Acoustic Glossary


 

Sound Waves, related terms, definitions, units etc.,


Sound Waves transfer sound energy from one point to another without any net movement of the air particles or other media they pass through.

To visualise this, 'create' some waves by moving your hand up and down while holding a length of rope. You can also make waves by moving your hand side-to-side, demonstrating there are two independent directions in which wave motion can occur, 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 disturbance propagated at a definite velocity in a material 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 (standing waves) which accumulates at walls. Sound wave reflections appear to make the localized sound level increase as all of the room modes terminate at the 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 Definition IEC 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 Definition

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 Wave, is 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. 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, 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 Wave, 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 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, travel near the surface of solids. Rayleigh waves 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.

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 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 reflection


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 under Shear Wave

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 characterized 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 characterized 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 Wavefront Definition IEC 801-23-02, locus of points of a progressive surface where the phase of a quantity characterizing 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.


Waveform

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) 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 is usually defined as the distance between sequential crests of a sound wave.
At 20 Hz the wavelength ≈ 17 m and at 20 kHz ≈ 17 mm, in air at 20 °C.
Frequency f, wavelength λ and wave velocity v are related by the formulae λ = v/f


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


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

 

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