Acoustic Glossary


Sound Waves : Definitions, Terms, Units and 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 and 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.

Sound waves in air are Longitudinal Waves.

see also : Particle:- Acceleration, Displacement, Velocity.

Beat : 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 : Transverse Wave in a plate or bar, a combination of Compression Wave and Shear Wave.

Compression : 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.

Cylindrical Wave : wave of which the wave fronts are coaxial cylinders.

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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 Factor : 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 : separation of the sinusoidal components of a wave that results from change of speed of sound with frequency.

Free Progressive Wave : wave propagating in a medium, free from boundary effects.

Grazing Incidence : the incidence of a wave on a surface at a very small grazing angle.

Incident Wave : a wave which propagates towards the surface separating two media.

Interference : 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

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

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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.

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.

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.

Rotational Wave : 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 : irregular Diffraction and Reflection of a sound wave in many directions.

Shear Wave : 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 : vector that specifies the speed and direction with which a sound wave travels.

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Spherical Wave : 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 Tube : a method for measuring Absorption Coefficients by means of Standing Waves in a tube.

Surface Acoustic Wave : SAW : 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 : 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 : 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. The simplest form of a wavefront is the Plane Wave.

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