Sound

Sound is a mechanical wave that propagates through a medium by means of alternating compression and rarefaction, transferring energy without the net movement of matter. As one of the most pervasive physical phenomena in nature, sound underlies acoustics, human communication, music, and a vast range of technological applications.

Physical nature of sound

Sound requires a material medium for propagation and cannot travel through a vacuum. Unlike electromagnetic radiation, it depends entirely on the elastic properties and density of the medium through which it moves.

Wave mechanics

Sound propagates as a longitudinal wave: the oscillation of particles occurs parallel to the direction of wave travel. Regions of higher-than-ambient pressure are called compressions; regions of lower-than-ambient pressure are called rarefactions. Together these alternating regions form the characteristic sinusoidal pressure pattern associated with a pure tone.

Key wave parameters include:

• Frequency — the number of complete oscillation cycles per second, measured in hertz (Hz). Frequency is perceived as pitch.
• Amplitude — the maximum displacement of particles from equilibrium, related to perceived loudness.
• Wavelength — the spatial distance between successive points of identical phase.
• Period — the time required for one complete cycle.
• Wave speed — the rate at which the wavefront advances through the medium.

Speed of sound

The speed of sound varies with the medium and its physical conditions. In dry air at 20 °C it is approximately 343 metres per second (about 1,235 km/h). Sound travels faster in liquids than in gases, and faster still in most solids, because greater elasticity in denser materials more than compensates for increased inertia. In water at 25 °C the speed is roughly 1,480 m/s; in steel it can exceed 5,000 m/s.

The ratio of an object's speed to the local speed of sound is the Mach number. An object or wavefront exceeding the speed of sound produces a shock wave, the audible result of which is the sonic boom.

Frequency ranges

Sound is conventionally divided into three frequency bands:

1. Infrasound — below approximately 20 Hz; inaudible to humans but detectable by elephants, some cetaceans, and sensitive instruments. Associated with earthquakes, wind turbines, and large meteorological events.
2. Audible sound — approximately 20 Hz to 20,000 Hz; the range perceptible by the healthy human ear, though the upper limit typically decreases with age.
3. Ultrasound — above 20,000 Hz; used extensively in medical imaging, industrial non-destructive testing, and echolocation by bats and dolphins.

Sound production and sources

Any vibrating object can serve as a sound source. A loudspeaker cone, a vocal cord, a plucked string, or a detonating explosive all generate pressure waves by imparting kinetic energy to surrounding particles. The character of a sound — its timbre, pitch, and loudness — depends on the geometry, material, and mode of vibration of the source.

Musical instruments

Musical instruments exploit controlled vibration to produce tones with defined spectral content. String instruments vibrate tensioned strings whose fundamental frequency depends on length, tension, and linear density. Wind instruments shape resonant air columns. Percussion instruments rely on the vibrational modes of membranes or rigid bodies.

Natural and environmental sources

Thunder, wind, flowing water, animal calls, and tectonic activity all generate sound. Bioacoustics studies the production and reception of sound by living organisms, spanning whale song, bird calls, and insect stridulation.

Propagation and behaviour

When a sound wave encounters a boundary between media, several phenomena can occur:

• Reflection — the wave bounces back from a surface; in enclosed spaces this produces echoes and reverberation.
• Refraction — the wave changes direction when passing between media of different acoustic properties, analogous to the refraction of light.
• Diffraction — the wave bends around obstacles or through apertures, allowing sound to be heard around corners.
• Absorption — some wave energy is converted to heat as the wave passes through a medium; soft, porous materials are particularly effective absorbers.
• Interference — two or more coherent waves superpose to produce regions of constructive and destructive interference, heard as beats when the frequencies are close.

The Doppler effect describes the perceived shift in frequency when the source and observer are in relative motion; an approaching source appears to emit a higher frequency, a receding one a lower frequency.

Measurement and units

Sound pressure level (SPL) is expressed in decibels (dB), a logarithmic scale referenced to the threshold of human hearing at 1 kHz (20 micropascals). Representative SPL values include:

• 0 dB — threshold of hearing
• 60 dB — normal conversation at one metre
• 85 dB — sustained exposure begins to risk hearing damage
• 120 dB — pain threshold for most humans
• 194 dB — theoretical maximum undistorted SPL in air at standard atmospheric pressure

Instrumentation for measuring sound includes microphones, sound level meters, and spectrum analysers. Acoustic engineering uses these measurements to design concert halls, recording studios, and noise-reduction systems.

Human perception of sound

The outer ear collects pressure waves and funnels them to the eardrum, which vibrates in response. These vibrations pass through the ossicles of the middle ear to the cochlea, where mechanoreceptor hair cells convert mechanical motion into electrochemical signals transmitted via the auditory nerve to the brain. The brain interprets these signals as sound, assigning attributes of pitch, loudness, timbre, and spatial location.

Prolonged exposure to sounds above approximately 85 dB can cause noise-induced hearing loss through damage to cochlear hair cells, which do not regenerate in mammals.

Applications of sound

Sound and its manipulation underpin numerous fields:

• Medicine — ultrasonic imaging (sonography) visualises internal organs and foetal development without ionising radiation; lithotripsy uses focused ultrasound to break kidney stones.
• Engineering — Sonar uses reflected sound pulses to map ocean floors and detect submerged objects.
• Telecommunications — voice signals are converted to electrical or digital representations and reconstructed as sound at the receiving end.
• Noise control — passive barriers, absorptive panels, and active noise cancellation (which superimposes anti-phase sound waves) reduce unwanted noise in vehicles, aircraft cabins, and buildings.
• Seismology — acoustic and seismic methods probe subsurface geology and locate hydrocarbon reservoirs.

Frequently asked questions

Is sound a transverse or longitudinal wave?

Sound in fluids (gases and liquids) is a longitudinal wave: particle displacement is parallel to the direction of propagation. In solids, shear modes allow transverse sound waves as well, which are significant in seismology.

Is sound faster than light?

No. The speed of sound in air (approximately 343 m/s) is roughly 874,000 times slower than the speed of light in a vacuum (approximately 299,792,458 m/s).

Is sound a form of energy?

Sound carries mechanical energy in the form of kinetic energy of particle motion and potential energy of elastic compression. This energy is gradually dissipated as heat through absorption as the wave propagates.

Is infrasound dangerous to humans?

High-intensity infrasound can cause discomfort, disorientation, and physiological effects, but the evidence for harm from typical environmental infrasound levels — such as that produced by wind turbines — remains contested among researchers as of the early 2020s.

Is the Doppler effect only relevant to sound?

No. The Doppler effect applies to all wave phenomena, including light; it is exploited in radar speed measurements and in cosmological redshift observations. However, it was first described and is most intuitively understood in the context of sound.