What are its harmful effects? Give any two ways to control it.

Solution:

Noise pollution is the presence of unwanted, excessively loud, or unpleasant sounds in the environment that are harmful to humans and other living organisms.

Harmful effects:

Prolonged exposure to loud noise (above 85 dB) causes permanent hearing loss
High noise levels cause stress, anxiety, and irritability
Disturbed sleep patterns and fatigue
In extreme cases: elevated blood pressure and heart disease

Control measures:

Plantation of trees: Trees act as natural sound barriers by absorbing and scattering sound waves. Planting trees along roads and near factories reduces noise effectively.
Acoustic tiles: Using sound-absorbing materials on walls and ceilings of factories, schools, and airports dampens reflected sound.

Other methods: noise-cancelling headphones, enforcing quiet zones near hospitals and schools, limiting use of loudspeakers.

PYQ 3: (CBSE 2022, 3 marks)

Q: A sound wave has a frequency of 500 Hz and wavelength 0.6 m. What is the speed of sound?

Solution:

Using $v = f \times \lambda$ :

$v = 500 \times 0.6 = 300$ m/s

(Note: Speed of sound varies with medium and temperature; this hypothetical wave travels at 300 m/s.)

Difficulty Distribution

Difficulty	%	Question Type
Easy (45%)	Definitions, fill in blanks, state properties	MCQ, 1-mark
Medium (40%)	Explain concepts, list effects, short calculations	2–3 mark short answers
Hard (15%)	Numerical problems, compare sounds, diagram interpretation	3–4 mark questions

Expert Strategy

For long-answer questions, structure the answer with clear headings and numbered points. “Explain noise pollution” expects: definition (1 mark), sources (1 mark), effects/harms (1–2 marks), control measures (1 mark). Practice writing in this structured format rather than in continuous paragraphs — it’s easier to grade and typically earns full marks.

For the sound wave diagram: draw a transverse wave shape (sinusoidal curve). Label: crest (top peak), trough (bottom dip), amplitude (distance from equilibrium to crest), wavelength (distance between two consecutive crests). The equilibrium line should be clearly marked. This diagram appears almost every year for 2-3 marks.

For numerical problems using $v = f\lambda$ : if two out of three variables are given, simply substitute and solve. Common variants: find wavelength given speed and frequency, find frequency given speed and wavelength.

Common Traps

Trap 1: Confusing pitch and loudness. Pitch depends on frequency; loudness depends on amplitude. A high-pitched sound (high frequency, like a whistle) is not necessarily loud. A low-pitched sound (low frequency, like a bass drum) is not necessarily soft. Exam questions often test exactly this distinction.

Trap 2: Stating that sound travels fastest in gases. Sound actually travels fastest in solids (tightly packed particles transmit vibrations quickly), slower in liquids, and slowest in gases. This is opposite to what students sometimes assume. The order is: solid > liquid > gas.

Trap 3: Forgetting that sound cannot travel in vacuum. Unlike light (which is an electromagnetic wave and travels through vacuum), sound requires a material medium. This is why astronauts in space cannot hear each other through sound — space is a vacuum.