CBSE Class 9 Science Notes: Sound

Nature of Sound Waves

Sound, at its core, is a form of energy that travels through a medium as vibrations. These vibrations are caused by the movement of particles in the medium, creating areas of compression (high pressure) and rarefaction (low pressure). Sound waves are **longitudinal waves**, meaning the particles of the medium vibrate parallel to the direction of the wave’s propagation.

• Definition: Sound waves are mechanical waves, meaning they require a medium to travel (solid, liquid, or gas).
• Core Principle: The vibrations of a sound source cause the air molecules (or molecules of another medium) to vibrate, which then transmits the sound energy outwards.
• Key Terms: Wavelength (λ), Frequency (f), Amplitude.

Propagation of Sound in Different Media

Sound needs a material medium to propagate; it cannot travel through a vacuum. The speed of sound varies depending on the medium, influenced by factors like density, elasticity, and temperature.

• Solids: Sound travels fastest in solids due to the close proximity and strong bonding of particles, facilitating efficient vibration transfer.
• Liquids: Sound travels at a moderate speed in liquids.
• Gases: Sound travels slowest in gases, as the particles are farther apart, making the transfer of vibrations less efficient.
• Vacuum: Sound cannot travel through a vacuum, as there are no particles to vibrate.

Speed of Sound

The speed of sound is the distance a sound wave travels per unit of time. It’s affected by the properties of the medium.

• Formula: $v = f \lambda$ where ‘v’ is the speed of sound, ‘f’ is the frequency, and ‘λ’ is the wavelength.
• Factors affecting speed: Temperature (higher temperature, faster speed in gases), Density and Elasticity of the medium.
• Example: The speed of sound in air at 20°C is approximately 343 m/s.

Range of Hearing in Humans

Humans can only perceive sound within a specific frequency range.

• Audible Range: Approximately 20 Hz (Hertz) to 20,000 Hz (20 kHz). This range decreases with age.
• Infrasound: Sounds with frequencies below 20 Hz. These are often inaudible to humans. Examples: Earthquakes, some animal communications.
• Ultrasound: Sounds with frequencies above 20,000 Hz. These are used in various applications.

Ultrasound and its Uses

Ultrasound refers to sound waves with frequencies higher than the upper limit of human hearing (above 20 kHz).

• Medical Applications:
  • Ultrasound Scanning (Sonography): Imaging internal organs (e.g., during pregnancy).
  • Echocardiography: Imaging the heart.
  • Ultrasound Therapy: Treatment of certain medical conditions.
• Industrial Applications:
  • Non-destructive testing: Detecting flaws in materials.
  • Cleaning: Ultrasound baths can clean delicate objects.

Reflection of Sound and Echo

Sound, like light, can be reflected. When a sound wave encounters a surface, it bounces back.

• Echo: An echo is the repetition of a sound caused by the reflection of sound waves from a surface.
• Conditions for hearing an echo: The reflecting surface must be far enough away that the reflected sound reaches the ear at least 0.1 seconds after the original sound. This is because the persistence of hearing in human ears is 0.1 seconds.
• Calculation of Distance: The distance to the reflecting surface can be calculated using the speed of sound and the time taken for the echo to return. $distance = \frac{speed \times time}{2}$ (dividing by 2 because the sound travels to the surface and back).
• Applications: Used in sonar (sound navigation and ranging) to measure distances underwater.