NCERT Class 9 Physics Formula and Important Terms Board Sample Problems Part 5 (For CBSE, ICSE, IAS, NET, NRA 2022)

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What is sound: It is a form of energy which produces the sensation of hearing in our ears.

What is Sound: It is a Form of Energy Which Produces the Sensation of Hearing in Our Ears
S. noTermsDescriptions
1SoundIt is a form of energy which produces the sensation of hearing in our ears.
2WaveThe movement of the disturbance through a medium due to the repeated periodic motion of the particles of the medium about their mean position is known as wave. Wave transfers energy and not matter
3Mechanical WaveIt is a periodic disturbance which requires a medium (solid, liquid or gas) for its propagation

Example Sound waves

Two types Transverse and Longitudinal waves

4Transverse wavesIn this case, particles of a medium vibrate or oscillate about their mean position at right angles to the direction of the

Crest — The point on the elevation of the medium whose distance is maximum from the mean position is called crest.

Through — The point on the depressed part of the medium whose distance is maximum from the mean position is called trough.

Wavelength: The distance between two successive crests or troughs It is represented by (lambda) .

5Longitudinal wavesIn this case, the particles of the medium vibrate or oscillate to and fro about their mean position parallel to the direction of the propagation of the disturbance

When longitudinal waves passes through a medium, the medium is divided into regions of Compression and Rarefaction

Compression -It is the region of the medium where the density of the medium is high the particles of the medium are very close to each other.

Rarefaction — It is the region of the medium where the density of the medium is low i.e.. . the particles of the medium are far apart from each other.

Wavelength-The distance between two successive compressions or rarefactions

Characteristics of Sound Waves

Characteristics of Sound Waves
S. noTermsDescriptions
1WavelengthThe distance between two consecutive compressions or two consecutive rarefactions is known as the wavelength.
2FrequencyIts SI unit is metre (m) .
3AmplitudeThe number of complete oscillations per second is known as the frequency of a sound wave. It is measured in hertz (Hz) .
4Relationship between frequency, speed and wavelengthSpeed, wavelength, and frequency of a sound wave are related by the following equation: Speed (v)
5Time periodThe time interval between two successive compressions is equal to the time period of the wave. This time period is reciprocal of the frequency of the wave and is given by the relation

T = 1/Frequency

6PitchIt is the characteristic property of a sound which depends on frequency of the sound wave

More is the frequency, more is the pitch and vice versa.

High pitch is characterized by a shrill voice.

7LoudnessLoudness is a measure of the response of the ear to the sound. The loudness of a sound is defined by its amplitude. The amplitude of a sound decides its intensity, which in turn is perceived by the ear as loudness
8Quality/TimbreTimbre is the quality of sound which allows us to distinguish between different sound sources producing sound at the same pitch and loudness. The vibration of sound waves is quite complex; most sounds vibrate at several frequencies simultaneously. The additional frequencies are called overtones or harmonics. The relative strength of these overtones helps determine a sound՚s timbre.
9Intensity of SoundIntensity of a sound wave is defined as the amount of sound energy passing through a unit area per second

Range of Frequencies

Range of Frequencies
InfrasoundIt has frequencies less than 20 Hz.
Audible RangeIt has frequencies between 20 Hz and 20000Hz
UltrasoundIt has frequencies more than 20,000 Hz.

Reflection of Sound

When a sound waves travelling in a medium bounce back to the same medium after striking the second medium (a solid) , reflection of sound wave is said to take place.

Laws of reflection

  • The angle of incidence of sound wave is equal to the angle of incidence of the sound wave.
  • The incident direction of sound, reflected direction of sound and the normal to the point of incidence, all lie in the same plane.

Echo in Sound

It is the repetition of sound due to the reflection of original sound by a large and hard obstacle.

Condition for echo

  • To hear a distinct echo, the time interval between the original sound and the reflected one must be at least 0.1 s.
  • This is because we can hear two sounds distinctly when the time gap between two sounds is more than 0.1 s as our persistence of hearing is 1/ 10th of a second i.e.. 0.1 s. It means that the impression of a sound remains for 0.1 s in our brain.
  • For hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be 17.2 m.


  • It is the repeated reflection of sound which results in persistence of sound for a long time after the source of sound has stopped producing sound and its gradual fading away until it is no longer audible.
  • Reverberation time — the time during which the audible sound persists after the production of sound.

Ultrasonic Sounds

The term “ultrasonic” applied to sound refers to anything above the frequencies of audible sound, and nominally includes anything over 20,000 Hz. Frequencies used for medical diagnostic ultrasound scans extend to 10 MHz and beyond.

  • Sounds in the range 20 - 100 kHz are commonly used for communication and navigation by bats, dolphins, and some other species.
  • Much higher frequencies, in the range 1 - 20 MHz, are used for medical ultrasound.
  • Bats produce high-pitched ultrasonic squeaks. These high-pitched squeaks are reflected by objects such as preys and returned to the bat՚s ear. This allows a bat to know the distance of his prey.
  • It is also used in diagnosing diseases in human body, to kill bacteria in liquids like milk, to detect faults and cracks

Practical Application of Sounds

  • Reflection of sound is used to measure the distance and speed of underwater objects. This method is known as SONAR.
  • Working of a stethoscope is also based on reflection of sound. In a stethoscope, the sound of the patient՚s heartbeat reaches the doctor՚s ear by multiple reflection of sound.

Interesting Facts About Sounds

Interesting Facts About Sounds
S. noPoints
1Our ears vibrate in a similar way to the original source of the vibration, allowing us to hear many different sounds.
2Dogs can hear sound at a higher frequency than humans, allowing them to hear noises that we can՚t.
3Sound is used by many animals to detect danger, warning them of possible attacks before they happen.
4Sound can՚t travel through a vacuum (an area empty of matter) .
5When traveling through water, sound moves around four times faster than when it travels through air.
6The scientific study of sound waves is known as acoustics.
7The loudest natural sound on Earth is that of an erupting volcano
8The sound of thunder is produced by rapidly heated air surrounding lightning which expands faster than the speed of sound.

SI Units

SI Units
Physical QuantitySymbolNameUnit
Linear position

Length, Distance


Linear velocity-m/s
Linear momentum-Kg ⚹ m/s
Linear acceleration-m/
ForcenewtonN = kg ⚹ m/
Impulse-N ⚹ s


jouleJ = N ⚹ m
PowerwattW = J/s

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