Lesson 7: Propagation of Waves, and Energy Transmission and Sound Waves
Lesson 1 Objective
At the end of this section, you should be able to:
- differentiate between mechanical, electromagnetic, longitudinal, and
transverse waves; - identify sources of sound wave and explain how sound is produced;
- compare the speed of sound in different materials;
- determine the speed of sound in air at a given temperature;
- explain reflection, refraction, diffraction, and interference of sound waves.
Brainstorming Question
During a thunder storm you may see a distant lightning flash some seconds before you hear the thunder. What do you think is the reason?
key terms and concepts
- Mechanical Waves: Require a medium to travel through.
- Electromagnetic Waves: Do not require a medium to travel through.
- Transverse Waves: The direction of wave propagation is perpendicular to particle vibration.
- Longitudinal Waves: The direction of wave propagation is parallel to particle vibration.
- Sound Generation: Produced by vibrations of particles; every sound source vibrates.
- Speed of Sound: Travels faster in solids than in liquids, and faster in liquids than in gases; not significantly affected by temperature in liquids and solids.
- Echo: Reflection of sound from hard surfaces.
Propagation of Waves and Energy Transmission
Waves are disturbances that propagate energy through space or matter. Waves can be categorized based on whether they require a material medium for their propagation and the direction of particle vibrations relative to the direction of wave movement.
Mechanical Waves
Mechanical waves require a material medium to propagate, as they rely on the vibration of particles within the medium. Examples include: Sound Waves, Water Waves, Waves on Strings, etc.
Electromagnetic Waves
Electromagnetic waves do not require a medium to travel, as they consist of oscillating electric and magnetic fields. They can propagate through a vacuum. Examples include: Light Waves, radio waves, X-Rays, etc.
Transverse Waves
In transverse waves, the direction of particle vibration is perpendicular to the direction of wave propagation. Examples include: Electromagnetic Waves: such as light and radio waves, Waves on Strings, etc.
Longitudinal Waves
In longitudinal waves, the direction of particle vibration is parallel to the direction of wave propagation, resulting in compressions and rarefactions. Examples include: Sound Waves, Pressure Waves, etc.
Simulation
Sound Waves
Production and Propagation
Sound waves are longitudinal waves created by the vibration of objects. For example, when you pluck a guitar string or strike a tuning fork, the vibrations produce sound waves that travel through a medium (such as air, water, or solids). These waves carry energy, which eventually reaches your ears, allowing you to hear the sound. Sound waves require a medium to propagate, meaning they cannot travel through a vacuum.
Speed of Sound
The speed of sound varies depending on the medium through which it travels. It moves fastest in solids, slower in liquids, and slowest in gases because particles are more closely packed in solids, allowing quicker transmission of vibrations. The speed of sound in air at 00C is approximately 331 m/s. The speed can be calculated using the formula:
v = $331 \, \text{m/s} \times \sqrt{1 + \frac{T_c}{273 \, \text{K}}}$
Example: What is the speed of sound in air at 200C?
v = $331 \, \text{m/s} \times \sqrt{1 + \frac{20^\circ\text{C}}{273^\circ\text{C}}}$ ≈343m/s
Reflection and Echo
When sound waves hit a hard surface, they reflect back, creating an echo. This is the principle behind SONAR (Sound Navigation and Ranging), which measures distances underwater by timing the echo’s return. Hard surfaces like walls, rocks, or buildings are good reflectors of sound, while soft surfaces like cloth absorb sound, reducing echoes.
Refraction and Diffraction
Sound waves can bend (refraction) when traveling through mediums with varying temperatures, or spread out (diffraction) when passing through openings or around obstacles. This is why you can hear sounds around corners or from adjacent rooms
Interference
Interference occurs when two or more sound waves meet and combine. This can result in constructive interference, where the waves amplify each other, making the sound louder, or destructive interference, where the waves cancel each other out, reducing the sound. This phenomenon is observed in various situations, such as when multiple speakers play sound in a room, creating areas of louder and quieter sound due to interference patterns.
Simulation