Sound waves and the speed of sound
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 2 worked examples, 4 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Lightning first, thunder later
- What sound is
- Compressions and rarefactions
Read the transcript of the teaching part
Last time
Before we start: last time, in Sound, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Lightning first, thunder later
You see the lightning flash, and the thunder arrives seconds later. The light has arrived almost instantly, but sound travels about a million times slower. That gap lets you work out how far away the storm is, and it tells you a lot about what sound is.
What sound is
Sound is made when something vibrates, such as a guitar string, a drum skin, or your vocal cords. The vibrating object pushes and pulls on the nearby air particles. These pass the vibration on, from particle to particle. The particles themselves only vibrate to and fro. They do not travel with the wave, but energy does.
Compressions and rarefactions
Sound is a longitudinal wave. The particles vibrate back and forth parallel to the direction the wave travels. Where the particles are squashed together, the pressure is high. This is a compression. Where they are spread apart, the pressure is low. This is a rarefaction. The wavelength is the distance from one compression to the next.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Speed of sound and echoes · The speed of sound.
Chapter 3.4: Sound
- Sound waves and the speed of sound 5 min
- Echoes, loudness, pitch and ultrasound 5 min
Also for O Level Physics 5054
Which topics come up most in O Level Physics 5054 past papers, counted from every paper since 2004.