Notes on Class 9 Science Chapter Sound for SEBA/CBSE

Notes on Class 9 Science Chapter Sound for SEBA/CBSE

Here we have provided summary and revision notes on Class 9 Science Chapter 12 Sound. These notes include key points, short revision notes, diagrams, and easy explanations from the entire chapter. If you are a Class 9 student studying from the NCERT Science textbook, this chapter is very important. Whether you are a student of CBSE or State Board like SEBA, these notee very helpful to all class 9 students.

You will find everything about Notes on Class 9 Science Chapter Sound in one place. For a better understanding of the chapter, also read the NCERT Solutions for Class 9 Science Chapter 12.

What is Sound?

• Sound is a form of energy that produces a sensation of hearing in our ears.
• Sound follows the law of conservation of energy.  The law of conservation of energy states that energy cannot be created or destroyed; it can only be changed from one form to another.

How is Sound Produced?

Sound is produced by vibrating objects.

What is vibration?

Vibration means a kind of rapid to and fro motion of an object.

Examples :

• Sound of human voice comes from vibrations in vocal cords.

Propagation of Sound (How Sound Travels)

What is a Medium?

The matter or substance through which sound is transmitted is called a medium.

• It can be: Solid (metal, wood), Liquid (water), or Gas (air).
• Sound CANNOT travel through vacuum (empty space with no particles)
• This is why astronauts on the moon cannot hear each other even if they shout.

How Does Sound Travel Through a Medium?

When an object vibrates, it makes the adjacent particles vibrate too. These particles do not move all the way from the source to the ear. A particle near the vibrating object first moves from its rest position. It then pushes the neighbouring particle and transfers the vibration to it. After this, the first particle returns to its original position. In the same way, vibrations pass from one particle to the next. This process continues through the medium until the sound reaches the ear.

Sound Waves and Compressions/Rarefactions

What is a Wave?

A wave is a disturbance that travels through a medium. When particles of the medium start vibrating, they make the neighbouring particles vibrate as well. These particles pass the disturbance to others in the same way. The particles of the medium do not move forward themselves; only the disturbance moves ahead through the medium.

Types of Waves

Waves are broadly classified as Mechanical Wave and Electromagnetic Wave.

What are mechanical waves?

A mechanical wave is a wave that needs a material medium (like air, water, or solid) to propagate by vibrating particles of that medium. Sound waves are mechanical waves.

What are Electromagnetic waves?

An electromagnetic wave is a wave that does not require any material medium to propagate. Examples include Light waves and radio waves.

Mechanical waves are further classified as Longitudinal waves and Transverse waves.

Longitudinal Waves (Sound Waves)

A longitudinal wave is a wave in which the individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance. Or

Any wave where particles move parallel to the direction of wave propagation is called a Longitudinal Wave.

• Example: Sound waves are longitudinal waves.

Characteristics:

• Particles oscillate along the direction of wave travel
• Particles don’t move from one place to another

Transverse Waves       

A transverse wave is a wave in which the particles of the medium vibrate perpendicular to the direction in which the wave travels. OR

A transverse wave is produced when particles move perpendicular to the direction of wave propagation. The particles in a transverse wave oscillate in an up and down motion.

• Example: Ripples on water surface, when we drop a pebble in a pond.
• Example: Light is a transverse wave.
• Sound is NOT a transverse wave.

COMPRESSION (C):-

When a vibrating object moves forward, it pushes and compresses the air in front of it creating a region of high pressure. This region is called a compression.

 Forward motion → Creates COMPRESSION (C)

• Region of high pressure
• Particles are tightly packed together
• Shown as crests in wave diagrams

RAREFACTION (R):

When the vibrating object moves backwards, it creates a region of low pressure called rarefaction (R).

Backward motion → Creates RAREFACTION (R)

• Region of low pressure
• Particles are spread apart
• Shown as troughs in wave diagrams

Characteristics of Sound Waves

A peak is called the crest and a valley is called the trough of a wave.

Sound waves can be described by three main properties:

• frequency
• amplitude and
• speed

1. FREQUENCY (\(\nu\) – Nu)

Definition: The number of complete oscillations per unit time is the frequency of the sound wave.

Symbol: ν (Greek letter nu)

SI Unit: Hertz (Hz)

• 1 Hz =\(\frac{1}{s}\) =1 oscillation per second
• Named after scientist H.R. Hertz

Common units:

• 1 kHz = 1000 Hz
• 1 MHz = 1,000,000 Hz

Key Point: Higher frequency = Higher Pitch of the sound

2. WAVELENGTH (λ – Lambda)

Definition: The distance between two consecutive compressions (C) OR two consecutive rarefactions (R) is called the wavelength.

Or: The distance between one crest and the next crest (or trough to trough)

Symbol: λ (Greek letter lambda)

SI Unit: Meter (m)

Key Point: Shorter wavelength = Higher frequency (closer compressions)

3. AMPLITUDE (A)

Amplitude is the maximum displacement of a vibrating particle from its mean (rest) position.

Symbol: It is usually represented by the letter A.

Units: Same as density or pressure

Relation to Sound:

• Larger amplitude = LOUDER sound
• Smaller amplitude = SOFTER sound

4. Time Period

The time taken for one complete oscillation of the medium.

Or:  The time taken by two consecutive compressions or rarefactions to pass a fixed point is called the time period of the wave.

Symbol: T

SI Unit: Second (s)

Relation between Frequency and Time Period:

\( \nu = \frac{1}{T} \)
or
\( T = \frac{1}{\nu} \)

• \(\nu\) = frequency
• λ = wavelength
• T = time period

• If frequency is high, time period is short
• If frequency is low, time period is long

Example: If frequency = 2 Hz, then T = 1/2 = 0.5 seconds

5. Speed of Sound (v)

Speed of sound = The speed of sound is defined as the distance which compression or a rarefaction, travels per unit time. OR

Speed of sound is the distance travelled by a sound wave per unit time.

We know, \( \text{speed}, v = \frac{distance}{time}\)
\( ⇒ v=\frac{\lambda}{t}\) (Here λ is the wavelength of the sound wave)
Since, \(\nu =\frac{1}{T}\) (Here \(\nu\) is the frequency of the sound wave)
\( ⇒ v=\lambda\times\nu\)

So, speed = wavelength × frequency.

Speed of Sound in Different Media

The speed of sound depends on:

1. Nature of the medium (solid/liquid/gas)
2. Temperature of the medium

Important Pattern:

• Speed is highest in solids, less in liquids, and least in gases. So, speed of sound depends on the density of the medium. More density of medium= More speed of sound.
• In any medium as we increase the temperature, the speed of sound also increases.

Speed of Sound at 25°C:

Speed of Sound vs Light

• The speed of light is about \( 3 \times 10^8 \) m/s.
• While the speed of sound in air is only about 344 m/s.
• So, light travels much faster than the sound.
• That is the reason why we hear the thunder a few seconds after the flash of thunder is seen, though flash and thunder occur simultaneously.

Supersonic Speed

When an object travels faster than the speed of sound, it is said to be travelling at supersonic speed. Examples: Bullets, jet aircraft.

Sonic Boom:

A sonic boom is the loud, explosive sound produced when an object moves faster than the speed of sound, creating shock waves. These shock waves carry significant energy, causing intense air pressure variations.

Effects: Can shatter glass. May damage buildings

Pitch, Loudness, and Quality of Sound

1. PITCH

Definition: How the brain interprets the frequency of an emitted sound is called its pitch.

Pitch of a sound depends on the frequency of the sound.

Relation:

• Larger frequency = high-pitched sound
• Smaller frequency = Low-pitched sound

Examples:

• A child’s voice has higher pitch (higher frequency)
• A deep voice has lower pitch (lower frequency)
• A whistle has high pitch
• A drum has low pitch

2. LOUDNESS

Loudness is the property of sound that tells whether a sound is loud or soft as heard by our ears. It is a physiological response of the ear to intensity (subjective).

Loudness of sound mainly depends on the amplitude of the sound wave.

Relation:

• Larger amplitude = LOUDER sound
• Smaller amplitude = SOFTER sound

Examples:

• Striking a table hard = Loud sound (large amplitude)
• Striking a table gently = Soft sound (small amplitude)

3. QUALITY (or TIMBRE)

The characteristic that enables us to distinguish one sound from another having the same pitch and loudness is called the quality or timber of sound.

Examples:

• We can distinguish between guitar and flute even if they play the same note.

Related Terms:

Tone = A sound of single frequency is called a tone.

Note = The sound which is produced due to a mixture of several frequencies is called a note.

Noise = Noise is unpleasant to the ear.

Music = Music is pleasant to hear and is of rich quality.

4. Intensity of Sound:

The amount of sound energy passing each second through unit area is called the intensity of sound.

Difference between Intensity and Loudness of Sound

Reflection of Sound

Sound reflects off a solid or liquid surfaces just like light reflects off mirrors. The laws of reflection of light are also applicable to reflection of sound.

1. The incident sound wave, the reflected sound wave and normal at the point of incidence lie in the same plane.
2. Angle of reflection of sound is always equal to the angle of incidence of sound

Echo

When we shout or clap near a suitable reflecting surface like a tall building or a mountain, we hear the same sound again after a short time. This repeated sound is called an echo.

Therefore, to hear a clear echo, the time interval between the original sound and the reflected sound must be at least 0.1 second, as the sensation of sound remains in our brain for about 0.1 second.  For hearing clear echo, the reflecting obstacle must be at a minimum distance of about 17.2 metres from the source of sound.

Reverberation

Reverberation is the persistence of sound in an enclosed space due to repeated reflections from walls, ceilings, and floors of a big hall or room.

To reduce reverberation, halls use:

• Compressed fibreboard
• Rough plaster
• Draperies/curtains
• Special seat materials with sound-absorbing properties

Audible Range and Hearing

The audible range for the human ear is from 20 Hz to 20,000 Hz

Important notes:

• Young children and animals like dogs can hear up to 25 kHz
• As people age, sensitivity to higher frequencies decreases

Infrasound (Infrasonic Sound)

Infrasound is sound having frequency less than 20 Hz

Characteristics:

• Cannot be heard by humans
• But some animals can hear them

Animals that produce infrasound:

• Rhinoceroses communicate at 5 Hz.
• Whales and elephants produce low-frequency infrasound.

Interesting fact: Some animals get disturbed before earthquakes because earthquakes produce low-frequency infrasound before the main shock waves begin.

Ultrasound (Ultrasonic Sound)

Frequencies higher than 20 kHz (20,000 Hz)are called ultrasonic sound or ultrasound.

Animals that produce/hear ultrasound:

• Dolphins: Use ultrasound for navigation and communication
• Bats: Use ultrasound for echolocation
• Porpoises: Use ultrasound for navigation and communication
• Moths: Have sensitive ears to detect bat ultrasound (helps them escape)
• Rats: Play games using ultrasound

Hearing Aid

People with hearing loss often need a hearing aid to hear sounds clearly. A hearing aid is an electronic device that works with the help of a battery. It receives sound through a microphone. The microphone changes sound waves into electrical signals. These signals are then made stronger by an amplifier. After amplification, the signals are sent to a small speaker inside the hearing aid. The speaker converts the electrical signals back into sound and delivers them to the ear, making the sound louder and clearer.

Applications of Ultrasound

1. Industrial Cleaning

Ultrasound is used to clean objects placed in hard-to-reach areas, such as spiral tubes, oddly shaped parts, and electronic components. The objects are placed in a cleaning solution, and ultrasonic waves are sent through the solution. The high-frequency waves make dust, grease, and dirt particles detach and fall off, cleaning the objects thoroughly.

Used for: Spiral tubes, odd-shaped parts, electronic components, jewelry cleaning

2. Detection of Flaws in Metal Blocks

Ultrasound can be used to detect cracks or flaws in metal blocks. These metal blocks are often part of structures like buildings, bridges, machines, or scientific instruments. Any internal cracks or holes, which cannot be seen from the outside, weaken the structure. When ultrasonic waves are sent through the metallic block, a detector on the other side monitors the waves that pass through. If there is even a small defect, the ultrasound get reflected back instead of passing through, indicating the presence of the flaw or defect, as shown in the following Figure.

Used in: Construction of buildings, bridges, machines, scientific equipment

3. Medical Applications

Echocardiography

In echocardiography, ultrasonic waves are reflected from different parts of the heart to create its image , helping doctors study heart function.

Ultrasonography (Ultrasound Scanning)

An ultrasound scanner uses ultrasonic waves to get images of internal organs like the liver, gall bladder, uterus, and kidneys. The waves travel through body tissues and reflect from areas where tissue density changes. These reflected waves are converted into electrical signals, which are used to generate images displayed on a monitor or printed on film. This imaging technique is called ultrasonography and helps detect abnormalities such as kidney stones, gall bladder stones, or tumors. It is also used to examine a foetus during pregnancy to monitor growth and detect congenital defects.

Kidney Stone Treatment

Ultrasound can also be used for treatment, such as breaking kidney stones into tiny grains, which are later flushed out through urine.

SONAR

Working of SONAR:

SONAR stands for Sound Navigation and Ranging. SONAR consists of a transmitter and a detector and is installed in a boat or a ship. The transmitter produces and transmits (sends) ultrasonic waves into the water. These waves travel through seawater and hit an object on the seabed. After striking the object, the waves are reflected back and received by the detector. The detector converts these ultrasonic waves into electrical signals, which are then analysed. The distance of the object can be calculated by knowing the speed of sound in water and the time taken for the sound to go and return. If the time taken is t and the speed of sound in seawater is v, then the total distance travelled by the sound waves is 2d = v × t.

This method is called echo-ranging.

Applications of SONAR:

The sonar technique is used to measure the depth of the sea and to locate underwater hills, valleys, submarine, icebergs, sunken ship etc.

Structure of Human Ear

The outer part of the ear is called the ‘pinna’. It collects sounds from the surroundings and passes through the auditory canal. At the end of the auditory canal, there is a thin membrane called the eardrum. When a compression of sound reaches the eardrum, it moves the eardrum inward, and when a rarefaction arrives, it moves outward. In this way the eardrum vibrates. These vibrations are increased in strength by three bones in the middle ear—the hammer, anvil, and stirrup. The middle ear then passes these stronger vibrations to the inner ear. In the inner ear, the vibrations are converted into electrical signals by the cochlea. These electrical signals are sent to the brain via the auditory nerve. The brain interprets these signals as sound.

Important Formulas and Relationships

1. Speed, Wavelength, and Frequency
   \( v=\lambda\times\nu\)
   
2. Frequency and Time Period
   \( \nu = \frac{1}{T} \)
   or
   \( T = \frac{1}{\nu} \)

3. Distance Calculation

Distance = Speed × Time

4. For Echo Problems

Distance to reflecting surface = (Speed × Time) / 2

Summary of Key Points

✓ Sound is produced by vibrating objects.
✓ Sound travels through a medium (solid, liquid, or gas).
✓ Sound cannot travel through a vacuum.
✓ Sound travels as compressions and rarefactions.
✓ Sound waves are longitudinal waves.
✓ Particles don’t move with sound; only the disturbance travels.
✓ Pitch depends on frequency.
✓ Loudness depends on amplitude.
✓ Speed of sound is same for all frequencies in a medium.
✓ Sound also follows the laws of reflection.
✓ Humans can hear 20 Hz to 20 kHz.
✓ Infrasound: below 20 Hz.
✓ Ultrasound: above 20 kHz.
✓ Ultrasound has many medical and industrial uses

Important Definitions (Quick Reference).

NCERT Notes for Class 9 Science

• Chapter 1 Matter in Our Surroundings Class 9 Notes
• Chapter 2 Is Matter Around Us Pure Class 9 Notes
• Chapter 3 Atoms and Molecules Class 9 Notes
• Chapter 4 Structure of the Atom Class 9 Notes
• Chapter 5 The Fundamental Unit of Life Class 9 Notes
• Chapter 6 Tissues Class 9 Notes
• Chapter 7 Diversity in Living Organisms Class 9 Notes
• Chapter 8 Motion Class 9 Notes
• Chapter 9 Force and Laws of Motion Class 9 Notes
• Chapter 10 Gravitation Class 9 Notes
• Chapter 11 Work, Power And Energy Class 9 Notes
• Chapter 12 Sound Class 9 Notes
• Chapter 13 Why Do we Fall ill Class 9 Notes
• Chapter 14 Natural Resources Class 9 Notes
• Chapter 15 Improvement in Food Resources Class 9 Notes