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Sound Class 9 Notes Science Chapter 11

In our everyday lives, we come across different sounds from various sources such as people, birds, bells, machines, vehicles, televisions, radios, and more. Sound is a type of energy that makes us hear things through our ears. Just like there are other forms of energy like mechanical energy and light energy, sound is also a form of energy. When we clap our hands, we produce a sound. Have you ever wondered if it's possible to make a sound without using our own energy? Also, which form of energy do we use to create sound? In this chapter, we will explore how sound is produced and how it travels through a medium (like air or water) to reach our ears.

Production of Sound

Sound is produced when the objects are set into vibration which is to and fro motion of an object which results in production of sound.

  • Sound can be produced by striking a tuning fork and through various actions like plucking, scratching, rubbing, blowing, or shaking different objects.

Sound Class 9 Notes Science Chapter 11

  • These activities involve setting the objects into vibration, resulting in sound production.
  • Vibration refers to the rapid to and fro motion of an object.
  • The human voice produces sound through vibrations in the vocal cords.
  • The buzzing sound accompanying a bee is generated through a specific mechanism.
  • When a stretched rubber band is plucked, it vibrates and produces sound.

Question for Chapter Notes: Sound
Try yourself:What is vibration?
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Propagation of Sound

Sound Propagation and Waves:

  • Sound is created when objects vibrate  and can travel through solids, liquids, or gases.
  • When something vibrates, it makes the particles around it vibrate too.
  • These vibrating particles then pass the vibrations to nearby particles.
  • This passing on of vibrations continues, causing sound to travel through the medium.
  • Sound waves are like ripples moving through water, but in this case, it's particles in the medium that carry the disturbance.
  • Sound waves are mechanical waves because they rely on the movement of particles in the medium.

Sound Propagation in Air:

  • Air is the most common medium for sound transmission.
  • When a vibrating object moves forward, it creates a region of high pressure called compression.
  • Compressions move away from the vibrating object, while backward motion creates a region of low pressure called rarefaction.
  • Rapid back-and-forth motion of the object creates a series of compressions and rarefactions in the air, forming the sound wave.
  • Compression represents a region of high pressure, while rarefaction represents a region of low pressure.

Compression (C) & Rarefaction (R) of soundCompression (C) & Rarefaction (R) of sound

  • Pressure is related to the density of particles in the medium: higher density results in higher pressure and vice versa.

Question for Chapter Notes: Sound
Try yourself:What is compression and rarefaction in sound propagation?
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Sounds Waves are Longitudinal Wave

Longitudinal Waves:

  • Sound propagates in the medium through a series of compressions (C) and rarefactions (R).
  • These regions of closer and further apart coils create longitudinal waves.
  • In longitudinal waves, particles of the medium move parallel to the direction of the wave propagation.
  • The particles oscillate back and forth around their position of rest without moving from one place to another.
  • Sound waves are an example of longitudinal waves.

Sound Class 9 Notes Science Chapter 11

Transverse Waves:

  • Transverse waves are a different type of wave and In transverse waves, particles do not oscillate along the direction of wave propagation but move up and down about their mean position.
  • The individual particles of the medium move in a direction perpendicular to the direction of wave propagation.
  • Water waves on a pond's surface when a pebble is dropped are an example of transverse waves.
  • Light is also a transverse wave, but its oscillations are not related to medium particles, pressure, or density.
  • Light waves are not mechanical waves.

Question for Chapter Notes: Sound
Try yourself:In longitudinal waves, how do particles of the medium move in relation to the direction of wave propagation?
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Characteristics of a Sound Wave

Sound Class 9 Notes Science Chapter 11We can describe a sound wave by its:

  • Frequency 
  • Amplitude
  • Speed

Key Characteristics of Sound Waves:

  • Sound waves can be described by their frequency, amplitude, and speed.
  • The density and pressure of the medium vary with distance as the sound wave propagates.
  • Compressions are regions of high density and pressure, while rarefactions are regions of low pressure.
  • Wavelength is the distance between two consecutive compressions or rarefactions. Wavelength is represented by λ (lambda) and it’s SI unit is metre.
  • Frequency represents the number of oscillations per unit time and is measured in hertz (Hz) It is usually represented by ν (Greek letter, nu).
  • Time period is the time taken for one complete oscillation and is represented by the symbol T.
  • Frequency and time period are inversely related.
    Sound Class 9 Notes Science Chapter 11

Pitch, Amplitude, and Loudness:

  • Pitch is determined by the frequency of the sound wave, where higher frequency corresponds to a higher pitch.

Sound Class 9 Notes Science Chapter 11

  • Amplitude refers to the magnitude of the maximum disturbance in the medium.
  • Loudness is determined by the amplitude of the sound wave, with greater amplitude producing a louder sound.
  • The loudness of a sound decreases as it travels farther from its source.

Sound Class 9 Notes Science Chapter 11

Quality and Speed of Sound:

  • Quality or timber refers to the characteristic that distinguishes one sound from another with the same pitch and loudness.
  • Sound waves with a single frequency are called tones, while those with a mixture of frequencies are called notes.
  • The speed of sound is the distance travelled by a point on a wave per unit time.
    Sound Class 9 Notes Science Chapter 11
    Sound Class 9 Notes Science Chapter 11
    So, Speed of sound = wavelength × frequency.
  • The speed of sound remains constant for all frequencies in a given medium under the same conditions.

Intensity of Sound:

  • Intensity of sound refers to the amount of sound energy passing through a unit area per second.
  • Loudness is the subjective perception of sound intensity by the ear.
  • Even sounds with the same intensity can be perceived as different loudness due to variations in the ear's sensitivity.

Speed of Sound In Different Media

  • Sound travels through a medium at a finite speed, which is slower than the speed of light.
  • The speed of sound depends on the properties of the medium.
  • The speed of sound in a medium is influenced by temperature.
  • As temperature increases, the speed of sound in the medium also increases.
  • The speed of sound varies in different media at a given temperature.
  • The speed of sound decreases when transitioning from a solid to a gaseous state.
  • Increasing the temperature in a medium generally leads to an increase in the speed of sound.

Sound Class 9 Notes Science Chapter 11

Reflection of Sound

Sound waves behave similar to a rubber ball bouncing off a wall when they encounter a solid or liquid surface. Just like light, sound also follows the laws of reflection that you have learned about in previous classes. When sound is incident on a surface, it reflects in a way that the angles of incidence and reflection are equal with respect to the normal (a line perpendicular to the surface) at the point of incidence. These angles and the normal lie in the same plane. For sound waves to reflect, they require a relatively large obstacle, whether it is smooth or rough in texture.

Reflection of SoundReflection of Sound


  • Shouting or clapping near a suitable reflecting object can produce an echo.
  • An echo is the sound we hear when the original sound is reflected back to us.
  • Our brain retains the sensation of sound for approximately 0.1 seconds.

Man producing echoMan producing echo

Conditions for Hearing a Distinct Echo:

  • To perceive a clear echo, there must be a time interval of at least 0.1 seconds between the original sound and the reflected sound.
  • Assuming the speed of sound is 344 m/s at a temperature of 22 ºC in air, the total distance travelled by the sound should be at least 34.4 metres (speed × time).
  • For a distinct echo, the minimum distance between the sound source and the reflecting surface should be half of the total distance, i.e., 17.2 metres.
  • The required distance for hearing echoes may vary with changes in air temperature.

Echoes can occur more than once due to successive reflections. The rolling of thunder is caused by the sound waves reflecting off multiple surfaces, such as clouds and land.


When sound is produced in a large hall, it continues to exist due to multiple reflections from the walls until its intensity decreases to the point where it cannot be heard anymore. This prolonged presence of sound caused by reflections is known as reverberation. Excessive reverberation in an auditorium or large hall is considered undesirable. To minimise reverberation, the walls and roof of the auditorium are typically covered with materials that absorb sound, such as compressed fibreboard, rough plaster, or draperies. Additionally, the choice of seat materials takes into account their ability to absorb sound.

Reverberation of SoundReverberation of Sound

Question for Chapter Notes: Sound
Try yourself:What is the minimum time interval required for a distinct echo to be heard?
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Uses of Multiple Reflection Of Sound

  1. Megaphones, loudhailers, horns, and musical instruments like trumpets and shehanais are designed to direct sound in a specific direction instead of spreading it in all directions. These instruments have a tube and a conical opening that reflect sound waves one after another, guiding most of the sound towards the audience.
  2. A stethoscope is a medical tool used by doctors to listen to sounds produced inside the body, particularly in the heart or lungs. The sound of the patient's heartbeat reaches the doctor's ears through multiple reflections of sound within the stethoscope.
  3. In concert halls, conference halls, and cinema halls, the ceilings are often curved to ensure that sound reaches all corners of the hall. This helps to distribute sound evenly throughout the space. Sometimes, a curved soundboard is placed behind the stage to reflect sound and ensure it spreads across the entire width of the hall.Curved ceiling of conference hall
    Curved ceiling of conference hall

Range of Hearing

  • The audible range of sound for humans is between 20 Hz to 20,000 Hz (cycles per second).
  • Children under five years old and some animals, like dogs, can hear frequencies up to 25 kHz (kilohertz).Sound Class 9 Notes Science Chapter 11
  • As people age, their ears become less sensitive to higher frequencies.
  • Sounds below 20 Hz are called infrasonic sound or infrasound.
  • Infrasound is like hearing the vibrations of a pendulum or the wings of a bee.
  • Rhinoceroses, whales, and elephants communicate using infrasound.
  • Animals can sense low-frequency infrasound before earthquakes, possibly alerting them to the impending quake.
  • Frequencies above 20 kHz are called ultrasonic sound or ultrasound.
  • Animals like dolphins, bats, and porpoises produce ultrasound.
  • Certain moths can hear high-frequency squeaks of bats, allowing them to avoid capture.
  • Rats engage in games using ultrasound.

Question for Chapter Notes: Sound
Try yourself:What is the upper limit of the audible range for children under five years old and some animals?
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Applications of Ultrasound

Cleaning Hard-to-Reach Objects:

  • Ultrasound is used for cleaning parts located in difficult-to-reach places.
  • Objects are placed in a cleaning solution, and ultrasonic waves detach and remove particles of dust, grease, and dirt.
  • This method ensures thorough cleaning, even in complex shapes like spiral tubes or electronic components.

Detecting Cracks and Flaws:

  • Ultrasound is used to detect cracks and flaws in metal blocks used in construction.
  • Ultrasonic waves pass through the metal block, and detectors detect transmitted and reflected waves.
  • The presence of reflected waves indicates the presence of flaws or defects that may weaken the structure.Sound Class 9 Notes Science Chapter 11

Medical Imaging: Echocardiography:

  • Ultrasonic waves are made to reflect from various parts of the heart, creating an image.
  • Echocardiography helps in diagnosing heart conditions and abnormalities.

Medical Imaging: Ultrasonography:

  • Ultrasonic waves are used to image internal organs of the human body.
  • Changes in tissue density cause ultrasonic waves to reflect, which are then converted into electrical signals.
  • These signals generate images of organs, aiding in the detection of abnormalities, such as stones or tumours.
  • Ultrasonography is particularly useful for examining the foetus during pregnancy to detect congenital defects and growth abnormalities.

Medical Treatment: Kidney Stone Breakage:

  • Ultrasound can be employed to break small kidney stones into fine grains.
  • The fragmented stones can then be flushed out with urine, avoiding the need for invasive procedures.
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FAQs on Sound Class 9 Notes Science Chapter 11

1. What are the characteristics of a sound wave?
Ans. A sound wave is a longitudinal wave that consists of compressions and rarefactions. It travels through a medium by vibrating particles in the same direction as the wave's motion.
2. How does sound propagate through different media?
Ans. Sound can travel through solids, liquids, and gases. The speed of sound varies depending on the density and elasticity of the medium. It travels faster in solids compared to liquids and gases.
3. What is the difference between an echo and reverberation?
Ans. An echo is a distinct reflection of sound that is heard separately from the original sound, usually due to a significant distance between the source and the reflecting surface. Reverberation, on the other hand, is the persistence of sound in an enclosed space due to multiple reflections off surfaces.
4. How is the speed of sound different in various media?
Ans. The speed of sound is fastest in solids, followed by liquids, and slowest in gases. This is because the particles in solids are closer together and can transmit vibrations more efficiently than in liquids and gases.
5. What are some of the uses of multiple reflections of sound?
Ans. Multiple reflections of sound are utilized in various applications such as in architectural acoustics to design concert halls, in medical imaging techniques like ultrasound, and in sonar systems used for underwater navigation and communication.
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