An echo is the repetition of sound caused by reflection of sound waves from a distant surface.

What is the relation between velocity, frequency, and wavelength?

The relation is v = nλ, where v is velocity, n is frequency, and λ is wavelength.

Class 10

Tamil Nadu Class 10 Science Chapter 5 — Acoustics

Samacheer Kalvi Class 10 Science Acoustics guide covering sound waves, echo, frequency, amplitude, velocity, human ear basics, important questions, and revision checklist.

Updated 6 June 20263 min read

Quick Summary

Acoustics is the study of sound. For exams, focus on wave terms, propagation of sound, echo, reflection of sound, applications, and basic numerical problems.

Key Definitions

Sound: Energy produced by vibrating bodies.
Frequency: Number of vibrations per second.
Amplitude: Maximum displacement of a vibrating particle from its mean position.
Wavelength: Distance between two consecutive compressions or rarefactions.
Echo: Reflected sound heard after a short interval.

Formulae

Concept	Formula
Wave velocity	`v = nλ`
Frequency	`n = v/λ`
Wavelength	`λ = v/n`

Properties of Sound

Sound needs a medium to travel.
Sound travels faster in solids than in liquids and gases.
Loudness depends on amplitude.
Pitch depends on frequency.
Sound can be reflected, absorbed, and transmitted.

Echo

An echo is heard when reflected sound reaches the ear after a minimum time gap. Large halls, hills, and empty buildings can produce echoes.

Echo — the key condition

To hear a distinct echo the reflected sound must reach the ear at least 0.1 s after the original (the persistence of hearing). Since sound travels ~344 m/s in air:

minimum distance to reflector = (v × t)/2 = (344 × 0.1)/2 ≈ 17.2 m

(The sound covers the distance twice — to the wall and back — hence ÷2.)

Applications — SONAR & echolocation

SONAR sends ultrasonic pulses into water; the echo time gives the depth: depth = (v × t)/2. Bats and dolphins use the same echolocation principle.

Worked Examples

Example 1 — Wave relation

A sound wave has frequency 500 Hz and wavelength 0.66 m. Find its velocity.

v = nλ = 500 × 0.66 = 330 m/s

Example 2 — Find wavelength

Velocity of sound is 340 m/s, frequency 170 Hz. Find wavelength.

λ = v/n = 340/170 = 2 m

Example 3 — Echo distance

A person hears an echo 0.8 s after shouting. Speed of sound = 340 m/s. Find the distance to the wall.

distance = (v × t)/2 = (340 × 0.8)/2 = 136 m

Example 4 — SONAR depth

An ultrasonic pulse returns from the seabed in 4 s. Speed in water = 1500 m/s. Find the depth.

depth = (1500 × 4)/2 = 3000 m

Textbook Solution Strategy

1. Identify whether the question is about wave terms, echo, or applications. 2. For numerical questions, write v, n, and λ. 3. Use v = nλ. 4. Keep units consistent. 5. For application questions, connect the property of sound to the use.

Important Questions

One Mark

Define frequency.
What is an echo?
Write the unit of frequency.

Two Marks

Differentiate between loudness and pitch.
Why does sound not travel in vacuum?
State two applications of reflection of sound.

Five Marks

Explain the propagation of sound through a medium.
Describe echo and its applications.
Solve numerical problems using v = nλ.

Common Mistakes

Confusing amplitude with frequency.
Writing that sound travels in vacuum.
Forgetting the unit hertz for frequency.
Using wavelength formula without converting units.

Revision Checklist

Wave terms memorised
v = nλ formula practised
Echo definition revised
Loudness vs pitch differences prepared
Applications of sound reflection revised

Frequently Asked Questions

Sound is a form of energy produced by vibrating bodies and transmitted through a material medium as waves.