Sound travels by propagating a mechanical disturbance through a material medium. In air, a vibrating source causes nearby molecules to oscillate, creating alternating regions of compression and rarefaction. These pressure disturbances cause neighbouring molecules to move and interact, transferring the disturbance onward. The individual molecules generally do not travel from the source to the listener as a group at the speed of sound.
Instead, each region of the medium undergoes local motion while the pattern of disturbance moves through the medium. Energy and information associated with the acoustic wave propagate from one region to the next. In gases, ordinary sound is primarily longitudinal, so particle motion is approximately parallel to the direction in which the wave travels. The exact propagation mechanism differs in solids and liquids because their particles and internal forces respond differently to disturbances.
The speed at which sound travels depends on the medium and its physical state. In an ideal gas, sound speed depends strongly on temperature and can be approximated by a relationship involving the ratio of specific heats, the gas constant, absolute temperature and molar mass. For dry air near room temperature, a commonly used approximate value is around 343 metres per second at 20 degrees Celsius, although the precise speed changes with temperature, humidity and composition.
In liquids and solids, the speed is generally higher because the relevant restoring forces are stronger. For a fluid, acoustic speed is related to compressibility and density, while in a solid it depends on elastic moduli and density and differs for longitudinal and shear waves. These relationships explain why sound does not have one universal speed and why changing the medium can change propagation behaviour.
The propagation of sound involves restoring forces. When air is compressed locally, pressure increases and the resulting pressure gradient pushes neighbouring material away from the compressed region. When the material becomes rarefied, pressure decreases and surrounding pressure gradients drive motion in the opposite direction. In a solid, deformation produces elastic restoring forces that cause the material to respond to disturbance.
A wave exists because the medium can both store energy associated with deformation and transfer that energy through interactions between neighbouring regions. A simple mathematical model treats the medium as many coupled elements, each influencing adjacent elements. The resulting equations support travelling wave solutions. Real materials are more complicated because viscosity, thermal conduction, structural heterogeneity and nonlinear behaviour can cause attenuation, dispersion and distortion as the wave propagates.
Sound can propagate through gases, liquids and solids, but the characteristics differ substantially. In gases and liquids, ordinary acoustic waves are primarily longitudinal because these fluids do not support static shear stress in the same way as solids. Solids can support both longitudinal compression waves and shear waves because their elastic structure can resist shear deformation. Earthquakes generate several types of seismic waves, including primary waves that are longitudinal and secondary waves that involve shear motion.
The existence and propagation of different wave modes allow scientists to infer properties of materials. At interfaces between different materials, some acoustic energy may be reflected while some is transmitted and refracted. Differences in acoustic impedance strongly influence the amount of reflection. These principles are exploited in ultrasound imaging and industrial nondestructive testing.
Sound does not travel instantaneously. A disturbance takes time to move from its source to an observer. The travel time depends on distance and propagation speed, so a distant sound source can be heard after a measurable delay. The familiar delay between seeing lightning and hearing thunder is an everyday example because light reaches an observer vastly faster than sound under ordinary atmospheric conditions.
The delay can be used to estimate distance if the sound source time is known and the propagation speed is estimated accurately. In scientific measurements, timing methods can be used to determine acoustic distances, locate sources and characterise materials. Temperature gradients, wind and atmospheric structure can change the propagation speed and path, so precise acoustic ranging requires environmental corrections.
Sound propagation is affected by boundaries and obstacles. Reflection occurs when a sound wave encounters a surface and part of its energy returns. Diffraction allows sound to spread around edges and through openings, particularly when the obstacle or opening has dimensions comparable with the wavelength. Refraction can bend sound when the propagation speed varies spatially, such as in an atmosphere with temperature gradients or wind. Absorption converts some acoustic energy into other forms, reducing the amplitude with distance.
Scattering redirects sound when it encounters objects or irregularities whose dimensions and properties interact with the wavelength. These processes can occur simultaneously. The sound field in a real room, city, forest or underwater environment is therefore the result of direct propagation combined with reflection, diffraction, refraction, absorption and scattering.
Sound carries both energy and information. A speech signal modifies the amplitude and frequency content of a carrier acoustic disturbance, allowing information to be encoded in temporal and spectral patterns. Musical sounds contain organised frequencies and changing amplitudes that convey pitch, rhythm and timbre. Sonar systems transmit acoustic signals and analyse returning echoes to determine information about objects or environments. Medical ultrasound sends controlled acoustic pulses into tissue and interprets reflected signals to construct images or estimate movement.
Industrial acoustic monitoring can detect changes in machines because faults alter vibration and emitted sound. The ability of sound to carry information depends on propagation conditions, bandwidth, noise and attenuation. The further sound travels, the more its signal may be weakened or altered by the environment.
Sound propagation is sometimes described as if air particles simply pass the sound onward like a line of objects passing a ball. This analogy is useful only at a very basic level. The actual process is governed by continuous or microscopic mechanical interactions, pressure gradients, elastic forces and conservation laws. The molecules oscillate around equilibrium positions rather than travelling from source to listener with the wave.
The propagation speed is determined by the physical properties of the medium, not primarily by the strength of the source. A louder sound generally has greater pressure amplitude and carries more acoustic energy, but under ordinary linear conditions it does not travel substantially faster merely because it is louder. Nonlinear effects can occur at very high amplitudes, including shock formation and waveform distortion. Thus ordinary sound travel is best understood as the propagation of a mechanical wave through matter, modified by the medium and environment.
