Sound Waves: Characteristics and Applications · Lesson 3 of 14
Sound Waves
“Sound travels as moving compressions and rarefactions while medium particles merely oscillate about their mean positions.”
• Explain the slinky analogy for sound propagation. • Define compression rarefaction and sound wave. • Explain why medium particles do not travel with the wave. • Describe a sound wave as longitudinal. • Define mechanical wave. • Compare longitudinal and transverse waves.
The fact that sound needs matter raises another question: does the matter itself travel all the way from the source to the listener? A slinky provides a useful analogy because it makes the movement of a disturbance visible.
Sound Waves
Activity: Observing a Disturbance in a Slinky
Stretch a slinky along a horizontal surface and mark one of its turns. Push and pull one end sharply. Regions where the turns crowd together and regions where they spread apart travel along the slinky. The marked turn, however, does not move from one end to the other. It oscillates around approximately the same position.
This distinction is crucial. The disturbance travels through the material, while individual parts of the material undergo local motion.
Compression and Rarefaction
A region of a sound wave in which particles of the medium are closer together than average, producing higher density.
A region of a sound wave in which particles of the medium are farther apart than average, producing lower density.
Imagine air in a tube with a piston at one end. When the piston moves forward, it pushes nearby air particles together and creates a high-density compression. Collisions pass this compressed region forward. When the piston moves backward, nearby air becomes less dense, creating a rarefaction. Repeated motion creates alternating compressions and rarefactions.
A disturbance consisting of alternating compressions and rarefactions propagating through a medium without the actual flow of the medium's particles from source to listener.
If a source is small and the medium is not confined to a tube, the disturbances can spread outward in many directions. A point-like source can therefore produce approximately spherical sound waves.
Longitudinal Waves
A wave in which particles of the medium vibrate parallel to the direction in which the wave propagates.
In a sound wave through air, the particles move back and forth along the same general direction in which the density disturbance travels. This makes sound a longitudinal wave in the treatment used here.
Mechanical Waves
A wave that requires a material medium for propagation.
Because sound cannot propagate without a material medium, it is a mechanical wave. Mechanical waves may be longitudinal or transverse. In a transverse wave, the particles vibrate perpendicular to the direction of wave propagation.
Earthquakes can produce both longitudinal and transverse seismic waves. Light is different: it can travel through vacuum and is therefore not a mechanical wave.
Sudden Sound Pulses
A firecracker explosion or thunder can create a sudden loud sound when gases expand very rapidly, producing a strong density disturbance that moves outward. A supersonic aircraft creates a more complex disturbance perceived as a sonic boom.
In sound propagation, the disturbance and energy travel. The particles of the medium do not continuously travel from the source to the listener.
Quiz
Which description best matches Compression?
Which description best matches Rarefaction?
Which term matches this description: A region of a sound wave in which particles of the medium are closer together than average, producing higher density.
Which term matches this description: A region of a sound wave in which particles of the medium are farther apart than average, producing lower density.
Which statement is a key takeaway from this lesson?
Practice Problems
- Use a marked turn of a slinky to explain why particles do not travel with a sound wave.
- Define compression and rarefaction in terms of density.
- Why is sound described as a longitudinal wave?
- Why is sound a mechanical wave?
- State one difference between longitudinal and transverse waves.
Key Takeaways
• Sound in air travels as a longitudinal wave. • The medium contains alternating regions of compression and rarefaction. • In compressions, particles are closer together and pressure is higher; in rarefactions, particles are farther apart and pressure is lower. • The wave moves forward while individual particles oscillate back and forth about their mean positions.