Science Exploration Notes Class 9 English Medium 2026-27

Science Exploration Chapter 10. Sound Waves: Characteristics and Applications

CBSE Notes New Syllabus: Class 9

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Chapter 10. Sound Waves: Characteristics and Applications

This section explains the reflection of sound and its important applications. It describes the laws of reflection, echo, reverberation, multiple reflection of sound and their practical uses in everyday life.

Reflection of Sound

Like light, sound waves are also reflected when they strike a hard surface. The reflected sound follows the same laws of reflection as light.

Definition

The bouncing back of sound waves after striking a hard surface is called the reflection of sound.

Laws of Reflection of Sound

• First Law – The incident sound wave, the reflected sound wave and the normal at the point of incidence lie in the same plane.
• Second Law – The angle of incidence is equal to the angle of reflection.

Echo

An echo is the repetition of a sound due to the reflection of sound waves from a distant surface.

Definition

An echo is the reflected sound heard separately after the original sound.

Conditions for Hearing an Echo

• The reflected sound should reach the listener at least 0.1 second after the original sound.
• The reflecting surface should be approximately 17 metres or more away from the listener.
• The reflecting surface should be large, smooth and hard.

Applications of Echo

• Measuring the depth of oceans.
• Locating distant objects.
• Medical imaging using ultrasonic echoes.
• Navigation systems such as SONAR.

Reverberation

When reflected sound mixes with the original sound due to repeated reflections inside a room or hall, the sound persists for a short time. This phenomenon is called reverberation.

Definition

Reverberation is the persistence of sound in an enclosed space due to multiple reflections.

Methods to Reduce Reverberation

• Using sound-absorbing materials.
• Covering walls with curtains or acoustic panels.
• Using carpets and cushioned seats.
• Installing false ceilings in auditoriums.

Multiple Reflection of Sound

Sound undergoes repeated reflections between surfaces. This phenomenon is called multiple reflection of sound.

Applications of Multiple Reflection

• Megaphones
• Horns
• Hearing aids
• Stethoscopes
• Sound boards in auditoriums

Difference Between Echo and Reverberation

Applications of Reflection of Sound

• Designing auditoriums and theatres.
• Medical diagnosis using ultrasound.
• Communication through megaphones.
• Navigation using SONAR.
• Construction of concert halls with proper acoustics.

ATP Education Concept Builder

Do not confuse echo with reverberation. An echo is heard as a separate repeated sound, whereas reverberation is the persistence of sound caused by multiple reflections in an enclosed space.

ATP Education Exam Booster

• Remember the laws of reflection of sound.
• Learn the conditions required to hear an echo.
• Differentiate clearly between echo and reverberation.
• Know the applications of multiple reflection of sound.
• Remember how reverberation can be reduced in auditoriums.
• CBSE competency-based questions often ask students to explain why curtains, carpets and acoustic panels are used in theatres and auditoriums.