Sound is a phenomenon in which pressure disturbances propagate through an elastic material medium. In the context of physics, it is characterised as a mechanical wave of pressure or related quantities (e.g. displacement), whereas in physiological-psychological contexts it implies a hearing process. Though sensitivity to sound varies among all organisms, the human ear is sensitive to audio frequencies ranging from 20 Hz to 20 kHz. More broadly, acoustic waves encompass mechanical longitudinal waves in general. Examples of the significance and application of sound include music, medical imaging techniques, oral language and parts of science.
Definition According to the technical standard established by ANSI/ASA S1.1‑2013, the American National Standard for Acoustical Terminology, sound is defined as: "(a) Oscillation in pressure, stress, particle displacement, particle velocity, etc., propagated in a medium with internal forces (e.g., elastic or viscous), or the superposition of such propagated oscillation. (b) Auditory sensation evoked by the oscillation described in (a)."
This two-part definition of sound states that sound can be taken as a wave motion in an elastic medium, making it also a stimulus, or as an excitation of the hearing mechanism that results in the perception of sound, making it a sensation.
While ANSI/ASA S1.1‑2013 provides the standard terminological definition of sound used in U.S. acoustics, and is one of the few major standards organisations to define the concept explicitly, physics and engineering texts often instead define sound as a kind of propagating mechanical disturbance—i.e., a wave—rather than as a vibration. Such sources typically describe sound as a perturbation in mechanical properties (e.g., pressure or particle motion) that travels through a medium and consists of medium variations (compressions and rarefactions), a formulation adopted for conceptual clarity and scientific precision.
Acoustics
Acoustics is the interdisciplinary scientific study of mechanical waves, vibrations, sound, ultrasound, and infrasound in gaseous, liquid, or solid media. A scientist who works in the field of acoustics is called an acoustician, while an individual specialising in acoustical engineering may be referred to as an acoustical engineer.ANSI S1.1‑1994. American National Standard: Acoustic Terminology. Sec. 3.03. An audio engineer, by contrast, is concerned with the recording, manipulation, mixing, and reproduction of sound.
Applications of acoustics are found in many areas of modern society. Subdisciplines include aeroacoustics, audio signal processing, architectural acoustics, bioacoustics, electroacoustics, environmental noise, musical acoustics, noise control, psychoacoustics, speech, ultrasound, underwater acoustics, and vibration.
Physics
Sound travels as a mechanical wave through a medium (e.g. water, crystals, air). Sound waves are generated by a sound source, such as a vibrating diaphragm of a loudspeaker. As the sound source vibrates the surrounding medium, mechanical disturbances propagate away from the source at the local speed of sound, thus resulting in a sound wave. At a fixed distance from the source, the pressure, velocity, and displacement of the medium's particles vary in time. At an instant in time, the pressure, velocity, and displacement vary spatially. The particles of the medium do not travel with the sound wave; instead, the disturbance and its mechanical energy propagate through the medium. Though intuitively expectable for solids, this also applies for liquids and gases.
The matter that supports the transmission of a sound is named the transmission medium. Media may be any form of matter, whether solids, liquids, gases or plasmas. However, sound cannot propagate through a vacuum because there is no medium to support mechanical disturbances.Is there sound in space? Northwestern University.Can you hear sounds in space? (Beginner) . Cornell University.
The propagation of sound in a medium is influenced primarily by: A complicated relationship between the density and pressure of the medium. This relationship, also affected by temperature, determines the speed of sound within the medium. Motion of the medium itself. If the medium is moving, this movement may increase or decrease the absolute speed of the sound wave depending on the direction of the movement. For example, sound moving through wind will have its speed of propagation increased by the speed of the wind if the sound and wind are moving in the same direction. If the sound and wind are moving in opposite directions, the speed of the sound wave will be decreased by the speed of the wind. The viscosity of the medium. Medium viscosity determines the rate at which sound is attenuated. For many media, such as air or water, attenuation due to viscosity is negligible.
Theoretical work indicates that sound waves carry an extremely small effective gravitational mass. This mass arises from nonlinear corrections to the stress–energy of the wave, and implies that sound waves both respond to gravity and generate a very weak gravitational field of their own. For ordinary equations of state, the effective mass is negative, meaning that sound waves in such media act as if they carry a tiny negative gravitational mass. The effect is extremely small because it appears only at nonlinear order in the equations governing wave motion.
Waves Sound waves exhibit behaviours such as reflection, transmission, refraction, diffraction, absorption, and attenuation.Kinsler, L. E.; Frey, A. R.; Coppens, A. B.; Sanders, J. V. (2000). Fundamentals of Acoustics (4th ed.). Wiley. pp. 128–130. When sound is moving through a non‑homogeneous medium , it may be refracted (either dispersed or focused).
Sound is transmitted through fluids (e.g. gases, plasmas, and liquids) as longitudinal waves, also called compression waves. Through solids, however, sound can be transmitted as both longitudinal waves and transverse waves. Longitudinal sound waves are waves of alternating pressure deviations from the equilibrium pressure, causing local regions of compression and rarefaction, while transverse waves (in solids) are waves of alternating shear stress perpendicular to the direction of propagation. Unlike longitudinal sound waves, transverse sound waves have the property of polarisation.
Sound waves may be viewed using parabolic mirrors and objects that produce sound.
The energy carried by a periodic sound wave alternates between the potential energy of the extra compression (in the case of longitudinal waves) or lateral displacement strain (in the case of transverse waves) of the matter, and the kinetic energy of the particles' displacement velocity in the medium.
Although sound transmission involves many physical processes, the signal received at a point (such as a microphone or the ear) can be fully described as a time‑varying pressure. This pressure‑versus‑time waveform provides a complete representation of any sound or audio signal detected at that location.
Sound waves are often simplified as sinusoidal plane waves, which are characterized by these generic properties: Frequency, or its inverse, period. Wavelength, or its inverse, wavenumber. Amplitude, sound pressure or Intensity Speed of sound Direction
Sometimes speed and direction are combined as a velocity vector; wave number and direction are combined as a wave vector.
To analyse audio, a complicated waveform—such as the one shown on the right—can be represented as a linear combination of sinusoidal components of different frequencies, amplitudes, and phases.Handel, S. (1995). Timbre perception and auditory object identification . Hearing, 425–461.Kendall, R.A. (1986). The role of acoustic signal partitions in listener categorization of musical phrases. Music Perception, 185–213.Matthews, M. (1999). Introduction to timbre. In P.R. Cook (Ed.), Music, cognition, and computerized sound: An introduction to psychoacoustic (pp. 79–88). Cambridge, Massachusetts: The MIT press.
Speed
The speed of sound depends on the medium the waves pass through, and is a fundamental property of the material. The first significant effort towards measurement of the speed of sound was made by Isaac Newton. He believed the speed of sound in a particular substance was equal to the square root of the pressure acting on it divided by its density:
This was later disproven and the French mathematician Laplace corrected the formula by deducing that the phenomenon of sound travelling is not isothermal, as believed by Newton, but adiabatic. He added another factor to the equation—gamma—and multiplied
by , thus coming up with the equation . Since , the final equation came up to be , which is also known as the Newton–Laplace equation. In this equation, K is the elastic bulk modulus, c is the velocity of sound, and is the density. Thus, the speed of sound is proportional to the square root of the ratio of the bulk modulus of the medium to its density.
Those physical properties and the speed of sound change with ambient conditions. For example, the speed of sound in gases depends on temperature. In air at sea level, the speed of sound is approximately using the formula . The speed of sound is also slightly sensitive, being subject to a second-order anharmonic effect, to the sound amplitude, which means there are non-linear propagation effects, such as the production of harmonics and mixed tones not present in the original sound (see parametric array). If relativistic effects are important, the speed of sound is calculated from the relativistic Euler equations.
In fresh water the speed of sound is approximately . In steel, the speed of sound is about . Sound moves the fastest in solid atomic hydrogen at about .
Sound pressure level
Sound pressure is the difference, in a given medium, between average local pressure and the pressure in the sound wave. A square of this difference (i.e., a square of the deviation from the equilibrium pressure) is usually averaged over time and/or space, and a square root of this average provides a root mean square (RMS) value. For example, 1 Pa RMS sound pressure (94 dBSPL) in atmospheric air implies that the actual pressure in the sound wave oscillates between (1 atm Pa) and (1 atm Pa), that is between 101323.6 and 101326.4 Pa. As the human ear can detect sounds with a wide range of amplitudes, sound pressure is often measured as a level on a logarithmic decibel scale. The sound pressure level (SPL) or Lp is defined as
where p is the root-mean-square sound pressure and is a reference sound pressure. Commonly used reference sound pressures, defined in the standard ANSI S1.1-1994, are 20 μPa in air and 1 μPa in water. Without a specified reference sound pressure, a value expressed in decibels cannot represent a sound pressure level.
Since the human ear does not have a flat spectral response, sound pressures are often frequency weighted so that the measured level matches perceived levels more closely. The International Electrotechnical Commission (IEC) has defined several weighting schemes. A-weighting attempts to match the response of the human ear to noise and A-weighted sound pressure levels are labeled dBA. C-weighting is used to measure peak levels.
Perception