Acoustic Glossary


Sound Wave Terms, Definitions, Units, Measurements ...

Sound Waves also known as acoustic waves, transfer sound energy from one point to another by vibrating the particles in the various media the waves pass through. In air there is little resistance (impedance) to sound propagation, other 'materials' will absorb, reflect or impede the sound energy in accordance with their acoustic properties.

You can simulate waves while holding a length of rope and moving your hand(s) up and down or side-to-side, demonstrating the two main directions in which sound wave motion occurs, longitudinal and transverse. In both cases the rope particles are involved but without any net particle movement. Observe also the nodes and antinodes, found in all waves as they move along the rope. In the special case of standing waves these node(s) would be stationary.

Sound wavelengths range from more than 17 metres to less than 17 mm, so react differently with 'objects' in their paths, resulting in the complex sound fields detailed below.

Sound Waves in air are longitudinal waves.

Sound Wave Definition IEC 801-23-01 disturbance propagated at a definite velocity in a medium in such a manner that at any point in the medium the quantity serving as the measure of disturbance is a function of the time, while at any instant the same quantity at a point is a function of the co-ordinates of the point.

Sound Wave Velocity Definition IEC 801-23-19 vector that specifies the speed and direction with which a sound wave travels

See also • particle acceleration • particle displacement • particle velocity

Related Terms - listed alphabetically

Antinode is where the amplitude of a sound wave is a maximum and halfway between two adjacent nodes (zeros)

See also standing wave.

Antinode Definition IEC 801-23-17 point, line or surface in a standing wave where some specified characteristic of the wave field has maximum amplitude.
Note : the appropriate modifier should be used before the word antinode to signify the type that is intended; e.g. displacement antinode, particle velocity antinode, sound pressure antinode.

Bending Wave Definition IEC 801-23-11 transverse wave in a plate or bar, a combination of compression wave and shear wave

Boundary Effect is a sound reflection effect due to room modes and or standing waves which accumulate at walls. Sound wave reflections appear to make the localized sound levels increase as all of the room modes terminate at a boundary (wall)

Compression is a point in the medium through which a longitudinal wave is travelling that has the maximum density.
Compression Wave is a longitudinal sound wave propagated by the elastic compression of the medium.
Compressional Wave Definition IEC 801-23-04 wave in an elastic medium that causes an element of the medium to change its volume without undergoing rotation
Note : mathematically, a compressional wave is one whose velocity field has zero curl.

Cylindrical Wave DefinitionIEC 801-23-08 wave of which the wave fronts are coaxial cylinders.

Diffraction is the distortion of a wavefront caused when an incident sound wave encounters an obstacle in the sound field. Depending on the size of the object and the wavelength of the sound, the sound wave bends or diffuses around the object and the diffraction or interference is significant.

Similarly when sound waves pass through a gap they spread out depending on the gap size and the wavelength (frequency).

Diffraction Definition IEC 801-23-25 phenomenon by which a sound wave is changed in direction by an obstacle or other heterogeneity in the medium.
Diffraction Factor Definition IEC 801-25-55 for a specified frequency and specified direction of incident sound, it is the ratio of the sound pressure acting on the part of a transducer designed to receive sound, to the free-field sound pressure at that place in the absence of the transducer.

Dispersion Definition IEC 801-23-22 separation of the sinusoidal components of a wave that results from change of speed of sound with frequency.

Divergence Loss
Free Progressive Wave
Grazing Incidence Definition IEC 705-04-05 the incidence of a wave on a surface at a very small grazing angle.

Incident Wave Definition IEC 103-10-22 a wave which propagates towards the surface separating two media.

Interference in acoustics, occurs when two sound waves interact and form a resultant wave of greater or lower amplitude.
Interference Definition IEC 801-23-13 phenomenon that results from the superposition of two or more waves of the same frequency but different in phase or direction of propagation.

Longitudinal Sound Waves occur when the particle displacement is in the same direction in which the wave is travelling, as opposed to transverse waves. The media particles do not move along the wave, they transfer energy to the next particle along the path, then return to their equilibrium point. Sound Waves in air are longitudinal waves.
Longitudinal Wave Definition IEC 801-23-05 wave in which the direction of particle displacement at each point of the medium is normal to the wavefront.

Node is where the amplitude of sound wave is zero, also known as a zero crossing. Antinodes are where the amplitude (positive or negative) is a maximum, halfway between two adjacent nodes - see standing wave
Node Definition IEC 801-23-16 a point, line or surface in a standing wave where some specified characteristic of the wave field has essentially zero amplitude
Note 1 : in practice, this amplitude is generally not zero but simply a minimum. The node is then said to be partial.
Note 2 : the appropriate modifier should be used before the word "node" to signify the type that is intended; e.g. displacement node, particle velocity node, sound pressure node.

See also • antinode

Plane Wave Definition IEC 801-23-06 wave of which all the wavefronts are parallel planes normal to the direction of propagation, also known as a planewave.

Progressive Sound Waves distribute energy from a source to the surrounding area. They move energy in the form of vibrating particles or sound fields.
Free Progressive Wave Definition IEC 801-23-03 wave propagating in a medium, free from boundary effects

Random Incidence Definition IEC 801-31-16 incidence of sound waves from all directions with equal probability.

Rarefaction a point in the medium through which a longitudinal wave is travelling that has the minimum density.

Rayleigh Waves also known as Surface Acoustic Waves (SAWs), travel near the surface of solids and include both longitudinal and transverse motions that decrease exponentially in amplitude as distance from the surface increases. There is a phase difference between these component motions. Surface acoustic wave sensors have many modern applications, for example 'touch screen' displays
Rayleigh Wave Definition IEC 801-23-12 surface wave associated with the free boundary of a solid or liquid such that a surface particle describes an ellipse whose major axis is normal to the surface, and whose centre is situated on the initially undisturbed surface
Note 1 : a maximum particle displacement away from this initially undisturbed surface the motion of the particle is opposite to that of the wave.
Note 2 : the propagation velocity of a Rayleigh wave is slightly less than that of a shear wave in the solid. The wave amplitude of the Rayleigh wave diminishes exponentially with depth.

Reflected Sound Wave when a wave encounters an object, some energy is absorbed by the object but most is reflected or diffused around the object. Where the wavelength is small compared to the object dimension the angle of incidence equals the angle of reflection, similar to light reflection in a mirror.

The reflected wave may interfere with the incident sound wave and cancellation or amplification may occur across the spectrum.

See also • specular reflectiontransmission impedance and radiation

Refraction the bending of a wave from its original path, either because it is passing from one medium to another with different velocities or by changes in the physical properties of the medium, for example, a temperature or wind gradient in the air.
Refraction Definition IEC 801-23-23 phenomenon by which the direction of propagation of a sound wave is changed due to spatial variation in the speed of sound.
Refraction Loss Definition IEC 801-23-42 that part of the transmission loss due to refraction resulting from non-uniformity of the medium

Rotational Wave Definition IEC 801-23-10 wave propagating in an elastic medium that causes an element of the medium to change its shape without a change of volume, also known as a shear wave.
Note : mathematically, the particle velocity of a rotational wave has zero divergence

SAW - surface acoustic waves, under rayleigh waves

Scattering Definition IEC 801-23-26 irregular diffraction and reflection of a sound wave in many directions.

Shear Wave Definition IEC 801-23-10 wave propagating in an elastic medium that causes an element of the medium to change its shape without a change of volume, also known as a rotational wave.
Note : mathematically, the particle velocity of a shear wave has zero divergence

Sine Wave or Pure Tone is characterised by it's frequency (number of cycles per second) or it's wavelength (distance it travels within a period) and the amplitude.

Sound Diffraction

Specular Reflection Definition IEC 801-23-24 phenomenon by which a sound wave is returned from a surface separating two media, at an angle to the normal equal to the angle of incidence

Spherical Wave Definition IEC 801-23-07 wave of which the wavefronts are concentric spheres.

Standing Wave a phenomenon when a sound is reflected back and forth between two parallel surfaces, such as two side walls in a room. If the incident wave and the reflected wave are in-phase they combine and 'resonance' occurs. The combined wave is stationary across the room and the sound pressure at the maximum is known as the antinode and the minimum is called a node.

When sound waves are 180 degrees out of phase and have the same amplitude, they cancel each other out.

Standing Wave Definition IEC 801-23-15 periodic wave having a fixed distribution in space that is the result of interference of progressive waves of the same frequency and kind.
Note : such waves are characterised by the existence of nodes or partial nodes and anti-nodes that are fixed in space.
Standing Wave Tube method for measuring absorption coefficients by means of standing waves in a tube.

Superposition of Waves occur whenever two (or more) waves are travelling through the same medium at the same time, the waves combine. The net displacement of the medium at any point in space or time, is the sum of the individual wave displacements.

See also • antinodeNodestanding waves

Surface Acoustic Wave (SAW), under rayleigh wave

Surface Wavefront Definition IEC 801-23-02 locus of points of a progressive surface where the phase of a quantity characterising the wave is the same at a given instant.

Transverse Wave Definition IEC 801-23-09 wave in which the direction of particle displacement at each point of the medium is parallel to the wavefront.

Wave Front or Wavefront is the surface of a propagating wave, made up of all points in the wave having the same phrase. It is usually perpendicular to the direction of propagation. The simplest form of a wavefront is the plane wave.

Wavelength (λ) is the distance between sequential crests of a sound wave. At 20 Hz the wavelength ≈ 17 m and at 20 kHz the wavelength is ≈ 17 mm (in dry air at 20 °C).

Wavelength (λ) for a sound wave = c/f, where c is the speed of sound (343 m/s in dry air at 20 °C) and f is the frequency - the units are metres.

Wavelength Definition IEC 103-10-10 distance, in the direction of propagation of a sinusoidal wave, between two successive points where the phases of the characteristic quantity differ by 2π radians.

Wave Number or Wavenumber is the spatial frequency of a wave, either in cycles per distance or radian per unit distance.

It can be envisaged as the number of waves that exist over a specified distance (analogous to frequency being the number of cycles or radian per unit time).

Wave Velocity is equal to the product of its wavelength and frequency.
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