What Is Sound Class 9 Physics: Definition, Properties & Examples

Definition and Nature of Sound in Class 9 Physics

Sound is a mechanical wave produced when an object vibrates. These vibrations cause the surrounding medium’s particles to oscillate, creating waves that travel to our ears. In Class 9 NCERT Science, sound is defined as a form of energy that requires a medium—solid, liquid, or gas—to propagate.

Key points:

• Sound originates from vibrating objects.
• It travels through a medium by particle vibration.
• It cannot travel through a vacuum as no particles are present.

For example, when a guitar string vibrates, it disturbs air particles, producing sound waves that reach our ears.

Understanding this definition is fundamental for grasping further concepts like wave types, speed, and properties of sound.

How Sound Travels: Mediums and Wave Types

Sound travels as longitudinal waves where particles vibrate parallel to the wave direction. These waves need a medium to carry the energy from the source to the receiver.

Mediums for Sound Travel:

• Solids: Particles are closely packed, so sound travels fastest.
• Liquids: Particles are less tightly packed than solids; sound speed is moderate.
• Gases: Particles are far apart; sound travels slowest.

This table shows how particle arrangement affects sound speed. The denser the medium, the faster sound travels.

Why Sound Cannot Travel in Vacuum:

Vacuum lacks particles, so no medium exists for vibrations to propagate, making sound transmission impossible.

Properties of Sound: Pitch, Loudness, and Quality

Sound has several important properties that affect how we perceive it:

• Pitch: Related to frequency. Higher frequency means higher pitch (e.g., a whistle), lower frequency means lower pitch (e.g., drum).
• Loudness: Depends on amplitude. Larger amplitude means louder sound.
• Quality (Timbre): Distinguishes different sounds with the same pitch and loudness (e.g., violin vs. flute).

Frequency and Pitch

Frequency ($f$) is the number of vibrations per second, measured in Hertz (Hz). The pitch is how high or low a sound seems.

Amplitude and Loudness

Amplitude is the maximum displacement of particles from their rest position. Greater amplitude means more energy and louder sound.

These properties help us identify and differentiate sounds in our environment.

Speed of Sound: Factors Affecting It

The speed of sound depends on several factors:

• Medium: Sound travels fastest in solids, slower in liquids, and slowest in gases.
• Temperature: Higher temperature increases particle energy, increasing sound speed.
• Humidity: More moisture in air increases sound speed.

Formula for Speed of Sound

In air, speed of sound can be approximated by:

$$ v = 331 + 0.6 imes T $$

where $v$ is speed in m/s and $T$ is temperature in °C.

Example:

If the air temperature is 30 °C, speed of sound is:

$$ v = 331 + 0.6 imes 30 = 331 + 18 = 349 \, m/s $$

This shows how temperature affects sound speed, important for experiments and understanding sound behaviour.

Reflection of Sound and Echo

Sound waves reflect when they hit a surface and bounce back. This reflection causes echoes.

What Is Echo?

An echo is the reflected sound heard after the original sound. It occurs if the reflecting surface is at least 17 meters away (since sound travels about 340 m/s).

Uses of Echo:

• Measuring distances in water bodies using sonar.
• Bats use echolocation to navigate.
• Ultrasound imaging in medicine.

Conditions for Hearing Echo:

• The reflected sound must reach the listener after at least 0.1 seconds delay.
• The reflecting surface must be large and hard.

Understanding echo helps in practical applications and explains sound behaviour in enclosed spaces.

Worked Example: Calculating Distance Using Echo

Problem: A person hears an echo 2 seconds after shouting. Calculate the distance of the reflecting surface. (Speed of sound = 340 m/s)

Solution:

Time for sound to travel to the surface and back = 2 seconds

Distance travelled by sound = speed × time = 340 × 2 = 680 meters

Since this is the round trip distance, the distance to the surface is:

$$ \text{Distance} = \frac{680}{2} = 340 \, \text{meters} $$

Answer: The reflecting surface is 340 meters away.

This example illustrates the practical use of sound reflection and echo in measuring distances.