Sound
Pressure Waves You Can Hear
Introduction
Sound is a longitudinal pressure wave. It needs a medium - air, water, steel, anything made of particles that can push against their neighbors. When something vibrates, it compresses nearby particles together, then pulls back, leaving a gap. Those compressed regions push on the next layer, which pushes the next, and a wave of compression and rarefaction propagates outward. No medium, no sound. Space is silent.
Speed of sound depends on the medium. In air at room temperature, about 343 meters per second. In water, roughly 1,480. In steel, 5,960. Denser and stiffer materials transmit sound faster because particles are closer together and spring back more quickly. When you hear a distant thunderclap, those pressure variations traveled from cloud to ear at roughly a fifth of a mile per second.
Pitch, Volume, and Timbre
Three properties define what a sound sounds like. Frequency determines pitch - higher frequency means higher pitch. Human hearing spans roughly 20 to 20,000 hertz. Middle C on a piano vibrates at 262 hertz. Amplitude determines loudness - bigger pressure variations mean louder sound. We measure loudness on a logarithmic decibel scale because our ears respond to enormous ranges: the softest audible whisper is a trillion times less powerful than a jet engine at close range.
Timbre - what makes a violin sound different from a flute playing the same note at the same volume - comes from the specific mix of harmonic overtones. Every real sound is a combination of many pure frequencies. Joseph Fourier showed in 1807 that any complex waveform can be decomposed into a sum of simple sine waves. This Fourier decomposition is one of the most powerful ideas in physics, used far beyond acoustics - in signal processing, quantum mechanics, and image compression.
Doppler Effect
When a sound source moves toward you, each successive wavefront is emitted slightly closer. The waves bunch up, their frequency increases, and pitch rises. When the source moves away, wavefronts stretch apart, frequency drops, and pitch falls. This is why an ambulance siren sounds higher as it approaches and lower as it recedes. The effect was first described by Christian Doppler in 1842 and confirmed experimentally by placing musicians on a moving train.
A similar effect applies to light. Nearby galaxies moving away from us have their light Doppler-shifted to longer wavelengths - redshifted. Nearby galaxies moving toward us are blueshifted. For distant galaxies, the redshift has a different cause: space itself is expanding, stretching light wavelengths during transit. This cosmological redshift is not a Doppler effect but produces a similar observational signature. Edwin Hubble's discovery that nearly all distant galaxies are redshifted was the first evidence that universe is expanding. Doppler effect in sound is an everyday experience. In light, its cosmic cousin revealed the expansion of the cosmos.
Sonic Boom
When an object exceeds the speed of sound, it outruns its own pressure waves. Wavefronts pile up into a conical shock front - a Mach cone. The sudden pressure jump at this cone is the sonic boom. It is not a one-time event at the moment of breaking the barrier. The boom travels along behind the object continuously, heard as a sharp crack wherever the cone sweeps across the ground.
There is an optical equivalent. When a charged particle moves faster than the speed of light in a medium (light slows in glass or water), it produces a cone of electromagnetic radiation called Cherenkov radiation - the eerie blue glow seen in nuclear reactor pools. Same geometry, same physics, different wave. Even the crack of a bullwhip is a miniature sonic boom - the tip of the whip exceeds the speed of sound.
The Bigger Picture
In quantum mechanics, vibrations in a crystal lattice are quantized into discrete packets called phonons. Phonons behave like particles - they carry momentum, they can scatter, they have energy proportional to frequency. They are the quantum of sound in a solid. Phonons carry heat through crystals and are central to understanding thermal conductivity.
Most remarkably, phonon exchange between electrons is what produces superconductivity. At low temperatures, two electrons can interact through lattice vibrations, forming Cooper pairs that flow through a material with zero electrical resistance. Sound, at its deepest level, connects to the quantum nature of matter. What begins as pressure pushing particles against their neighbors ends as a quantum field that enables current to flow forever.
