Grade 7 Chapter 18: Sound: Production of Sound

Introduction to Sound

Sound is a form of energy that produces the sensation of hearing. We hear sounds all around us, from the gentle rustle of leaves to loud music and human speech. But how is sound produced, and how does it travel to our ears? In this chapter, we will explore the fascinating world of sound, its production, and its characteristics.

18.1 How is Sound Produced?

Sound is produced by vibrations. When an object vibrates, it causes the particles of the medium around it to vibrate, which then transmit the sound energy.

• Vibration: A rapid back-and-forth or to-and-fro motion of an object.
• Examples of Sound Production:
  • Stretched String: When you pluck a guitar string, it vibrates, producing sound.
  • Drum Membrane: When you hit a drum, its stretched membrane vibrates, creating sound.
  • Vocal Cords: In humans, sound is produced by the vibration of vocal cords in the larynx (voice box).
  • Flute/Whistle: In wind instruments, the air column inside vibrates to produce sound.

18.2 Propagation of Sound (Medium for Sound)

Sound needs a medium to travel. It cannot travel through a vacuum (empty space). Sound travels by creating disturbances (vibrations) in the particles of a medium, which then transmit these disturbances from one particle to the next.

• Medium: A substance through which sound waves can travel. This can be a solid, liquid, or gas.
• Speed of Sound: Sound travels at different speeds in different media. Generally, sound travels fastest in solids, slower in liquids, and slowest in gases.
  • Solid: Example: Sound travels faster through a railway track than through air.
  • Liquid: Example: Sound travels faster in water than in air.
  • Gas: Example: Sound travels slowest in air.
• Vacuum: Sound cannot travel in a vacuum because there are no particles to vibrate and transmit the sound energy. This is why you cannot hear sounds in outer space.

18.3 Characteristics of Sound

Sound has several important characteristics that help us distinguish between different sounds:

1. Loudness (Intensity)

Loudness is the characteristic of sound that depends on the amplitude of the vibration. A larger amplitude of vibration produces a louder sound, while a smaller amplitude produces a softer sound.

• Amplitude: The maximum displacement of a vibrating particle from its mean position.
• Unit: Loudness (or sound level) is measured in decibels (dB).
• Relationship: As the amplitude of sound decreases, its loudness also decreases.

2. Pitch

Pitch is the characteristic of sound that depends on the frequency of the vibration. Frequency determines how high or low a sound is.

• Frequency: The number of oscillations or vibrations per second.
• Unit: Frequency is measured in Hertz (Hz).
• Relationship:
  • Higher frequency = Higher pitch (e.g., a child's voice, a flute sound).
  • Lower frequency = Lower pitch (e.g., a man's voice, a drum sound).
• Factors affecting Pitch of a Stretched String:
  • Length: A shorter string vibrates faster, producing a higher pitch. A longer string vibrates slower, producing a lower pitch.
  • Tension: A tighter (more tensed) string vibrates faster, producing a higher pitch. A looser string vibrates slower, producing a lower pitch.
  • Thickness/Mass: A thinner/lighter string vibrates faster, producing a higher pitch. A thicker/heavier string vibrates slower, producing a lower pitch.

3. Quality (Timbre)

Quality or Timbre is the characteristic of sound that allows us to distinguish between two sounds of the same loudness and pitch, produced by different sources. It depends on the waveform of the sound.

• Example: The sound of a tabla and a sitar playing the same note (same pitch) and at the same loudness will still sound different due to their unique quality or timbre.

18.4 Audible, Infrasonic, and Ultrasonic Sound

The human ear can only hear sounds within a specific range of frequencies. This range defines audible sound.

• Audible Sound:
  • Frequency Range: Approximately 20 Hz to 20,000 Hz (20 kHz).
  • These are the sounds that humans can hear.
• Infrasonic Sound (Infrasound):
  • Frequency Range: Less than 20 Hz.
  • These sounds are too low in frequency for humans to hear.
  • Examples: Sounds produced by large animals (like elephants, whales, rhinoceroses), earthquakes, volcanoes.
• Ultrasonic Sound (Ultrasound):
  • Frequency Range: More than 20,000 Hz (20 kHz).
  • These sounds are too high in frequency for humans to hear.
  • Examples: Sounds produced by bats, dolphins, dogs (can hear higher frequencies than humans). Used in medical imaging (sonography), cleaning delicate instruments, and in SONAR.

Note: The pitch of sound is directly related to its frequency. Higher frequency means higher pitch.