Class 9 Science Notes Chapter 11 on Sound provides students with a clear and detailed explanation of all key concepts related to sound.
These sound class 9 notes cover what sound is, sound waves, types of waves, characteristics of sound, echo, reverberation, the human ear, and SONAR. Designed for CBSE Class 9 students, these are ideal for exam preparation, quick revision, and understanding important concepts effectively.
What is Sound in Class 9?
Sound is a crucial topic in CBSE Class 9 Science Notes Chapter 11. It explains sound as a sensation of hearing produced by vibrating objects.
These notes aim to simplify complex ideas about sound waves, their propagation, and their properties.
Understanding what sound is class 9 is fundamental to grasping various natural phenomena and technologies around us.
Detailed Notes of Sound
These notes explain Sound (Class 9, Chapter 11) in a clear and structured way, covering concepts like sound waves, their characteristics, echo, SONAR, and the human ear. They are based on the NCERT and CBSE Class 9 Science syllabus, making them useful for both understanding concepts and exam preparation.
Sound and Medium
Sound is energy that makes us hear. It originates from vibrations. Sound requires a material medium to travel. This medium can be a solid, liquid, or gas. Sound cannot travel through a vacuum. The speed of sound changes with the medium; it is fastest in solids and slowest in gases.
Sound Waves
Sound travels as a wave. A wave is a disturbance moving through a medium. Particles in the medium vibrate to transfer this disturbance. Sound waves are mechanical waves. They require a medium for propagation.
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Compression (C): A region of high pressure formed when a vibrating object pushes air particles together.
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Rarefaction (R): A region of low pressure formed when a vibrating object moves back, spreading air particles apart.
Types of Waves
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Longitudinal Waves: Particles of the medium move parallel to the wave's direction of travel. Sound waves are longitudinal waves. They travel as compressions and rarefactions.
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Transverse Waves: Particles move perpendicular to the wave's direction of travel. Light waves are examples of transverse waves.
| Types of Waves | ||
| Feature | Transverse Waves | Longitudinal Waves |
| Particle Motion | Perpendicular to wave direction | Parallel to wave direction |
| Visual Pattern | Crests & Troughs | Compressions & Rarefactions |
| Examples | Light, String vibrations | Sound, Spring compression |
| Medium | Solids/surface liquids | All states (solid, liquid, gas) |
Characteristics of Sound Waves
We describe sound waves using several properties.
| Characteristics of Sound Waves | ||
| Term | Definition | Related Concept |
| Wavelength (λ) | Distance between two consecutive compressions or rarefactions represents one complete wave. | Spatial length of a wave |
| Frequency (f) | Number of oscillations per unit time, measured in Hertz (Hz). | Pitch of sound (higher frequency → higher pitch) |
| Time Period (T) | Time taken for one complete oscillation; T = 1/f. | Reciprocal of frequency |
| Amplitude | Maximum disturbance of particles from their mean position. | Loudness of sound (greater amplitude → louder sound) |
| Pitch | Sensation of sound that depends on frequency as interpreted by the brain. | Frequency |
| Loudness | A measure of how strong or weak a sound appears to the human ear. | Amplitude |
| Quality / Timbre | A property that distinguishes sounds having same pitch and loudness. | Waveform |
| Tone | Sound produced by a single frequency. | Pure sound |
| Note | Sound is produced by a mixture of several frequencies. | Complex sound |
| Speed of Sound | Distance travelled by a point on a sound wave per unit time. | Depends on medium (density, elasticity, temperature) |
Intensity of Sound
Sound Intensity is the amount of sound energy that passes through a unit area per second. It is an objective physical quantity measured in watts per square meter (W/m²) and determines the strength of a sound. Unlike loudness, which is a subjective perception depending on human hearing, intensity quantifies the actual energy carried by sound waves.
Echo
An echo is the reflection of sound that is heard after a short time delay when sound waves strike a large, hard, and smooth surface such as a building, cliff, or wall. When you shout or clap near such a surface, the sound waves bounce back and reach your ears, producing the same sound again.
Reverberation
Reverberation is the persistence of sound in an enclosed space, such as a hall or auditorium, even after the original sound has stopped. It occurs because sound waves undergo multiple reflections from walls, ceilings, floors, and other surfaces. These repeated reflections keep the sound going for a short duration, creating a prolonged effect.
Range of Hearing
The audible range for humans is 20 Hz to 20,000 Hz.
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Infrasound: Frequencies below 20 Hz.
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Ultrasound: Frequencies above 20 kHz.
SONAR
SONAR stands for Sound Navigation and Ranging. It uses ultrasonic waves to measure distances, speeds, and directions of underwater objects. It involves a transmitter and a detector. This technology relies on the reflection of sound.
Human Ear Structure
The human ear processes sound. It has three parts:
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Outer Ear (Pinna): Gathers sound waves and directs them into the auditory canal.
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Middle Ear: Contains the eardrum and three small bones (hammer, anvil, stirrup). These bones amplify vibrations from the eardrum.
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Inner Ear: Includes the cochlea, which converts sound vibrations into electrical signals. The auditory nerve sends these signals to the brain.
Key Mechanisms of Sound
Sound is produced due to the vibration of objects and travels through a medium in the form of longitudinal waves consisting of compressions and rarefactions.
Propagation of Sound Waves
Sound waves travel through a medium by creating compressions and rarefactions. When an object vibrates, it pushes air particles, creating high-pressure regions (compressions). As it moves back, it creates low-pressure regions (rarefactions). These compressions and rarefactions move forward, transferring sound energy. The density of the medium plays a critical role in how efficiently these vibrations are transferred.
Echo Formation
An echo occurs when sound waves hit a hard surface and reflect back to the source. The ear can distinguish between the original sound and the reflected sound if there is a sufficient time delay. This delay depends on the distance to the reflecting surface and the speed of sound.
SONAR Working
SONAR systems transmit ultrasonic pulses underwater. These pulses travel through water, hit objects, and reflect back as echoes. A detector receives these echoes. By measuring the time taken for the echo to return and knowing the speed of sound in water, the distance to the object can be calculated.
Human Ear Function
The outer ear collects sound waves. These waves travel through the ear canal to the eardrum, causing it to vibrate. The middle ear bones amplify these vibrations.
The inner ear converts them into electrical signals. These signals then travel to the brain, where they are interpreted as sound. This complex process allows us to hear various sound chapter 11 class 9 notes that
Sound Notes Class 9 PDF Download
Access Sound Notes Class 9 PDF to study all important concepts like sound waves, characteristics of sound, echo, reverberation, and the human ear in one place.
These notes are NCERT-based and exam-oriented, making them ideal for quick revision. Download the PDF for easy offline study anytime, anywhere.