Sound is a mechanical wave phenomenon involving oscillations and disturbances that propagate through a material medium such as air, water or a solid. In ordinary fluids, audible sound consists primarily of longitudinal pressure and particle-velocity variations, although solids can support both longitudinal and transverse elastic waves. Sound requires a medium because its propagation involves mechanical interactions between particles or elements of the material; it cannot propagate through an ideal vacuum. The human auditory system is sensitive to only a particular frequency range, conventionally described as approximately 20 Hz to 20 kHz for young healthy listeners, although individual hearing limits vary and generally change with age and exposure.
The principal physical quantities used to describe sound include frequency, wavelength, amplitude, sound pressure, intensity and phase. Frequency determines the rate of oscillation and is closely related to perceived pitch for periodic sounds. Wavelength is related to frequency and propagation speed by \(v=f\lambda\). Sound pressure is the fluctuating pressure associated with the acoustic wave, while sound intensity describes acoustic power flow per unit area. Because the human ear responds nonlinearly to sound pressure and because acoustic pressures cover a very large numerical range, sound pressure level is commonly expressed logarithmically in decibels relative to a reference pressure.
The speed of sound depends strongly on the medium and its physical properties. In dry air under ordinary atmospheric conditions, sound travels at roughly 343 metres per second at 20 degrees Celsius, with the precise value affected by temperature, humidity and composition. In liquids and solids, sound can travel substantially faster because their elastic properties provide stronger restoring forces.
In a gas, the speed of small-amplitude sound is related to pressure and density and, for an ideal gas, to thermodynamic properties such as temperature and the ratio of specific heats. Wind can alter the ground-relative propagation of sound, while temperature gradients can refract sound by changing the local speed of propagation.
Sound can be periodic, quasi-periodic, transient or noise-like. A pure tone has a single sinusoidal frequency in an ideal model, whereas musical tones generally contain a fundamental frequency and harmonics whose relative amplitudes influence timbre. Speech contains rapidly changing spectral and temporal structures produced by the vocal tract, while environmental noise may contain broad frequency distributions. Transient sounds such as clicks and impulses contain wide frequency content because a short duration in time corresponds to a broad spectrum in frequency. Fourier analysis provides a mathematical framework for decomposing complex acoustic signals into frequency components and is extensively used in audio engineering, acoustics and signal processing.
Human perception of sound involves the outer, middle and inner ear followed by neural processing in the auditory system. The outer ear helps collect and filter incoming sound, the middle ear transfers mechanical vibrations through the ossicles, and the cochlea converts mechanical motion into neural signals through specialised sensory cells. Perception is not a direct one-to-one representation of physical pressure. Equal physical amplitudes at different frequencies can produce different perceived loudness, and masking can make one sound less detectable in the presence of another. Psychoacoustics therefore studies the relationship between measurable acoustic signals and human auditory perception.
Sound undergoes the same fundamental wave phenomena found in other wave systems, including reflection, refraction, diffraction, interference and resonance. Reflection from surfaces produces echoes and reverberation, while diffraction allows sound to bend around obstacles and spread through openings. Interference between coherent or partially coherent sound waves can produce constructive and destructive patterns, including regions of reduced sound pressure. Refraction can occur when temperature or wind gradients change sound speed, causing sound paths to curve. Resonance can strongly amplify sound in cavities, instruments and structures when excitation approaches a natural frequency.
Sound has extensive practical applications. It is central to speech communication, music, architectural design, medical diagnosis, underwater detection, industrial monitoring, non-destructive testing and animal communication. Microphones convert acoustic pressure variations into electrical signals, loudspeakers perform the reverse transformation, and digital audio systems sample and process these signals numerically. Sonar uses sound propagation and reflected signals to estimate underwater range or detect objects. Medical ultrasound uses frequencies above ordinary human hearing to form images and analyse biological structures. Industrial acoustic monitoring can detect machinery faults by identifying changes in spectral or temporal signatures.
Sound can also cause environmental and occupational problems when exposure is excessive or unwanted. Prolonged exposure to sufficiently high sound levels can damage hearing, while environmental noise can interfere with communication, concentration, sleep and other activities. Acoustic treatment can control sound through absorption, diffusion, isolation and barrier effects, but these mechanisms address different physical problems. Soundproofing primarily seeks to reduce transmission between spaces, whereas acoustic treatment within a room seeks to modify reflection, reverberation and spatial sound distribution. Understanding the distinction between source characteristics, propagation conditions, receiver sensitivity and human perception is essential for accurate analysis of any sound-related problem.
