Sound waves
Sound is a longitudinal mechanical wave. It travels through air (and other media) as a series of compressions (high-pressure regions) and rarefactions (low-pressure regions).
Where sound comes from
A vibrating object (vocal cords, guitar string, loudspeaker cone) pushes air molecules back and forth. These compressions travel outward as pressure waves, eventually reaching ears.
Properties
- Speed: ~340 m/s in air at room temperature; faster in water (~1500 m/s); much faster in solids (~5000 m/s in steel).
- Frequency: determines pitch. High f = high pitch.
- Amplitude: determines loudness. High A = loud.
- Sound waves cannot travel through a vacuum — no particles to vibrate.
Range of human hearing
- ~20 Hz to 20 kHz (in young people).
- This range narrows with age (especially the high end).
- Ultrasound: f > 20 kHz — used in medical imaging, industrial flaw detection, ultrasonic cleaning.
- Infrasound: f < 20 Hz — produced by earthquakes, large explosions; can be felt rather than heard.
How sound is converted in the ear
- Pressure waves enter the ear canal.
- Eardrum vibrates.
- Tiny bones (ossicles) amplify the vibration.
- Cochlea (snail-shaped fluid-filled tube) converts vibrations to nerve signals.
- Brain interprets as sound.
Why we hear different pitches and volumes
- High-frequency vibrations stimulate cells near the start of the cochlea; low-frequency near the end.
- Brain decodes which cells fire to give pitch.
- Stronger vibration = more cells firing more often = louder.
⚠Common mistakes
- Saying sound travels at "the speed of light" — vastly different speeds.
- Forgetting that sound needs a medium.
- Mixing up amplitude and frequency.
- Saying "we hear by detecting molecules of sound" — there are no sound molecules; we detect vibrations of air molecules.
AI-generated · claude-opus-4-7 · v3-deep-physics