Grade 8 Chapter 15: Sound

Introduction

Sound is a form of energy that travels in waves and enables us to hear. From the chirping of birds to the music we enjoy, sound is an integral part of our daily experience. This chapter will explore how sound is produced, how it travels through different mediums, its various characteristics like pitch and loudness, and phenomena such as echo and reverberation. We will also learn about the human ear, noise pollution, and the fascinating applications of ultrasound in technology and medicine.

Production and Propagation of Sound

• Production of Sound: Sound is produced by vibrations. When an object vibrates, it causes the particles of the surrounding medium to vibrate, which then transmit the sound energy.
  • Examples: Vibrating strings of a guitar, vibrating vocal cords, vibrating diaphragm of a speaker.
• Propagation of Sound: Sound needs a material medium (solid, liquid, or gas) to travel. It cannot travel through a vacuum.
  • Sound travels as a wave, specifically a longitudinal wave, meaning the particles of the medium vibrate parallel to the direction of wave propagation.
  • Speed of Sound: The speed of sound varies in different mediums. It is generally fastest in solids, slower in liquids, and slowest in gases.
    • Speed of sound in air (at 20°C) ≈ 343 m/s
    • Speed of sound in water ≈ 1480 m/s
    • Speed of sound in steel ≈ 5100 m/s

Characteristics of Sound

Sound waves have several characteristics that describe them:

• Amplitude: The maximum displacement of the particles of the medium from their mean position. It determines the loudness of the sound. Greater amplitude means louder sound.
• Frequency (f): The number of oscillations or vibrations per second. Its unit is Hertz (Hz). Frequency determines the pitch of the sound. Higher frequency means higher pitch (shriller sound).
• Wavelength (λ): The distance between two consecutive compressions or rarefactions of a sound wave.
• Speed (v): The distance covered by a sound wave per unit time. Relationship: v = f × λ (Speed = Frequency × Wavelength).
• Pitch: The characteristic of sound that depends on its frequency. A high-pitched sound has a high frequency (e.g., a child's voice, a whistle). A low-pitched sound has a low frequency (e.g., a lion's roar, a man's voice).
• Loudness: The characteristic of sound that depends on its amplitude. A sound with a larger amplitude is louder. Loudness is measured in decibels (dB).
• Quality or Timbre: The characteristic that distinguishes two sounds of the same pitch and loudness produced by different sources (e.g., distinguishing a guitar from a piano playing the same note). It depends on the shape of the sound wave.

Audible, Infrasonic, and Ultrasonic Sounds

• Audible Sound: Sounds that humans can hear. The human ear can detect frequencies between 20 Hz and 20,000 Hz (20 kHz).
• Infrasonic Sound: Sounds with frequencies below 20 Hz. Humans cannot hear them.
  • Examples: Sounds produced by earthquakes, volcanoes, some large animals (e.g., whales, elephants).
• Ultrasonic Sound (Ultrasound): Sounds with frequencies above 20,000 Hz (20 kHz). Humans cannot hear them.
  • Examples: Sounds produced by bats, dolphins, dogs (can hear up to 50 kHz).

Reflection of Sound

Like light, sound waves can also reflect off surfaces.

• Echo: The phenomenon of hearing the same sound again due to its reflection from a distant obstacle (like a wall, mountain). For a clear echo, the minimum distance to the reflecting surface should be about 17.2 meters (assuming speed of sound 344 m/s and persistence of hearing 0.1 s).
• Reverberation: The persistence of sound in a large hall or auditorium due to repeated reflections from the walls, ceiling, and floor. Excessive reverberation can make sound unclear.

Absorption of Sound

When sound waves strike a surface, a part of the sound energy is absorbed by the surface. Soft, porous materials are good absorbers of sound.

• Examples: Curtains, carpets, foam, acoustic panels are used in auditoriums and recording studios to reduce reverberation and improve sound quality by absorbing sound.

Human Ear: Structure and Function

The human ear is a delicate and complex organ responsible for hearing.

• Outer Ear (Pinna): Collects sound waves and directs them into the ear canal.
• Middle Ear: Consists of the eardrum (tympanic membrane) and three tiny bones (ossicles: malleus, incus, stapes). Sound waves cause the eardrum to vibrate, and these vibrations are amplified by the ossicles.
• Inner Ear: Contains the cochlea (a snail-shaped organ). The amplified vibrations from the middle ear are converted into electrical signals in the cochlea. These signals are then sent to the brain via the auditory nerve, where they are interpreted as sound.

Noise Pollution

Noise pollution is excessive or unwanted sound that can cause adverse effects on human health and the environment. It is measured in decibels (dB).

• Sources: Vehicular traffic, industrial machinery, loudspeakers, construction activities, firecrackers.
• Effects: Hearing loss, stress, high blood pressure, sleep disturbance, headaches, irritation, reduced concentration.
• Control Measures:
  • Using silencers in vehicles and machinery.
  • Planting trees (act as sound barriers).
  • Avoiding unnecessary use of horns and loudspeakers.
  • Creating 'silent zones' near hospitals and schools.
  • Implementing strict regulations for noise levels.

Uses of Ultrasound

Ultrasonic sounds, which are beyond the human hearing range, have numerous practical applications:

• Medical Applications:
  • Sonography (Ultrasonography): Used to image internal organs (e.g., fetus during pregnancy, kidney stones, heart). It is safe as it does not use harmful radiation.
  • Echocardiography: Imaging of the heart.
  • Breaking Kidney Stones: High-intensity ultrasound waves can be used to break kidney stones into smaller pieces that can be passed out with urine.
• Industrial Applications:
  • Cleaning: Used to clean hard-to-reach parts of objects (e.g., electronic components, jewelry) by creating high-frequency vibrations.
  • Detecting Flaws: Used to detect cracks and flaws in metal blocks and structures without damaging them (non-destructive testing).
• Navigation and Ranging:
  • SONAR (Sound Navigation And Ranging): Used in ships to measure the depth of the sea, locate underwater objects (submarines, shipwrecks), and map the seabed.
  • Bats and Dolphins: Use ultrasound for navigation and hunting (echolocation).

Summary

• Sound is produced by vibrations and needs a medium (solid, liquid, gas) to propagate. It cannot travel in vacuum.
• Speed of sound: Solids > Liquids > Gases.
• Characteristics:
  • Loudness depends on amplitude (measured in dB).
  • Pitch depends on frequency (measured in Hz).
  • Quality/Timbre distinguishes sounds from different sources.
• Audible range for humans: 20 Hz to 20,000 Hz. Below 20 Hz is Infrasonic; above 20,000 Hz is Ultrasonic.
• Reflection of Sound:
  • Echo: Reflection from a distant surface.
  • Reverberation: Multiple reflections in an enclosed space.
• Absorption of Sound: Soft, porous materials absorb sound.
• Human Ear: Outer ear collects, middle ear amplifies, inner ear converts to electrical signals for the brain.
• Noise Pollution: Excessive unwanted sound, causes health issues. Controlled by silencers, tree planting, silent zones.
• Uses of Ultrasound: Sonography, breaking kidney stones, cleaning, flaw detection in industries, SONAR, echolocation by bats/dolphins.

References

1. Maharashtra State Board Science and Technology Standard Eight Textbook (Specific Edition/Year) - Chapter 15: Sound.
2. Maharashtra State Board 8th Standard Science Syllabus.
3. Balbharati Science and Technology Textbook.