TwiddleLab / Physics / Waves and Sound

Sound Is a Pressure Wave

Lesson 3 of 10 · 7 min · Free, no account needed

What you will learn

• Sound is a pressure wave: squeezes and stretches of air that travel, while each bit of air only jiggles back and forth.
• It is a longitudinal wave: the air moves along the direction the wave travels.
• Sound needs a medium. In air at 20 °C it goes about 343 m/s, so thunder takes about 3 seconds per km.

Start the lesson

How the lesson goes

1. You see lightning, and hear the thunder 6 seconds later. How far away was it?

2. What does a speaker do to the air? Push it and see.

   Each dot is a little patch of air.

3. A bell in a jar. What happens to its sound when the air goes?

   Ring it full of air, then pump the air out and ring again.

4. What sound is.

   Sound is a pressure wave: squeezes (high pressure) and stretches (low pressure) of the air that travel outward.

   It is a longitudinal wave: the air jiggles along the direction the wave goes. On the rope, the bits moved across it: that kind is called transverse.

   Sound needs a medium. In air at 20 °C it travels about 343 m/s: 1 km in about 3 seconds.

   Simplified: we show air as dots in rows and slow everything down a lot. Light from the flash arrives almost at once (it is about a million times faster).

5. Game: Storm tracker. Find each lightning strike from the thunder delay.

   Four strikes. Your score is the metres you are off, in total.

6. Sound travels through other stuff too. Which is fastest?

   Sound speed depends on how hard the stuff springs back when squeezed compared with how heavy it is. Steel springs back enormously hard, so the push races through it.

7. Code: a microphone turns sound into a list of numbers.

   Step through one wave of a tone.

8. Lightning strikes above you. Where does your friend stand if they hear the thunder 3 s later?

   Distance = speed × time = 343 m/s × 3 s = 1029 m. The flash reaches them almost instantly, so the delay is all sound travel time.

9. Your storm map. Which strike did you nail?

Previous lesson: Frequency, Wavelength, Speed · Next lesson: Pitch and Loudness · Part of the course: Waves and Sound

Lessons are interactive and need JavaScript. Turn it on to play them.