SOUND, CBSE CLASS IX, PHYSICS NOTES PART I

 

CBSE CLASS IX, SOUND, SCIENCE (PHYSICS) NOTES-(PART I)

                               SOUND

According to the CBSE Syllabus 2025-26

Topics in the Chapter

1. Introduction
 2. Production of Sound
 3. Propagation of Sound
 4. Sound waves are Longitudinal waves
 5. Characteristics of a Sound Wave
 6. Wavelength
 7. Frequency
 8. Time Period
 9. Amplitude
 a. Pitch
 b. Loudness
 c. Quality or Timbre
 10. Velocity
 11. Speed of sound in various medium
 12. Sonic Boom
 13. Reflection of Sound
 14. Echo
 15. Reverberation
 16. Range of Hearing
 17. Hearing Aid
 18. Application of Ultrasound
 19. SONAR
 20. Structure of Human Ear
a. Working of Human Ear

 INTRODUCTION

The sensation felt by our ears is called sound. Sound is a form of energy that makes us hear. The law of conservation of energy is also applicable to sound. Sound travels in the form of a wave.

PRODUCTION OF SOUND

I. Sound is produced when an object vibrates, or sound is produced by vibrating objects.
 II. The energy required to make an object vibrate and produce sound is provided by some outside source (like our hand, wind, etc.).
Example: The Sound of our voice is produced by the vibration of two vocal cords in our throat.
III. The Sound of a drum or tabla is produced by the vibration of its membrane when struck.
IV. In laboratory experiments, sound is produced by a vibrating tuning fork. The vibrations of a tuning fork can be shown by touching a small suspended pith ball (cork ball) with a prong of the sounding tuning fork. The pith ball is pushed away with a great force.

 SOUND CAN BE PRODUCED BY THE FOLLOWING METHODS

(i) By vibrating string (sitar)
(ii) By vibrating air (flute)
(iii) By vibrating membrane (table, drum)
(iv) By vibrating plates (bicycle bell)
(v) By friction in objects
(vi) By scratching or scrubbing the objects, etc.

WAVES

A wave is a disturbance in a medium that moves from one point to another and carries energy without a net movement of particles. It may take the form of elastic deformation or a variation of pressure.
E.g., A rubber cork on the water that goes up and down when a rock falls into the water creates a ripple.

VELOCITY

The distance travelled by a wave in one second is called the velocity of the wave.
Its SI unit is metre per second (ms-1).
Velocity = Distance travelled/Time taken
⇒ v = λ/T
(λ is the wavelength of the waves travelled in one time period T)
v = λv (1/T = v)
So, Velocity = Wavelength × Frequency
This is the wave equation.

PROPOGATION OF SOUND

a. The substance through which sound travels is called a medium.
b. The medium may be solid, liquid, or gas.
c. When an object vibrates, then the air particles around it also start vibrating in exactly the same way and displaced from their stable position.
d.  These vibrating air particles exert a force on nearby air particles so they are also displaced from their rest position and start to vibrate.
e.  This process is continued in the medium till sound reaches our ears.
f. The disturbance produced by sound travels through the medium (not the particles of the medium).
g. Wave is a disturbance which travels through a medium and carries energy.
h. So sound travels in wave form known as mechanical waves.

 PARTICLE MOTION OF MECHANICAL WAVES


TRANSVERSE WAVES


Particle motion is perpendicular to the direction of wave motion.  This type of wave is a mechanical wave.
E.g., a Light and Mexican wave in a stadium.


LONGITUDINAL WAVES


Particles travel parallel to the direction of wave motion by means of successive compressions or elongations. This is also a mechanical wave.
E.g., Sound waves in the air.

COMPRESSION

When a body vibrates then it compresses the air surrounding it and forms an area of high density called compression (C).
>  Compression is the part of a wave in which particles of the medium are closer to one another, forming high pressure.
> This compression moves away from the vibrating body.

RAREFACTION

When a vibrating body vibrates back, an area of low pressure is formed called rarefaction (R).
> Rarefaction is the area of a wave in which particles of the medium are further apart from one another, forming a low-pressure or low-density area.
> When the body vibrates back and forth, a series of compression and rarefaction is formed in air, resulting in a sound wave.
> The propagation of a sound wave is the propagation of a density change.
Sound needs a Medium for propagation

INTRODUCTION TO SOUND WAVES

Sound needs a medium to propagate. The matter or material through which sound propagates is called a medium. When particles vibrate about their mean positions, it push a region of compressed air, creating a region of high pressure, followed by a region of low pressure as the particle retreats to its mean position. The sound wave propagates by compressions and rarefactions of particles in a medium. Sound propagation can be visualised as the propagation of pressure variations in the medium.

SOUND WAVES ARE MECHANICAL WAVES

a. It needs a material medium for propogation like air, water, steel etc.                      b. It cannot travel in a vacuum.                                                                                    c. An electric bell is suspended in airtight bell jar connected with vacuum pump.        d. When bell jar is full of air, we hear the sound but when air is pumped out from the bell jar by vacuum pump and we ring the bell, no sound is heard.                            e. So, a medium is necessary for propagation of sound.

 SOUND WAVES AS LONGITUDINAL WAVES

 A wave in which the particles of the medium vibrate back and forth in the same direction in which the wave is moving is called a longitudinal wave.                          a. When we push and pull the slinky, compression (number of turns is more or closer) and rarefaction (number of turns is less or farther) are formed.                    b. When a wave travels along with slinky, each turn moves back and forth by only a small distance in the direction of the wave. So the wave is longitudinal.                      c. The direction of vibrations of the particles is parallel to the direction of the wave.
 
TRANSVERSE WAVE

When one end of a slinky is moved up and down rapidly, while the other end is fixed, it produces transverse wave.                                                                              a. This wave possess along the slinky in horizontal direction, while turns of slinky (particles) vibrate up and down at right angle to the direction of wave.                        b. Thus in transverse wave particles of the medium vibrate up and down at right angles to the direction of wave.                                                                                    c. Light waves are transverse waves but they don’t need a material medium for propagation.
 

CHARACTERISTICS OF SOUND WAVE

The characteristics of sound waves are: wavelength, frequency, amplitude, time period and velocity.
a. When a wave travel in air the density and pressure of air changes from their mean position.
b. Compression is shown by crest while rarefaction is shown by trough.
c. Compression is the region of maximum density or pressure.
d. Rarefaction is the region of minimum density or pressure.

 WAVELENGTH

 The distance between two successive crests or troughs, successive compressions and rarefactions, is called the wavelength (λ).  The SI unit of wavelength is metres (m).
It is denoted by the Greek letter lambda (λ). Its SI unit is the metre.
 TIME PERIOD

 Time taken to complete one vibration is called time period. Time required to pass two consecutive compressions or rarefactions through a point is called time period.
• SI unit of time period is second (s). Time period is denoted by T.
• The frequency of a wave is the reciprocal of the time period.
• v = 1/T

 FREQUENCY

The number of complete waves produced in one second or the number of vibrations per second is called frequency(𝛎). The number of compressions or rarefactions passed in one second is also the frequency.
The frequency of the wave is the same as the frequency of the vibrating body that produces the wave.
• The SI unit of frequency is hertz (Hz). The symbol of frequency is v (nu).
• 1 Hertz: One Hz is equal to 1 vibration per second.
• A bigger unit of frequency is kilohertz, kHz = 1000 Hz.
 
v=1/T
 
Speed (v), wavelength (λ), and frequency (𝛎) are related as v=λ𝛎
 

AMPLITUDE

 The magnitude of disturbance in a medium on either side of the mean value is called an amplitude (A).
• Amplitude is denoted by A and its SI unit is metres (m).
Sounds have characteristics like pitch and loudness, and timbre.

PITCH

The pitch of sound depends on the frequency of sound (vibration). The number of compressions or rarefactions per unit time. Directly proportional to frequency.
a. It is directly proportional to its frequency. Greater the frequency, the higher is the pitch and lesser the frequency, lower is the pitch.
b. A woman’s voice is shrill having a high pitch while a man’s voice is flat having low pitch.
c. High pitch sound has large number of compressions and rarefactions passing a fixed point per unit time.

LOUDNESS OR VOLUME

The loudness depends on the amplitude of the sound wave.
Higher force → higher amplitude → louder sound
Loudness is the measure of the sound energy reaching the ear per sec.
a. The greater the amplitude of sound wave, greater is the energy, louder the sound; short is the amplitude, less is the energy, soft is the sound.
b. Loudness is measured in decibel ‘dB’.
c. The amount of sound energy flowing per unit time through a unit area is called the intensity of sound.


QUALITY OR TIMBRE

The timbre of a sound depends on the shape of the sound wave produced by it. It is the characteristic of musical sound.
a. It helps us to distinguish between two sounds of the same pitch & loudness.
• Sound of single (same) frequency is called a tone, while a mixture of different frequencies is called a note. 
 Noise is unpleasant to hear, while music is pleasant to hear, and it is of good quality.

NOTE AND TONE

A sound of a single frequency is called a tone. A sound produced with a mixture of several frequencies is called a note.




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