Know that a medium is needed to transmit sound waves

Objective

Know that a medium is needed to transmit sound waves.

What is Sound? 🎵

Sound is a vibration that travels through a medium (solid, liquid or gas) and reaches our ears. Think of a drum: when you hit it, the drum skin vibrates, pushing the air around it. That vibration travels as a wave until it reaches your ear.

How Sound Travels

Sound waves are longitudinal waves: particles of the medium move back and forth in the direction of the wave. The wave needs a material to push against; without a medium, there is nothing to transmit the vibration.

The speed of sound depends on the medium’s density ($\rho$) and bulk modulus ($B$):

$$v = \sqrt{\frac{B}{\rho}}$$

Because solids are dense but have a high bulk modulus, sound travels fastest in solids. Gases are less dense but also have a lower bulk modulus, so sound travels slower in air.

Why Can't Sound Travel in Space? 🚀

Space is a near-perfect vacuum; there are almost no particles to vibrate. Without a medium, the sound wave cannot propagate, so astronauts rely on radio (electromagnetic waves) to communicate.

Exam Tips 📚

1. Remember: Sound needs a medium. Write a short sentence explaining why.
2. Use the speed of sound table to answer questions about which medium transmits sound fastest.
3. When asked why sound cannot travel in space, mention the lack of particles and the need for a medium.
4. Use the formula $v = \sqrt{\frac{B}{\rho}}$ to show how density and bulk modulus affect speed.

Quick Quiz ❓

• Which medium transmits sound the fastest? (Answer: Steel)
• What happens to the speed of sound if the density of the medium increases but the bulk modulus stays the same? (Answer: Speed decreases)
• Why can't we hear the sound of a meteor hitting the surface of the Moon? (Answer: Lack of atmosphere to carry sound waves)