Sound Wave Terms, Definitions, Units, Measurements ...
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
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.
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.
Related Terms - listed alphabetically
is where the amplitude of a sound wave
is a maximum and halfway between two adjacent nodes
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
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)
is a point in the medium
through which a longitudinal wave is travelling that has the maximum density.
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.
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
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.
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
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
to the wavefront.
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
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
also known as Surface Acoustic Waves (SAWs)
, travel near the surface of solids and include both longitudinal
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.
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
SAW - surface acoustic waves, under rayleigh waves
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
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
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 • antinode • Node • standing 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.
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
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).