Sound
Sound is a longitudinal wave that needs a medium to travel through. It's characterised by frequency (pitch) and amplitude (loudness), can be measured using the wave equation, and has practical applications from medicine to engineering.
Sound Waves
What is Sound?
Sound is produced by vibrating objects. When something vibrates, it pushes the particles around it (usually air), which then bump into other particles, passing the energy forward. This chain reaction of particles pushing each other is what we experience as sound.
The key insight: sound is a longitudinal wave. This means the particles vibrate backwards and forwards in the same direction that the wave is travelling — not side to side like a ripple on water.
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Sound needs a medium to travel. A medium is any material made of particles (air, water, metal, rubber). In a vacuum (where there are no particles), sound cannot travel at all — not even a little. This is why space is silent.
Representing Longitudinal Waves
Longitudinal waves are tricky to draw because the particle motion isn't visible as a simple up-down pattern. Instead, we draw them as a series of vertical lines to show particle movement parallel to the direction of travel:
COMPRESSIONS (lines close together) RAREFACTIONS (lines far apart)
║ ║ ║ ║ ║ ║ ║ ║ ║ ║ ║ ║ ║
───────────────────────────────────────────────────────>
Direction of wave travel
When lines are bunched close: HIGH density of particles (compression)
When lines are spread out: LOW density of particles (rarefaction)
Sound waves shown as series of lines. Lines don't move left-right — they represent particles moving in the direction of the arrow.
The key to understanding this diagram: imagine the lines are people standing in a row. A compression is when everyone squeezes together. A rarefaction is when everyone spreads out. The "wave" is the pattern of squeezing and spreading moving along the row.
Compression and Rarefaction (Extended Tier)
Compression
A region where the particles are pushed tightly together, creating a zone of high density and high pressure. Think of a tightly packed crowd in one corner of a room.
Rarefaction
A region where the particles are spread far apart, creating a zone of low density and low pressure. Think of everyone in the room spreading out to opposite corners.
Sound is fundamentally a pressure wave. When compressions and rarefactions pass through a material, they cause rapid changes in pressure. When these pressure changes reach a solid object (like your eardrum or a window), the object vibrates in sync with the wave.
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Examiner's pet peeve:
Q1: Explain why sound cannot travel through a vacuum, even though it's an electromagnetic phenomenon.