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Sound as pressure waves
Beginner
Compressions and rarefactions in air.
Sound is a longitudinal pressure variation travelling through a medium. A loudspeaker or vocal cord compresses and rarefies the air layer by layer.
Normal conversation: pressure variations ~0.02 Pa out of 10⁵ Pa.
Air itself isn't carried across the room.
In more detail
Sound is a longitudinal pressure wave, with regions of higher and lower pressure travelling through the medium. Our ears detect these pressure changes.
Real-world uses
- Microphones convert pressure variations into electrical signals.
- Architects design rooms to control pressure waves for good acoustics.
- A loudspeaker pushes air back and forth, creating regions of high and low pressure that travel to your ear.
Open the interactive lesson, with simulation and quiz →
Frequently asked questions
Is sound a pressure or a displacement wave?
Both. The pressure and displacement vary together but are 90 degrees out of phase.
How fast do pressure changes travel?
At the speed of sound, not at the speed of the particles moving back and forth.
Do the air molecules travel with the sound?
No. They oscillate around their average positions, and only the pressure disturbance moves onward.
Related topics in Waves & Sound
End correction · Kundt's tube · Resonance tube experiment · Pitch and frequency · Loudness and intensity · Decibel scale