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Tamil Nadu (TNBSE)Class 8 Science★ 4-7 marks in the TN Class 8 annual exam through book-back MCQs, fill-ups and short answers

Sound

Samacheer Kalvi (TN State Board) Class 8 Science — Physics Chapter 6, Sound. How sound is produced and propagated, amplitude and loudness (decibel), frequency, time period and pitch, the audible range, types of musical instruments, and noise pollution. Notes with the actual book-back questions, common mistakes and quick revision.

⏱ 16 min readEasy difficulty✓ Free · NCERT-aligned

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By the end of this chapter you'll be able to…

1Explain how sound is produced and propagated
2Define amplitude and relate it to loudness (decibel)
3Relate frequency and time period to pitch
4State the audible range
5Classify musical instruments

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Why this chapter matters

Sound explains how we hear and how music is made. Amplitude and loudness (decibel), frequency and pitch, the audible range and instrument types are directly tested book-back content in the TN Class 8 exam.

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Class 7 — Sound basics

Vibration and hearing.

Sound — Class 8 Science (Samacheer Kalvi)

TN State Board (Samacheer Kalvi) Class 8 Science, Physics — Chapter 6. How sound is made, travels and is heard.

1. About this chapter

This chapter covers the production and propagation of sound, amplitude and loudness, frequency, time period and pitch, the audible range, musical instruments, and noise pollution.

2. Production and propagation

Sound is produced by a vibrating object and needs a material medium to travel — it cannot travel through vacuum.
Sound travels fastest in solids (metals), slower in liquids, slowest in gases.

3. Characteristics of sound

Amplitude: the maximum displacement of a vibrating particle from its mean position; its unit is the metre (m). Amplitude decides the loudness.
Loudness is measured in decibel (dB) — higher amplitude → louder sound.
Frequency: the number of vibrations per second; unit hertz (Hz). Frequency decides the pitch (higher frequency → higher/shriller pitch).
Time period: the time taken for one vibration (= 1 / frequency).

4. Audible range, instruments and noise

Audible range for humans: 20 Hz to 20,000 Hz (below = infrasonic, above = ultrasonic).
Musical instruments: string (sitar, guitar), wind (flute), and percussion (drum).
Noise pollution (traffic, loudspeakers) harms health; it is reduced by limiting loud sounds and planting trees.

5. Worked examples

Example 1. In which medium does sound travel fastest? In solids (metals).

Example 2. Which property of sound decides its loudness? The amplitude of the sound wave.

Example 3. To which type does a sitar belong? A string instrument.

6. Book-back questions (Samacheer Kalvi)

I. Choose the correct answer

Sound waves travel very fast in — (a) gases / (b) metals / (c) vacuum. Ans: (b) metals.
The amplitude of a sound wave decides its — (a) pitch / (b) loudness. Ans: (b) loudness.
A sitar is a — (a) string instrument / (b) wind instrument. Ans: (a) string instrument.
The range of audible frequency for the human ear is — (a) 0–10 Hz / (b) 20 Hz to 20,000 Hz. Ans: (b) 20 Hz to 20,000 Hz.
If the amplitude and frequency are increased — loudness increases and the pitch becomes higher.

II. Fill in the blanks 6. The unit of loudness is decibel (dB). 7. The maximum displacement of a vibrating particle from its mean position is its amplitude (unit: metre). 8. Higher the frequency, higher the pitch.

III. Answer briefly 9. Define amplitude and give its unit. 10. Differentiate loudness and pitch.

7. Common mistakes

Mistake: Saying sound travels fastest in air. Fix: Sound travels fastest in solids (metals).
Mistake: Confusing amplitude/loudness with frequency/pitch. Fix: Amplitude → loudness (decibel); frequency → pitch.
Mistake: Forgetting the unit of amplitude or loudness. Fix: Amplitude is in metre; loudness in decibel (dB).

8. Quick revision

Physics Ch 6 · production, amplitude/loudness, frequency/pitch.
Sound needs a medium; travels fastest in solids (metals).
Amplitude (unit metre) → loudness (decibel, dB).
Frequency (Hz) → pitch; time period = 1/frequency.
Audible range 20 Hz–20,000 Hz; instruments: string, wind, percussion.

Key formulas & results

Everything you need to memorise, in one card. Screenshot this for revision.

Amplitude

max displacement from mean position (unit: metre)

Decides loudness.

Loudness

measured in decibel (dB)

Higher amplitude → louder.

Frequency / time period

frequency (Hz); time period = 1/frequency

Frequency decides pitch.

Audible range

20 Hz to 20,000 Hz

Sound fastest in solids.

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Common mistakes & fixes

These are the exact errors that cost students marks in board exams. Read them once, save yourself the trouble.

WATCH OUT

✗ Saying sound travels fastest in air

✓ Sound travels fastest in solids (metals).

WATCH OUT

✗ Confusing amplitude/loudness with frequency/pitch

✓ Amplitude → loudness (decibel); frequency → pitch.

WATCH OUT

✗ Forgetting the unit of amplitude or loudness

✓ Amplitude is in metre; loudness in decibel (dB).

NCERT exercises (with solutions)

Every NCERT exercise from this chapter — what it covers and how many questions to expect.

Choose the correct answer

Book-back MCQs on propagation, amplitude, instruments and audible range.

Questions

Fill in the blanks

Decibel, amplitude and pitch.

Questions

Answer briefly / in detail

Define amplitude, compare loudness and pitch, audible range.

Questions

Practice problems

Try each one yourself before tapping "Show solution". Active recall > rereading.

Q1EASY· MCQ

Sound waves travel very fast in (a) gases (b) metals (c) vacuum.

▶ Show solution

(b) metals.

Q2EASY· MCQ

The amplitude of a sound wave decides its ____.

▶ Show solution

loudness.

Q3EASY· MCQ

A sitar is a ____ instrument.

▶ Show solution

string.

Q4EASY· Fill in the blanks

The unit of loudness is ____.

▶ Show solution

decibel (dB).

Q5MEDIUM· Answer briefly

Define amplitude and give its unit.

▶ Show solution

Amplitude is the maximum displacement of a vibrating particle from its mean position; its unit is the metre.

Q6MEDIUM· Answer briefly

Differentiate loudness and pitch.

▶ Show solution

Loudness depends on the amplitude (measured in decibel), while pitch depends on the frequency (higher frequency gives a higher pitch).

5-minute revision

The whole chapter, distilled. Read this the night before the exam.

•Physics Chapter 6 of Samacheer Kalvi Class 8 Science.
•Sound needs a medium; travels fastest in solids (metals).
•Amplitude (unit metre) → loudness (decibel, dB).
•Frequency (Hz) → pitch; time period = 1/frequency.
•Audible range 20 Hz–20,000 Hz (infrasonic/ultrasonic outside).
•Instruments: string (sitar), wind (flute), percussion (drum).

Tamil Nadu (TNBSE) marks blueprint

Where the marks come from in this chapter — so you can plan your prep.

Typical chapter weightage: 4-7 marks across book-back MCQ, fill-ups and short answers

Question type	Marks each	Typical count	What it tests
MCQ / Fill	1	3-5	Propagation, amplitude, instruments, range
Short Answer	2-3	1-2	Amplitude, loudness vs pitch
Application	1-2	1	Noise pollution

Prep strategy

Learn amplitude (metre) → loudness (decibel)
Relate frequency to pitch and time period
Memorise the audible range
Classify musical instruments

Where this shows up in the real world

This chapter isn't just an exam topic — it lives in the world around you.

Music

String, wind and percussion instruments use vibration, pitch and loudness.

Communication

Speech and hearing rely on sound.

Health

Controlling decibel levels prevents noise pollution.

Exam strategy

Battle-tested tips from teachers and toppers for this chapter.

State that sound is fastest in solids
Link amplitude→loudness, frequency→pitch
Give units (metre, decibel, hertz)
Classify instruments correctly

Going beyond the textbook

For olympiad aspirants and curious learners — topics that build on this chapter.

Calculate the time period from a given frequency.
Explain why we hear thunder after we see lightning.

Where else this chapter is tested

CBSE board isn't the only one — other exams test this chapter too.

TN Class 8 Annual ExamHigh

Foundation / NMMS ScienceMedium

School unit testsHigh

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Questions students ask

The real ones — pulled from the Q&A community and tutor sessions.

Loudness depends on the amplitude of a sound wave, and the decibel (dB) is the standard unit used to measure how loud a sound is.

Increasing the amplitude makes the sound louder, while increasing the frequency makes the pitch higher (shriller).

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