Class 9 Science Sound Notes with PDF | NCERT Science Notes - Monelitho

Class 9 Science Sound

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1. Introduction

Sound is one of the most familiar forms of energy in our daily life. We hear voices, music, birdsong, traffic, bells, and many other sounds all around us. Sound helps us communicate, enjoy music, detect danger, and understand our surroundings. In science, sound is studied as a form of energy produced by vibrating bodies and transmitted through a medium in the form of waves.

This chapter explains how sound is produced, how it travels, why it cannot travel in vacuum, how we hear it, and how it behaves when reflected. It also introduces concepts such as amplitude, frequency, time period, pitch, loudness, quality, echo, reverberation, audible range, ultrasound, and the working of the human ear. These ideas are very important because they connect physics with biology, communication, technology, and everyday experience.

Sound may appear simple, but its study reveals many deep principles of wave motion. By understanding sound, we also understand how waves carry energy, how different materials affect transmission, and how animals and humans use sound in different ways. This chapter is therefore both practical and conceptually important.

2. What Is Sound?

Sound is a form of energy that produces the sensation of hearing in our ears. It is produced when an object vibrates. A vibrating body sets the particles of the surrounding medium into vibration, and these vibrations travel outward as sound waves.

For example, when a bell is struck, it vibrates and produces sound. When the strings of a guitar are plucked, they vibrate and create sound. When we speak, the vocal cords in our throat vibrate and produce sound.

Sound cannot be seen, but its effects can be observed. The vibration of the source is the main reason sound exists. Without vibration, there is no sound.

Examples of Sound-Producing Sources

• Human vocal cords
• Strings of musical instruments
• Drums and membranes
• Birds, bells, and horns
• Tuned metal forks

3. Sound Is Produced by Vibrations

The most important idea in this chapter is that sound is produced by vibrations. A vibration is a to-and-fro motion about a mean position. When a body vibrates, it moves rapidly backward and forward, and this motion disturbs the surrounding air, producing sound waves.

If you strike a tuning fork and place it near your ear, you can hear a sound. If you touch the tuning fork immediately after striking it, you may feel it vibrating. This simple observation shows that vibration and sound are closely related.

In some cases, the vibrations are visible. A stretched rubber band, a tuning fork, or the membrane of a drum may visibly move. In other cases, the vibrations are too small to be seen without instruments. But in every case, vibration is the source of sound.

How Vibrations Create Sound

When a body vibrates, it compresses and rarefies the surrounding medium. These changes in pressure travel away from the source. Our ears detect these pressure changes and the brain interprets them as sound.

4. Propagation of Sound

Sound needs a material medium to travel. It cannot travel through vacuum. Sound travels by making the particles of the medium vibrate one after another. The disturbance moves through the medium, but the particles themselves do not travel with the wave over large distances.

Sound can travel through solids, liquids, and gases. However, the speed of sound is different in different media. In general, sound travels fastest in solids, slower in liquids, and slowest in gases. This is because particles are closest together in solids and can transfer vibrations more quickly.

Since sound is a mechanical wave, it requires a medium. Mechanical waves are waves that need matter to travel. Sound is therefore different from light, which can travel through vacuum.

Why Sound Cannot Travel in Vacuum

In vacuum, there are no particles to vibrate and pass the disturbance onward. Since sound depends on the vibration of particles of a medium, it cannot travel without matter. This is why sound from the Sun or stars cannot reach us directly through space.

Evidence for Sound Needing a Medium

An important experiment demonstrates this idea. If an electric bell is placed inside a closed glass jar and the air is slowly pumped out, the ringing sound becomes weaker and eventually cannot be heard, even though the hammer inside the bell still vibrates. This shows that sound cannot travel without a medium.

5. Longitudinal Nature of Sound Waves

Sound waves in air are longitudinal waves. In a longitudinal wave, the particles of the medium vibrate parallel to the direction in which the wave travels. This means the particles move back and forth in the same direction as the wave.

As the sound wave moves through air, it creates alternate regions of high pressure and low pressure. These regions are called compressions and rarefactions.

Compression

A compression is a region where particles of the medium are close together and pressure is high. It is represented by the crowded part of the wave.

Rarefaction

A rarefaction is a region where particles are spread apart and pressure is low. It is the less crowded part of the wave.

The succession of compressions and rarefactions carries energy from one place to another. This is how sound travels through air.

Why Sound Waves Are Longitudinal in Air

Because the particles of air move back and forth along the same direction as the wave, the sound wave is longitudinal. This is very different from water waves on the surface or light waves, which are not longitudinal in the same sense.

6. Mechanical Wave and Its Features

Sound is a mechanical wave because it requires a medium to propagate. Mechanical waves transfer energy through the oscillation of particles in the medium. They do not carry matter from one place to another over long distances; instead, they transfer disturbance and energy.

Waves have important properties such as amplitude, wavelength, frequency, and time period. These characteristics help describe sound scientifically.

Amplitude

Amplitude is the maximum displacement of a vibrating particle from its mean position. It tells us how strong the vibration is. Greater amplitude means greater energy in the sound wave.

Wavelength

Wavelength is the distance between two consecutive compressions or two consecutive rarefactions. It is the length of one complete wave cycle.

Frequency

Frequency is the number of vibrations or oscillations completed in one second. Its unit is hertz.

Time Period

Time period is the time taken to complete one vibration or one oscillation. It is the reciprocal of frequency.

Frequency and time period are related by:

Frequency = 1 / Time period

and

Time period = 1 / Frequency

Speed of Sound

Speed of sound is the distance travelled by sound in one second. It depends on the medium and also on the temperature of the medium. Sound travels fastest in solids, then liquids, and slowest in gases.

7. Characteristics of Sound

Sound has three important characteristics that help us distinguish one sound from another. These are loudness, pitch, and quality or timbre.

7.1 Loudness

Loudness is the characteristic by which we can tell whether a sound is loud or soft. It depends mainly on the amplitude of vibration. Larger amplitude produces a louder sound, while smaller amplitude produces a softer sound.

Loudness is related to the intensity of sound. A sound wave with greater energy produces a greater sensation of loudness.

Factors Affecting Loudness

• Amplitude of vibration
• Distance from the source
• Size and nature of the vibrating body
• Sensitivity of the ear

Loudness is not the same as intensity, although they are related. Intensity is a physical quantity, while loudness is the sensation produced in the ear.

7.2 Pitch

Pitch is the characteristic of sound that helps us distinguish between a shrill sound and a deep sound. It depends on frequency. A sound with higher frequency has a higher pitch, and a sound with lower frequency has a lower pitch.

For example, a woman’s voice is usually higher in pitch than a man’s voice because the vocal cords of women generally vibrate faster. Similarly, a mosquito has a high-pitched sound, while a large drum produces a low-pitched sound.

7.3 Quality or Timbre

Quality, also called timbre, is the property that allows us to distinguish between two sounds of the same loudness and pitch but produced by different sources. For example, a flute and a violin may play the same note at the same loudness, yet we can still tell them apart because of their different quality.

Quality depends on the mixture of harmonics and overtones present in the sound.

8. Amplitude, Frequency, and Their Relationship to Sound

The amplitude of a sound wave is related to loudness. Greater amplitude means the particles of the medium vibrate more strongly. This creates stronger compressions and rarefactions, resulting in a louder sound.

Frequency is related to pitch. Higher frequency means more vibrations per second and a higher pitch. Lower frequency means fewer vibrations per second and a lower pitch.

These two properties help us understand why different sounds feel different. A loud drumbeat has high amplitude, while a shrill whistle has high frequency. A soft whisper has low amplitude, and a bass sound has low frequency.

9. Human Ear and Hearing Process

The human ear is the organ of hearing. It converts sound waves into nerve impulses that the brain can interpret. The ear is a very delicate and highly efficient organ.

Main Parts of the Ear

• Outer ear: Collects sound waves.
• Middle ear: Amplifies vibrations and passes them onward.
• Inner ear: Converts vibrations into nerve signals and helps in balance.

Outer Ear

The outer ear includes the pinna and the ear canal. The pinna collects sound from the environment and directs it into the ear canal. The sound then reaches the eardrum.

Eardrum

The eardrum is a thin membrane that vibrates when sound waves strike it. These vibrations are transmitted to the middle ear.

Middle Ear

The middle ear contains three tiny bones called the ossicles: hammer, anvil, and stirrup. These bones amplify the vibrations and pass them to the inner ear.

Inner Ear

The inner ear contains the cochlea, which is filled with fluid. The vibrations are converted into electrical signals by sensory cells in the cochlea. These signals travel through the auditory nerve to the brain, which interprets them as sound.

How We Hear

Sound waves enter the ear, make the eardrum vibrate, vibrations are passed through the middle ear bones, and finally converted into nerve impulses in the cochlea. The brain receives these impulses and gives us the sensation of hearing.

10. Audible Range of Human Hearing

Human beings can hear sound only within a certain range of frequencies. The normal audible range of the human ear is from about 20 hertz to 20,000 hertz.

Sounds with frequency below 20 hertz are called infrasonic or infrasound. Sounds with frequency above 20,000 hertz are called ultrasonic or ultrasound.

Infrasonic Sounds

Infrasonic sounds are too low in frequency for humans to hear. Some animals such as elephants and whales can use such sounds for communication.

Ultrasonic Sounds

Ultrasonic sounds are too high in frequency for humans to hear. Bats, dolphins, and some insects can produce and detect ultrasonic sounds.

Ultrasound has many practical applications in medicine, industry, and navigation.

11. Reflection of Sound

Sound, like light, can be reflected. When sound waves strike a hard surface, they bounce back. This phenomenon is called reflection of sound. Reflection is responsible for echo and reverberation.

The laws of reflection of sound are similar to those of light. The angle of incidence is equal to the angle of reflection, and the incident sound, reflected sound, and the normal at the point of incidence lie in the same plane.

Hard surfaces reflect sound better than soft surfaces. That is why empty halls tend to produce more echoes and why curtains and carpets are used to reduce sound reflection.

Applications of Reflection of Sound

• Megaphones and loudhailers
• Stethoscopes
• Sound boards in auditoriums
• Listening devices for underwater sound

12. Echo

An echo is the repetition of sound caused by reflection of sound waves from a distant surface. When reflected sound reaches the ear after a short delay, we hear it as a separate sound.

For a distinct echo to be heard, the reflected sound must reach the ear at least 0.1 second after the original sound. This usually requires the reflecting surface to be at least about 17 metres away in air, assuming normal temperature conditions.

Conditions for Echo

• The reflecting surface must be large and hard.
• The distance between source and reflector must be sufficiently large.
• The sound must be short and sharp.
• The original and reflected sound must be separated by at least 0.1 second.

Echoes are commonly heard in hills, large halls, and empty buildings. They are also used in some devices for measuring distance.

13. Reverberation

Reverberation is the persistence or prolongation of sound in a hall or room due to repeated reflections from walls, ceiling, and floor. Unlike echo, reverberation does not produce a distinct repetition but rather a lingering sound.

In a large hall, too much reverberation makes speech unclear. That is why auditoriums use sound-absorbing materials such as curtains, cushions, and fibre boards to reduce it.

Difference Between Echo and Reverberation

• Echo: A distinct repetition of sound after reflection.
• Reverberation: Persistence of sound due to multiple reflections.

Soundproofing and acoustic design are based on controlling reflection and reverberation.

14. Uses of Ultrasonic Sound

Ultrasound refers to sound waves with frequencies greater than 20,000 hertz. These waves are beyond the range of human hearing but are useful in many fields.

Medical Uses

• Ultrasonography to view internal organs and babies in the womb
• Detecting stones in the kidney and gall bladder
• Some therapeutic procedures

Industrial Uses

• Detecting cracks in metal blocks
• Cleaning delicate parts of machines
• Measuring thickness and locating internal defects

Navigation and Underwater Uses

• SONAR uses ultrasonic waves to measure depth and locate underwater objects.
• Ships and submarines use ultrasound-based systems for detection and ranging.

15. SONAR

SONAR stands for Sound Navigation and Ranging. It is a technique used to determine the distance, direction, or speed of underwater objects using ultrasonic waves.

In SONAR, ultrasonic waves are sent from a ship or submarine toward the seabed or an underwater object. The waves reflect back after hitting the object. By measuring the time taken for the echo to return, the distance can be calculated.

Working of SONAR

1. Ultrasonic waves are transmitted into water.
2. They travel until they strike an underwater object or the seabed.
3. The reflected waves return as echoes.
4. The time difference is used to calculate distance.

SONAR is very important in ocean exploration, submarine navigation, and measuring sea depth. It is one of the best examples of practical physics.

16. Structure and Working of the Human Ear in Detail

The human ear not only detects sound but also helps maintain balance. Its structure is delicate and highly specialized.

Outer Ear

The pinna collects sound waves and funnels them into the ear canal. The ear canal guides the waves to the eardrum.

Middle Ear

The middle ear contains three ossicles: hammer, anvil, and stirrup. These tiny bones amplify the vibrations received from the eardrum and transmit them to the cochlea through the oval window.

Inner Ear

The cochlea is a coiled tube filled with fluid and sensory hair cells. When vibrations enter the cochlea, they cause the fluid to move and stimulate the hair cells. These cells convert mechanical vibrations into nerve impulses.

Auditory Nerve and Brain

The auditory nerve carries the impulses from the cochlea to the brain. The brain interprets these signals as sound. This is how hearing takes place.

Balance

The inner ear also helps maintain body balance by detecting the position and movement of the head. This is an important function that allows us to walk, stand, and move without falling.

17. Important Graphical and Wave Ideas

Sound waves can be represented graphically. In a wave diagram, compressions appear as peaks of high pressure and rarefactions appear as valleys of low pressure.

The number of compressions passing a point each second indicates frequency. The distance between two compressions or rarefactions indicates wavelength. Amplitude shows how strong the wave is.

These wave properties help explain why different sources of sound produce different sensations.

18. Applications of Sound in Real Life

Sound has many practical uses in science and daily life.

• Communication through speech and phones
• Music and musical instruments
• Medical imaging using ultrasound
• Sonar in ships and submarines
• Detecting faults in metals
• Animal communication and navigation

The study of sound is therefore not just about hearing, but about understanding a very useful form of wave energy.

19. Common Misconceptions Students Make

A few misunderstandings often come up in this chapter. It is helpful to clear them early.

• Sound is not a matter; it is a form of energy.
• Sound cannot travel in vacuum.
• Loudness is not the same as pitch.
• Higher amplitude means louder sound, not necessarily higher pitch.
• Higher frequency means higher pitch, not necessarily louder sound.
• Echo and reverberation are not the same.
• Ultrasound is beyond human hearing, not a special type of loud sound.

20. Quick Revision Notes

• Sound is produced by vibrations.
• Sound needs a material medium.
• Sound is a longitudinal mechanical wave in air.
• Compressions are high-pressure regions and rarefactions are low-pressure regions.
• Amplitude determines loudness.
• Frequency determines pitch.
• Quality or timbre helps us distinguish sounds of same pitch and loudness.
• The human ear has outer, middle, and inner parts.
• Audible range of humans is about 20 hertz to 20,000 hertz.
• Echo is a distinct reflection of sound.
• Reverberation is persistence of sound.
• Ultrasound has many medical and industrial uses.
• SONAR uses ultrasonic waves for underwater detection.

21. Practice Questions

1. What is sound? How is it produced?
2. Why is sound called a mechanical wave?
3. What are compressions and rarefactions?
4. Define amplitude, frequency, and time period.
5. How do loudness and pitch depend on wave properties?
6. Explain the working of the human ear.
7. What is the audible range of human beings?
8. What is the difference between echo and reverberation?
9. What is ultrasound? Mention its uses.
10. Explain the working of SONAR.

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22. Final Understanding

Sound is a wonderful example of how energy moves through the world. It begins with vibration, travels as waves through a medium, and finally reaches our ears where it becomes a sensation. Along the way, it shows many interesting properties such as reflection, loudness, pitch, and quality. It also has practical uses in medicine, industry, communication, and navigation.

The study of sound teaches us that waves are not just abstract scientific ideas. They are part of every conversation, every musical note, every echo in a valley, and every sonar signal in the sea. Understanding sound gives us a deeper appreciation of both nature and technology.

If you learn the basic definitions carefully, revise the diagram of the human ear, and relate sound properties to daily examples, this chapter becomes very interesting and easy to remember. Sound is one of the most enjoyable topics in physics because we experience it constantly, and science helps us understand it clearly.