Sound waves
The idea
Pluck a rubber band and you can see the blur of vibration that you hear as sound. A sound wave is that vibration passed along through a material: the source shoves nearby air particles, they shove their neighbors, and a chain of squeezes and stretches races outward at about 340 m/s in air. Because sound is particles shoving particles, it needs a medium — in the vacuum of space there is nothing to shove, so there is no sound at all. It travels faster through water and faster still through solids, where particles sit closer together.
Two wave properties map directly onto what you hear: frequency sets pitch (more vibrations per second sounds higher), and amplitude sets loudness (bigger vibrations sound louder). The misconception to correct is that sound arrives instantly. It is quick, but 340 m/s is nothing like light speed — that is why you see a firework burst before you hear the bang, and why thunder lags behind lightning by about 3 s for every kilometer of distance.
Worked example
You clap your hands facing a canyon wall and hear the echo 2 s later. Sound travels at 340 m/s in air. How far away is the wall?
- Find the total distance the sound covered: distance = speed × time = 340 m/s × 2 s = 680 m.
- Pause before answering — the sound made a round trip, traveling to the wall AND back to your ears. The 680 m covers both legs.
- Halve it for the one-way distance: 680 m ÷ 2 = 340 m to the wall.
- Sanity-check the trap: answering 680 m would only be right if the clock stopped the instant the sound reached the wall, but you timed until the echo returned, so dividing by 2 is essential.
Answer. The canyon wall is 340 m away — half of the 680 m round trip the echo traveled.
Check your understanding
- Why does an astronaut on the Moon need a radio to talk, even when standing right next to a crewmate?
- How could you estimate how far away a storm is using the gap between lightning and thunder, and what numbers would you use?
- What changes about a guitar string's vibration when the note gets higher versus when it gets louder?
- Why might you hear an approaching train through the steel rail before you hear it through the air?
Build the foundations first
Sound waves builds on these concepts. If any feel shaky, start there.