NCERT Science Notes - Class 9
Chapter 11 - Sound

Welcome to AJs Chalo Seekhen. This webpage is dedicated to Class 9 | Science | Chapter - 11 | Sound. In this chapter, students delve into the world of acoustic waves. This chapter covers how sound is produced, how it travels through different mediums, and how our ears perceive it. Key concepts include the characteristics of sound waves such as frequency, amplitude, and speed. The chapter also explores the phenomena of reflection, echo, and reverberation, along with the applications of sound in everyday life and technologies. With practical examples and experiments, it aims to give students a comprehensive understanding of the scientific principles behind sound, fostering a deeper appreciation for this essential aspect of the natural world.

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NCERT Science Notes - Class 9
Chapter 11 - Sound

Activity 11.1: Production of Sound

Materials Needed:

• Tuning fork
• Rubber pad
• Table tennis ball or small plastic ball
• Thread
• Needle
• Support for suspending the ball

1. Setting the Tuning Fork Vibrating:
   • Strike the prong of the tuning fork on a rubber pad to set it vibrating.
   • Bring the vibrating tuning fork near your ear.
2. Observation:
   • Do you hear any sound?
   • Explanation: When the tuning fork vibrates, it creates sound waves that travel through the air to your ear.
3. Touching the Vibrating Prong:
   • Touch one of the prongs of the vibrating tuning fork with your finger.
   • Share Your Experience: Describe what you felt and discuss it with your friends.
   • Explanation: Touching the vibrating prong stops the vibration, and you should feel the vibrations and notice the sound stopping.
4. Suspending the Ball:
   • Suspend a table tennis ball or a small plastic ball using a thread. Use a needle to thread the ball and tie a knot at one end.
5. Touching the Ball with the Tuning Fork:
   • Gently touch the suspended ball with the prong of the vibrating tuning fork.
   • Observation: What happens when the vibrating tuning fork touches the ball?
   • Explanation: The vibrations from the tuning fork are transferred to the ball, causing it to move. This demonstrates how sound waves can transfer energy to other objects.

• Sound: A form of energy that is produced by vibrating objects and travels through a medium (solid, liquid, or gas).
• Vibration: Rapid back and forth motion of particles or objects.
• Medium: Substance through which sound waves travel (e.g., air, water, solids).

Questions for Students:

1. Do you hear any sound when the tuning fork is brought near your ear? Why?
   • Answer: Yes, because the vibrating tuning fork produces sound waves that travel through the air and reach your ear.
2. What happens when you touch the vibrating tuning fork with your finger?
   • Answer: The vibrations stop, and the sound stops because your finger absorbs the vibrations.
3. What happens when the vibrating tuning fork touches the suspended ball?
   • Answer: The ball moves because it absorbs the vibrations from the tuning fork.

Activity 11.2: Understanding Sound Production

1. Activity Steps:
   • Step 1: Fill a beaker or glass with water up to the brim.
   • Step 2: Gently touch the water surface with one of the prongs of a vibrating tuning fork. Observe what happens.
   • Step 3: Dip the prongs of the vibrating tuning fork into the water. Observe the effect.
   • Discussion: Discuss with others why these effects are observed.
2. Observations and Conclusion:
   • Observation: In both cases, the water's surface reacts when touched by the vibrating tuning fork, indicating the transfer of energy from the vibrations of the tuning fork to the water.
   • Conclusion:
     • Sound is produced due to the vibrations of objects.
     • Answer to Question: No, you cannot produce sound without a vibrating object.
3. Methods of Producing Sound:
   • We can produce sound by various actions:
     • Striking: Example - Striking the tuning fork.
     • Plucking: Example - Plucking a rubber band.
     • Scratching, Rubbing, Blowing, or Shaking: Each action causes objects to vibrate, producing sound.
4. Definition of Vibration:
   • Vibration: A rapid to-and-fro motion of an object.
5. Examples of Sound Produced by Vibrations:
   • Human Voice: Produced by the vibration of vocal cords.
   • Birds: Wing flapping produces sound.
   • Buzzing Sound of a Bee: Created by the rapid vibrations of its wings.
   • Rubber Band: When a stretched rubber band is plucked, it vibrates, creating sound. (Try this activity if you haven’t done it before.)

Activity 11.3: Musical Instruments and Sound Production

11.2 - Propagation of Sound

1. Production and Propagation of Sound:
   • Sound Production: Sound is created by vibrating objects.
   • Medium for Transmission: The substance through which sound travels is called a medium. This medium can be solid, liquid, or gas.
2. How Sound Travels:
   • Sound travels through a medium from the vibrating object to the ear.
   • A particle of the medium near the vibrating object is first displaced from its original (equilibrium) position.
   • This particle then exerts a force on adjacent particles, causing them to displace. This chain reaction continues until the sound reaches the ear.
   • Key Point: It is the disturbance that moves through the medium, not the actual particles of the medium.
3. Definition of Wave:
   • A wave is defined as a disturbance that moves through a medium. It sets neighboring particles into motion without causing the particles themselves to move forward.
4. Sound Waves as Mechanical Waves:
   • Sound waves are mechanical waves because they require a medium to travel.
   • Example of Medium: Air is the most common medium through which sound travels.
5. Compression and Rarefaction:
   • Compression (C): A region of high pressure where particles are close together, created when a vibrating object pushes air particles forward.
   • Rarefaction (R): A region of low pressure where particles are spread out, created when a vibrating object moves backward.
   • As the object vibrates back and forth, a series of compressions and rarefactions is formed, creating a sound wave that travels to the listener.
6. Propagation of Sound as Pressure Variations:
   • High Pressure (Compression): High density of particles creates high pressure.
   • Low Pressure (Rarefaction): Lower density of particles results in lower pressure.
   • Sound propagates as variations in density and pressure in the medium.

11.2.1 - Sound Waves are Longitudinal Waves

1. Activity 11.4: Understanding Longitudinal Waves with a Slinky:
   • Instructions:
     • Stretch a slinky with one friend holding one end and you holding the other.
     • Push and pull the slinky sharply.
     • Observation: A dot on the slinky moves back and forth, parallel to the direction of the disturbance.
   • Result: Regions where coils are close together are called compressions (C), and where coils are spread apart are called rarefactions (R).
2. Comparison with Sound Propagation:
   • Sound Propagation: Sound travels in a medium as a series of compressions and rarefactions, similar to the motion observed in the slinky.
   • Longitudinal Waves: In longitudinal waves, particles of the medium move parallel to the direction of the disturbance.
   • Oscillation: The particles do not move forward but instead oscillate back and forth around their equilibrium position.
3. Definition of Longitudinal Waves:
   • Longitudinal waves are waves where particles of the medium move parallel to the wave’s direction of travel. Sound waves are longitudinal waves.
4. Transverse Waves:
   • In transverse waves, particles move perpendicular to the direction of wave propagation.
   • Example: Dropping a pebble in a pond creates transverse waves on the water’s surface.
   • Characteristics of Transverse Waves:
     • Particles oscillate up and down rather than back and forth.
     • Light is an example of a transverse wave, but it does not involve medium particles or pressure changes, so it is not a mechanical wave.
5. Key Differences Between Longitudinal and Transverse Waves:
   • Longitudinal Waves: Particle motion is parallel to wave direction (e.g., sound waves).
   • Transverse Waves: Particle motion is perpendicular to wave direction (e.g., water surface waves, light waves).

Heinrich Rudolph Hertz

1. Personal Background:
   • Name: Heinrich Rudolph Hertz
   • Birth: 22 February 1857
   • Place: Hamburg, Germany
   • Education: University of Berlin
2. Scientific Contributions:
   • Electromagnetic Theory:
     • Hertz confirmed J.C. Maxwell’s electromagnetic theory through his experiments.
     • His work laid the foundation for the development of radio, telephone, telegraph, and television.
   • Photoelectric Effect:
     • He discovered the photoelectric effect, a phenomenon later explained by Albert Einstein.
3. Honors:
   • The SI unit of frequency, named hertz (Hz), was designated in his honor to recognize his contributions to science.

Frequency, Time Period, Pitch, Amplitude, and Speed of Sound

1. Frequency (ν):
   • Definition: Frequency refers to how frequently an event occurs within a unit time. For sound, it is the number of oscillations or compressions/rarefactions per unit time.
   • Example: If you beat a drum multiple times in one second, the number of beats per second is the frequency of the drum.
   • Representation: Represented by the Greek letter ν (nu).
   • SI Unit: Hertz (Hz).
   • Relation to Time Period: Frequency and time period are inversely related: $$\nu =\frac{1}{T}$$
2. Time Period (T):
   • Definition: The time period is the time taken for one complete oscillation of the wave, or for two consecutive compressions or rarefactions to pass a fixed point.
   • SI Unit: Second (s).
   • Formula: Time period, $T = \frac{1}{ν}$
     
     
3. Pitch:
   • Definition: Pitch is how the brain interprets the frequency of a sound; it is a characteristic that determines the perceived "highness" or "lowness" of a sound.
   • Relation to Frequency: Higher frequency results in a higher pitch, while lower frequency results in a lower pitch.
   • Example: Different musical instruments (e.g., violin and flute) produce sounds with different pitches due to their distinct frequencies.
     
     
4. Amplitude (A):
   • Definition: Amplitude is the maximum displacement of particles in the medium from the mean position. It represents the magnitude of the maximum disturbance in the medium.
   • SI Unit: The unit of amplitude depends on the nature of the wave; for sound, it can be in terms of density or pressure.
   • Relation to Loudness: Larger amplitude produces louder sound, while smaller amplitude produces softer sound.
     
     
5. Quality (Timbre):
   • Definition: Quality, or timbre, is the characteristic that allows us to distinguish between sounds of the same pitch and loudness. It describes the richness or pleasantness of a sound.
   • Tone: Sound of a single frequency.
   • Note: Sound with a mixture of frequencies, usually pleasant.
   • Noise: Unpleasant, jumbled sound.
     
     
6. Speed of Sound (v):
   • Definition: The speed of sound is the distance a point on the wave (like a compression or rarefaction) travels per unit time.
   • Formula: $$v = λ × ν$$where $λ$ is the wavelength (distance sound travels in one time period, $T$).
   • Characteristics: The speed of sound in a given medium remains constant for all frequencies under consistent physical conditions.

Example 11.1: Calculation of Time for Sound to Travel a Given Distance

Problem: A sound wave has a frequency of $\nu = 2 \, \text{kHz} = 2000 \, \text{Hz}$ν=2kHz=2000Hz and a wavelength $\lambda = 35 \, \text{cm} = 0.35 \, \text{m}$λ=35cm=0.35m. We need to find the time it takes for the sound to travel a distance of 1.5 km.

Solution

1. Calculate Speed of Sound (v):
   • Formula: $v = \lambda \times \nu$
     
   • Calculation: $$v=0.35\text{m}\times 2000\text{Hz}=700\text{m/s}$$
2. Calculate Time Taken (t):
   • Given distance, $d = 1.5 \, \text{km} = 1500 \, \text{m}$
     
   • Formula: $t = \frac{d}{v}$
     
   • Calculation: $$t=\frac{1500\text{m}}{700\text{m/s}}=2.1\text{s}$$
   Answer: Sound will take 2.1 seconds to travel a distance of 1.5 km.

Key Concept: Intensity vs. Loudness

• Intensity of Sound: The amount of sound energy passing per second through a unit area. It is an objective measure of sound energy.
• Loudness: Refers to how the ear perceives the sound. It is a subjective quality and can vary depending on sensitivity of the ear, even if two sounds have the same intensity.

11.2.3 - Speed of Sound in Different Media

• Speed of Sound: Sound travels through a medium at a finite speed. Notably, the sound of thunder is heard after the flash of lightning is seen, indicating that sound travels much slower than light.

1. Properties of the Medium: The speed of sound varies based on the medium through which it travels:
   • State of Matter: Sound travels fastest in solids, slower in liquids, and slowest in gases.
   • Temperature: The speed of sound increases with an increase in the temperature of the medium. For example:
     • In air, the speed of sound is 331 m/s at 0ºC and 344 m/s at 22ºC.

Note: You are not required to memorize these values.

11.3 - Reflection of Sound

11.3.1 - Echo

• Definition: An echo is the reflection of sound that we hear after a delay. When we shout or clap near a suitable reflecting object (like a tall building or a mountain), we may hear the same sound again after some time.

• The sensation of sound lingers in our brain for about 0.1 seconds.
• To perceive a distinct echo, the time interval between the original sound and the reflected sound must be at least 0.1 seconds.

• Assuming the speed of sound is 344 m/s at 22 ºC in air:
  • The sound must travel to the reflecting obstacle and back to the listener within 0.1 seconds.
  • Total distance covered by sound: $$\text{Total Distance} = 344 \, \text{m/s} \times 0.1 \, \text{s} = 34.4 \, \text{m}$$
    
  • Therefore, the minimum distance of the obstacle from the sound source must be half of this distance: $$\text{Minimum Distance} = \frac{34.4 \, \text{m}}{2} = 17.2 \, \text{m}$$
    

• This minimum distance can change with the temperature of the air, as the speed of sound varies with temperature.

• Echoes may occur multiple times due to successive reflections from various surfaces.
• For example, the rolling of thunder is the result of sound reflecting off multiple surfaces, such as clouds and the ground, creating a prolonged echo effect.

11.3.2 - Reverberation

11.3.3 - Uses of Multiple Reflection of Sound

11.4 - Range of Hearing

• Audible Range: The audible range of sound for humans extends from approximately 20 Hz to 20,000 Hz (20 kHz). Here, 1 Hz equals one cycle per second.
• Children and Animals: Children under the age of five and some animals, such as dogs, can hear frequencies up to 25 kHz (1 kHz = 1000 Hz).
• Age-Related Hearing Loss: As individuals age, their sensitivity to higher frequencies typically decreases.
  
  
• Infrasonic Sound:
  • Frequencies below 20 Hz are referred to as infrasonic sound or infrasound.
  • If humans could hear infrasound, they might detect vibrations like the pendulum's oscillation or the wings of a bee.
  • Some animals, such as rhinoceroses, use infrasound for communication, with frequencies as low as 5 Hz.
  • Whales and elephants also produce sounds in the infrasound range.
  • It has been observed that certain animals may sense infrasound before earthquakes occur, as low-frequency sounds are generated before the main shock waves.
    
    
• Ultrasonic Sound:
  • Frequencies higher than 20 kHz are classified as ultrasonic sound or ultrasound.
  • Ultrasound is utilized by animals such as dolphins, bats, and porpoises.
  • Certain moths possess highly sensitive hearing and can detect the high-frequency sounds of bats, allowing them to evade predation. Rats also communicate using ultrasound.
    
    
• Hearing Aid:
  • Individuals with hearing loss may benefit from a hearing aid, which is an electronic, battery-operated device.
  • A hearing aid functions as follows:
    • It receives sound through a microphone that converts sound waves into electrical signals.
    • An amplifier amplifies these electrical signals.
    • The amplified signals are sent to a speaker, which converts them back into sound to facilitate clear hearing for the user.

11.5 - Applications of Ultrasound

Ultrasound refers to high-frequency sound waves that can travel along well-defined paths, even in the presence of obstacles. Its applications are extensive in both industrial and medical fields.

• Cleaning:
  • Ultrasound is used to clean parts located in hard-to-reach areas, such as spiral tubes, oddly shaped components, and electronic parts.
  • Objects are placed in a cleaning solution, and ultrasonic waves are sent into the solution. The high frequency causes dust, grease, and dirt particles to detach and fall away, resulting in thorough cleaning.
• Non-Destructive Testing:
  • Ultrasound can detect cracks and flaws in metal blocks used in large structures like buildings, bridges, machines, and scientific equipment.
  • Cracks or holes that are invisible externally can compromise structural integrity.
  • Ultrasonic waves are transmitted through the metal block, and detectors capture the reflected waves. If there is a defect, the ultrasound reflects back, indicating the presence of a flaw. Ordinary sound waves are ineffective for this purpose as they can bend around corners and enter the detector without detecting the defect.
• Medical Imaging:
  • Echocardiography: Ultrasonic waves are used to reflect off various parts of the heart to create an image of the heart, assisting in the diagnosis of heart conditions.
• Ultrasound Scanners:
  • These instruments utilize ultrasonic waves to obtain images of internal organs in the human body.
  • Doctors can examine organs such as the liver, gallbladder, uterus, and kidneys to detect abnormalities, including stones or tumors.
  • The ultrasonic waves travel through body tissues and reflect from areas where there are changes in tissue density, helping to create clear images.

NCERT Science Notes - Class 9 | Chapter 11 - Sound

NCERT Science Notes - Class 9 | Chapter 11 - Sound