Class 9 Science Notes Chapter 12 (Sound) – Science Book

Alright class, let's focus on Chapter 12, 'Sound', from your NCERT Class 9 Science textbook. This is a fundamental chapter, and understanding its concepts is crucial not just for your class exams but also for various government competitive examinations. We will cover the key points in detail.

Chapter 12: Sound - Detailed Notes for Competitive Exams

1. Introduction to Sound

• Sound is a form of energy that produces the sensation of hearing in our ears.
• It plays a vital role in communication.

2. Production of Sound

• Core Principle: Sound is produced by vibrating objects. Vibration is the rapid to and fro or back and forth motion of an object.
• Examples:
  • Human Voice: Produced by vibrations of the vocal cords in the larynx (voice box).
  • Musical Instruments:
    • String Instruments (Sitar, Guitar): Vibrating strings.
    • Wind Instruments (Flute, Trumpet): Vibrating air columns.
    • Percussion Instruments (Drum, Tabla): Vibrating membranes (skins).
  • Tuning Fork: A standard source of sound produced by vibrating prongs.
  • Stretching a rubber band and plucking it.
  • Striking a bell.

3. Propagation of Sound

• Medium Required: Sound needs a material medium (solid, liquid, or gas) to travel. It cannot travel through a vacuum.
  • Demonstration Concept: Bell jar experiment (electric bell inside a vacuum jar becomes inaudible).
• How Sound Travels: The particles of the medium do not travel all the way from the source to the ear. Instead, the disturbance created by the vibrating source travels through the medium.
  • Vibrating object sets adjacent particles of the medium into vibration.
  • These particles transfer their energy to the next particles, and so on.
  • This results in the propagation of the disturbance as a wave.
• Sound Waves are Longitudinal Waves:
  • In these waves, the individual particles of the medium move back and forth parallel to the direction of propagation of the disturbance.
  • Sound travels through a medium as a series of Compressions (C) and Rarefactions (R).
    • Compression: Region of high pressure and density where particles are crowded together.
    • Rarefaction: Region of low pressure and density where particles are spread apart.
  • One compression and one adjacent rarefaction constitute one complete sound wave.
• Analogy: Similar to pushing and pulling a spring (slinky).

4. Characteristics of a Sound Wave

• Sound waves can be described by their:
  • Frequency (ν - nu):
    • Definition: The number of complete oscillations (or cycles/vibrations) per second. It determines the pitch of the sound.
    • Unit: Hertz (Hz). 1 Hz = 1 oscillation per second.
    • Pitch: How the brain interprets frequency. High frequency = High pitch (shrill sound); Low frequency = Low pitch (grave/flat sound). (e.g., woman's voice generally has higher pitch than a man's).
  • Amplitude (A):
    • Definition: The maximum displacement of the particles of the medium from their mean (undisturbed) position. It determines the loudness of the sound.
    • Unit: For a wave, it relates to pressure/density variations. Loudness is often measured in decibels (dB), although amplitude itself is displacement.
    • Loudness: Depends on amplitude. Large amplitude = Loud sound; Small amplitude = Soft sound.
  • Time Period (T):
    • Definition: The time taken to complete one full oscillation (or cycle/vibration).
    • Unit: second (s).
    • Relation with Frequency: Frequency and Time Period are reciprocals.
      ν = 1 / T or T = 1 / ν
  • Wavelength (λ - lambda):
    • Definition: The distance between two consecutive compressions (C) or two consecutive rarefactions (R). It is the spatial period of the wave.
    • Unit: metre (m).
  • Speed (v):
    • Definition: The distance which a point on a wave (like a compression) travels per unit time.
    • Unit: metre per second (m/s).
    • Relation: Speed = Wavelength × Frequency
      v = λ × ν
    • Factors Affecting Speed:
      • Medium: Speed of sound depends primarily on the nature of the medium. Generally, v_solid > v_liquid > v_gas. (Sound travels fastest in solids, slowest in gases).
      • Temperature: Speed of sound increases with an increase in temperature. (e.g., Speed of sound in air is approx. 331 m/s at 0°C and 344 m/s at 22°C).
      • Humidity: Speed of sound in air increases slightly with humidity.
    • Note: Speed of sound is nearly independent of pressure for gases. It depends on amplitude only in extreme cases.

5. Reflection of Sound

• Like light, sound gets reflected at the surface of a solid or liquid and follows the same laws of reflection:
  • The angle of incidence equals the angle of reflection.
  • The incident sound wave, the reflected sound wave, and the normal to the reflecting surface at the point of incidence all lie in the same plane.
• Echo:
  • Definition: The repetition of sound caused by the reflection of sound waves from a hard surface (like a building, cliff, hill).
  • Condition for hearing a distinct echo: The time interval between the original sound and the reflected sound (echo) must be at least 0.1 seconds. This is due to the persistence of hearing of the human ear.
  • Minimum Distance Calculation:
    • Let d be the distance to the reflector, v be the speed of sound, t be the time interval (0.1 s).
    • Total distance travelled by sound = 2d (to the reflector and back).
    • Speed = Total Distance / Time => v = 2d / t
    • Minimum distance d = (v × t) / 2
    • Using v = 344 m/s (approx. speed in air at room temp) and t = 0.1 s,
    • d = (344 × 0.1) / 2 = 34.4 / 2 = 17.2 metres.
    • So, the minimum distance from a reflecting surface to hear a distinct echo is approximately 17.2 m. This distance changes with the temperature of the air.
• Reverberation:
  • Definition: The persistence of sound in a closed space (like a big hall or auditorium) as a result of multiple reflections from surfaces like walls, ceiling, and floor.
  • Effect: If reverberation is too long, sound becomes blurred, distorted, and confusing.
  • Reduction: To reduce undesirable reverberation, auditoriums and concert halls use sound-absorbing materials on walls and ceilings (like compressed fibreboard, rough plaster, draperies) and seats (upholstered).
• Applications of Reflection of Sound:
  • Megaphones, Horns, Trumpets, Shehnais: Designed to direct sound in a particular direction without spreading, using multiple reflections.
  • Stethoscope: A medical instrument used by doctors to listen to sounds produced within the body (heart, lungs). Sound undergoes multiple reflections in the tube of the stethoscope.
  • Soundboards: Curved surfaces placed behind the stage in concert halls or auditoriums to reflect sound evenly towards the audience.
  • Ceilings of Concert Halls: Often curved to reflect sound towards the audience.

6. Range of Hearing

• Audible Range (Human): The range of frequencies that humans can hear is typically from 20 Hz to 20,000 Hz (or 20 kHz).
• Infrasound (or Infrasonic Sound): Sounds with frequencies below 20 Hz.
  • Humans cannot hear infrasound.
  • Produced by: Earthquakes, volcanic eruptions, ocean waves, vibrating pendulums.
  • Some animals like whales, elephants, and rhinoceroses communicate using infrasound.
• Ultrasound (or Ultrasonic Sound): Sounds with frequencies above 20 kHz (20,000 Hz).
  • Humans cannot hear ultrasound.
  • Produced and detected by: Bats, dolphins, porpoises, rats, dogs (dogs can hear up to ~45 kHz).
  • Bats use ultrasound for navigation and finding prey (echolocation).

7. Applications of Ultrasound

• Ultrasound waves have high frequency and short wavelength, allowing them to travel along well-defined paths and penetrate materials.
• Industrial Uses:
  • Cleaning intricate parts (e.g., spiral tubes, electronic components) by immersing them in a cleaning solution and passing ultrasound; vibrations dislodge dirt.
  • Detecting cracks and flaws in metal blocks or structures without damaging them. Ultrasound pulses are sent; reflections from cracks indicate flaws.
• Medical Uses:
  • Ultrasonography: Used to create images of internal human organs (liver, gall bladder, uterus, kidney, heart). Works by detecting reflected ultrasound waves (echoes) from different parts of the organs. Used extensively in prenatal care (imaging the fetus).
  • Echocardiography: Using ultrasound waves to image the heart.
  • Breaking kidney stones into fine grains, which are then flushed out with urine.
• SONAR (SOund Navigation And Ranging):
  • Principle: Uses ultrasonic waves to measure the distance, direction, and speed of underwater objects.
  • Components: A transmitter (produces and transmits ultrasound) and a detector/receiver (receives the reflected ultrasound).
  • Working: Transmitter sends ultrasound pulses. These travel through water, strike an object (seabed, submarine, iceberg, fish shoal), and reflect back. The receiver detects the echo.
  • Calculation: The time interval (t) between transmission and reception is measured. The speed of sound in water (v) is known. The distance (d) to the object is calculated using 2d = v × t or d = (v × t) / 2. This technique is called echo-ranging.

8. Structure of the Human Ear

• The ear is the sense organ for hearing. It converts sound waves (pressure variations in air) into electrical signals that travel to the brain via the auditory nerve.
• Three Main Parts:
  • Outer Ear (Pinna and Auditory Canal):
    • Pinna: Collects sound waves from the surroundings.
    • Auditory Canal: Sound travels through this canal to reach the eardrum.
  • Middle Ear (Eardrum, Hammer, Anvil, Stirrup):
    • Eardrum (Tympanic Membrane): A thin membrane at the end of the auditory canal. Compressions and rarefactions cause it to vibrate inwards and outwards.
    • Three Bones (Ossicles): Hammer (Malleus), Anvil (Incus), and Stirrup (Stapes). These act as levers to amplify the vibrations received from the eardrum. The stirrup transmits the amplified vibrations to the oval window of the inner ear.
  • Inner Ear (Cochlea and Auditory Nerve):
    • Cochlea: A spiral-shaped, fluid-filled cavity. It converts the pressure variations (amplified vibrations) into electrical signals.
    • Auditory Nerve: Transmits these electrical signals from the cochlea to the brain, where they are interpreted as sound.

Multiple Choice Questions (MCQs)

1. Sound cannot travel through:
   (a) Solids
   (b) Liquids
   (c) Gases
   (d) Vacuum

2. The characteristic of sound that distinguishes a shrill sound from a flat sound is:
   (a) Amplitude
   (b) Frequency
   (c) Wavelength
   (d) Speed

3. A sound wave travels at 340 m/s. If its wavelength is 1.7 m, what is its frequency?
   (a) 20 Hz
   (b) 578 Hz
   (c) 200 Hz
   (d) 0.005 Hz

4. The minimum distance required between the source of sound and a reflector to hear a distinct echo (at approx. 22°C) is:
   (a) 10.0 m
   (b) 17.2 m
   (c) 34.4 m
   (d) 0.1 m

5. Which part of the human ear converts sound vibrations into electrical signals?
   (a) Eardrum
   (b) Hammer, Anvil, Stirrup
   (c) Cochlea
   (d) Auditory Canal

6. SONAR technology primarily uses:
   (a) Infrasonic waves
   (b) Audible sound waves
   (c) Ultrasonic waves
   (d) Radio waves

7. Reverberation in a large hall can be reduced by using:
   (a) Polished surfaces
   (b) Sound-absorbing materials like curtains and carpets
   (c) Hard reflecting surfaces
   (d) Increasing the volume of the sound source

8. Which of the following frequency ranges represents infrasound?
   (a) 0 Hz - 20 Hz
   (b) 20 Hz - 20,000 Hz
   (c) Above 20,000 Hz
   (d) 15 Hz - 15,000 Hz

9. When sound travels through air, the air particles:
   (a) Vibrate along the direction of wave propagation
   (b) Vibrate perpendicular to the direction of wave propagation
   (c) Move from the source to the listener
   (d) Do not vibrate

10. The loudness of a sound wave is primarily determined by its:
    (a) Frequency
    (b) Speed
    (c) Wavelength
    (d) Amplitude

Answers to MCQs:

1. (d) Vacuum
2. (b) Frequency
3. (c) 200 Hz (Calculation: ν = v / λ = 340 m/s / 1.7 m = 200 Hz)
4. (b) 17.2 m
5. (c) Cochlea
6. (c) Ultrasonic waves
7. (b) Sound-absorbing materials like curtains and carpets
8. (a) 0 Hz - 20 Hz (Technically, frequencies below 20 Hz)
9. (a) Vibrate along the direction of wave propagation (Longitudinal wave)
10. (d) Amplitude

Make sure you understand the definitions, units, relationships (like v=λν, T=1/ν), and applications thoroughly. This chapter often features numerical problems based on speed, frequency, wavelength, and echo calculations in exams. Good luck with your preparation!