Acoustic Glossary


 

Sound Waves : Definitions, Terms, Units, Measurements..

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 complex sound fields detailed below.

Sound waves in air are longitudinal waves.

Sound Wave IEC 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.

See also Particle Acceleration • Particle Displacement • Particle Velocity


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

Antinode IEC Definition, 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.


Beat IEC Definition, phenomenon that results from the linear or non-linear superposition of two or more waves of the same kind but of different frequencies.
Bending Wave IEC Definition, transverse wave in a plate or bar, a combination of compression wave and shear wave

Boundary Effect : 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 IEC Definition, is a point in the medium through which a longitudinal wave is travelling that has the maximum density.

Compression Wave a Longitudinal Sound Wave propagated by the elastic compression of the medium.

Compressional Wave IEC Definition, 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 IEC Definition, wave of which the wave fronts are coaxial cylinders.

Diffraction the distortion of a wave front 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 IEC Definition, phenomenon by which a sound wave is changed in direction by an obstacle or other heterogeneity in the medium.

Diffraction Factor IEC Definition, 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 IEC Definition, separation of the sinusoidal components of a wave that results from change of speed of sound with frequency.
Free Progressive Wave IEC Definition, wave propagating in a medium, free from boundary effects

Grazing Incidence IEC Definition, the incidence of a wave on a surface at a very small grazing angle.

Incident Wave IEC Definition, 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 IEC Definition, 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 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 waves are also known as l-waves and compression waves

Longitudinal Wave IEC Definition, 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 IEC Definition, 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.


Plane Wave or Planewave IEC Definition, wave of which all the wavefronts are parallel planes normal to the direction of propagation.

Progressive Wave distribute energy from a source to the surrounding area. They move energy in the form of vibrating particles or sound fields.


Rarefaction is 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 IEC Definition, 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 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 IEC Definition, phenomenon by which the direction of propagation of a sound wave is changed due to spatial variation in the speed of sound

Refraction Loss IEC Definition, that part of the transmission loss due to refraction resulting from non-uniformity of the medium


Rotational Wave IEC Definition, 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 Shear Wave

Scattering IEC Definition, irregular diffraction and reflection of a sound wave in many directions.

Shear Wave IEC Definition, 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 Rotational Wave.


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 Wave Velocity IEC Definition, vector that specifies the speed and direction with which a sound wave travels.


Specular Reflection IEC Definition, 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 IEC Definition, 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 IEC Definition, 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 a method for measuring absorption coefficients by means of Standing Waves in a tube.


Superposition of Waves IEC Definition, the principle of superposition may be applied to waves whenever two (or more) waves travelling through the same medium at the same time. The waves pass through each other without being disturbed. The net displacement of the medium at any point in space or time, is simply the sum of the individual wave displacements.


Surface Acoustic Wave IEC Definition, an acoustic wave travelling along the surface of a material, with an amplitude that typically decays exponentially with depth into the substrate, also known as a Rayleigh Wave.


Surface Wavefront IEC Definition, 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 IEC Definition, the direction of particle displacement at each point of the medium is perpendicular to the direction of wave propagation, as opposed to longitudinal waves. The medium particles do not move along the wave, they transfer energy to the next particle along the path, then return to their equilibrium point.


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

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