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Sound is a longitudinal mechanical wave — alternating compressions and rarefactions of a medium propagated by collisions between particles. It requires a material medium and cannot travel through a vacuum. The speed of sound in air at 20°C is approximately 343 m/s and increases with temperature (v ≈ 331 + 0.6T m/s). The frequency of a sound wave determines its pitch; amplitude determines loudness.
Ring a bell inside a bell jar and evacuate the jar to demonstrate that sound needs a medium. Measure the speed of sound by timing an echo from a distant wall.
From transverse waves, you know that waves carry energy through a medium by having particles oscillate around equilibrium positions. Sound waves are longitudinal waves — the particles oscillate back and forth in the same direction the wave is traveling, rather than perpendicular to it. Imagine a row of dominoes: pushing the first creates a pulse that travels down the line as each domino pushes the next. Sound in air works similarly: a vibrating speaker cone pushes adjacent molecules together (creating a compression), and when it pulls back, it leaves behind a region of lower density (a rarefaction). This alternating compression-rarefaction pattern propagates outward at the speed of sound.
The speed of sound depends entirely on the medium, not on the frequency or amplitude of the wave. In air at 20°C it is approximately 343 m/s — a number worth memorizing. The formula v ≈ 331 + 0.6T shows that warmer air has faster-moving molecules that transmit disturbances more quickly, raising the speed. In denser materials with stronger intermolecular forces — water (~1480 m/s) or steel (~5000 m/s) — sound travels far faster. The key lesson from wave-speed-medium applies directly: the medium's properties set the speed, not the source.
Frequency and amplitude are the two independent variables that describe a sound. Frequency — how many compressions pass a point per second — determines pitch: 440 Hz is the musical note A. Amplitude — how large the pressure excursions are — determines loudness. These are completely independent: you can have a loud high-pitched sound (a piccolo at full volume) or a quiet low-pitched sound (a distant bass note). This independence is why audio engineers can boost bass frequencies without making everything louder.
One implication of sound being a mechanical, longitudinal wave is that it cannot travel through a vacuum — there are no molecules to compress and rarefy. This is why the classic film trope of deafening space explosions is physically wrong. Sound also takes time to travel: the familiar three-second delay between a lightning flash and thunder tells you the storm is roughly one kilometer away (343 m/s × 3 s ≈ 1 km). Appreciating this finite propagation speed becomes crucial when you encounter the Doppler effect.