Sound Waves: Characteristics and Applications · Lesson 2 of 14
Propagation of Sound
“Sound can travel through solids, liquids and gases, but it cannot propagate without a material medium.”
• Explain that sound can propagate through solids liquids and gases. • Define medium and vacuum. • Use the desk and water activities as evidence for propagation. • Explain the vacuum bell jar experiment. • Apply the need for a medium to communication in outer space.
A vibrating source can produce sound, but a listener may be some distance away. Something must carry the disturbance from the source to the ear. The next question is therefore not how sound begins, but how it travels.
Propagation of Sound
Sound Through Solids
Two people can stand on opposite sides of a desk. If one person gently scratches or knocks the desk, the other first listens through air and then places an ear against the desk. The sound can be heard through the solid material, showing that sound is capable of propagating through solids.
Sound Through Liquids
When two metal spoons are tapped together under water, their sound can still be heard. The sound travels through water and then through air before reaching the ear. This provides evidence that liquids can also transmit sound.
The material through which a sound wave propagates. A medium may be a solid, liquid or gas.
A region where there is no material medium, or where matter is nearly absent.
Sound Needs a Medium to Propagate
The vacuum bell jar experiment gives a clear demonstration. An electric bell is placed inside a bell jar and switched on. At first the sound is clearly audible because air is present. As a vacuum pump removes air from the jar, the sound becomes fainter even though the bell can still be seen vibrating. In a near vacuum, almost no sound is heard. When air is returned to the jar, the sound becomes audible again.
The source is still vibrating during the experiment, so the disappearance of sound cannot be explained by the bell stopping. What has changed is the amount of matter available to transmit the disturbance.
Sound in Outer Space
Outer space is close to a vacuum. Astronauts performing a spacewalk cannot directly hear each other's voices through the surrounding space, nor can they hear two external metal objects clanking in the same way they would through air on Earth. Communication therefore relies on electronic devices in the spacesuits.
A vibrating source alone is not sufficient for sound to reach a listener. A material medium is required between the source and listener.
Quiz
Which description best matches Medium?
Which description best matches Vacuum?
Which term matches this description: The material through which a sound wave propagates.
Which term matches this description: A region where there is no material medium, or where matter is nearly absent.
Which statement is a key takeaway from this lesson?
Practice Problems
- Give one observation showing that sound travels through a solid.
- Give one observation showing that sound travels through a liquid.
- Explain why the ringing bell becomes faint when air is removed from a bell jar.
- Why cannot ordinary sound travel from one astronaut directly to another through the vacuum of space?
Key Takeaways
• Sound requires a material medium such as a solid, liquid or gas to travel. • Sound cannot propagate through a vacuum because there are no particles to transfer the disturbance. • Particles of the medium transfer the disturbance from one region to another. • The particles themselves oscillate around their mean positions rather than travelling with the sound.