How the physics of sound works

01Sound begins with a push

Every sound starts with something moving. A drumhead flexes, a vocal cord vibrates, a speaker cone thrusts forward. That motion compresses the air immediately in front of it, squeezing molecules closer together than they would otherwise be. The compressed region pushes outward against its neighbors, creating a traveling zone of higher pressure that moves away from the source at a fixed speed.

This is the first and most important fact: sound is not a substance that travels. It is a disturbance that propagates. The air molecules at your ear did not come from the speaker. They were already there. What traveled was the pattern of compression and rarefaction, passing through the medium like a wave through a rope.

02Waves carry energy, not matter

A sound wave is a longitudinal wave: the particles oscillate parallel to the direction of travel, unlike the transverse waves on a string. Each molecule shifts only a tiny distance from its equilibrium position before the restoring forces of the gas pull it back. The energy, however, continues forward, handed from particle to particle like a bucket brigade.

This distinction matters because it explains why sound requires a medium. In a vacuum there is nothing to compress, nothing to oscillate, nothing to pass the energy along. Sound cannot cross empty space, which is why explosions in space movies are scientifically silent, however dramatic the soundtrack.

03Frequency determines pitch

The rate at which the source vibrates sets the frequency of the wave, measured in hertz: one cycle per second equals one hertz. A human ear can typically detect frequencies from roughly 20 Hz to 20,000 Hz, though this range narrows with age and exposure to loud sounds.

Frequency is a physical property independent of perception, but pitch is the brain's interpretation of it. Double the frequency and you raise the pitch by one octave. The mapping is logarithmic, which is why a jump from 220 Hz to 440 Hz sounds like the same musical interval as a jump from 440 Hz to 880 Hz.

04Amplitude determines loudness

The size of the pressure variation, not the speed of the wave, determines how loud a sound seems. Larger oscillations carry more energy and produce greater force on the eardrum. Amplitude is often measured on a logarithmic decibel scale, where an increase of 10 dB corresponds to roughly ten times the acoustic intensity.

This logarithmic scaling reflects how the ear works: it compresses a vast range of physical pressures into a manageable perceptual range. The threshold of hearing corresponds to molecular displacements smaller than the diameter of an atom, while a jet engine at close range produces pressure swings millions of times larger.

05The medium sets the speed

Sound travels at different speeds through different materials. In air at room temperature, it moves at approximately 343 meters per second. In water, roughly four times faster. In steel, nearly fifteen times faster than in air. The stiffer the medium and the less dense it is, the more quickly the restoring forces can relay the disturbance forward.

Temperature also matters. In air, the speed of sound rises by roughly 0.6 meters per second for each degree Celsius of warming. Warmer air molecules move faster, which means they collide more frequently and transmit pressure changes more quickly. This is why distant sounds on a hot day can behave differently than on a cold one.

06Superposition lets waves combine

When two sound waves meet, they do not bounce off each other like solid objects. They pass through each other, and at every point the total displacement is the sum of the individual displacements. This principle, called superposition, is what makes interference, beats, and standing waves possible.

Constructive interference occurs when two crests align, producing a louder sound. Destructive interference occurs when a crest meets a trough, producing silence or a reduction in volume. Active noise-canceling headphones exploit this by generating a wave that is the exact inverse of incoming noise, so the two cancel at the ear.

07Everything is a spectrum

Most real sounds are not single pure tones. A musical note, a spoken word, and a breaking glass are each composed of multiple frequencies layered together. The mathematical tool that separates these components is the Fourier transform, which decomposes any waveform into a sum of sine and cosine waves at different frequencies.

This decomposition is not just a mathematical trick. The inner ear performs something analogous: the basilar membrane is tuned so that different locations respond best to different frequencies, physically sorting a complex sound into its constituent tones before the brain even processes it.