Acoustic wave equation

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In physics, the acoustic wave equation governs the propagation of acoustic waves through a material medium. The form of the equation is a second order partial differential equation. The equation describes the evolution of acoustic pressure or particle velocity u as a function of position x and time . A simplified form of the equation describes acoustic waves in only one spatial dimension, while a more general form describes waves in three dimensions.

The acoustic wave equation was an important point of reference in the development of the electromagnetic wave equation in Kelvin's master class at Johns Hopkins University.[1]

For lossy media, more intricate models need to be applied in order to take into account frequency-dependent attenuation and phase speed. Such models include acoustic wave equations that incorporate fractional derivative terms, see also the acoustic attenuation article or the survey paper.[2]

In one dimension[edit]


Richard Feynman[3] derives the wave equation that describes the behavior of sound in matter in one dimension (position ) as:

where is the acoustic pressure (the local deviation from the ambient pressure), and where is the speed of sound.[4]


Provided that the speed is a constant, not dependent on frequency (the dispersionless case), then the most general solution is

where and are any two twice-differentiable functions. This may be pictured as the superposition of two waveforms of arbitrary profile, one () travelling up the x-axis and the other () down the x-axis at the speed . The particular case of a sinusoidal wave travelling in one direction is obtained by choosing either or to be a sinusoid, and the other to be zero, giving


where is the angular frequency of the wave and is its wave number.


Derivation of the acoustic wave equation

The wave equation can be developed from the linearized one-dimensional continuity equation, the linearized one-dimensional force equation and the equation of state.

The equation of state (ideal gas law)

In an adiabatic process, pressure P as a function of density can be linearized to

where C is some constant. Breaking the pressure and density into their mean and total components and noting that :


The adiabatic bulk modulus for a fluid is defined as

which gives the result


Condensation, s, is defined as the change in density for a given ambient fluid density.

The linearized equation of state becomes

where p is the acoustic pressure ().

The continuity equation (conservation of mass) in one dimension is


Where u is the flow velocity of the fluid. Again the equation must be linearized and the variables split into mean and variable components.

Rearranging and noting that ambient density changes with neither time nor position and that the condensation multiplied by the velocity is a very small number:

Euler's Force equation (conservation of momentum) is the last needed component. In one dimension the equation is:


where represents the convective, substantial or material derivative, which is the derivative at a point moving with medium rather than at a fixed point.

Linearizing the variables:


Rearranging and neglecting small terms, the resultant equation becomes the linearized one-dimensional Euler Equation:


Taking the time derivative of the continuity equation and the spatial derivative of the force equation results in:


Multiplying the first by , subtracting the two, and substituting the linearized equation of state,


The final result is

where is the speed of propagation.

In three dimensions[edit]


Feynman[3] provided a derivation of the wave equation that describes the behavior of sound in matter in three dimensions as:

where is the Laplace operator, is the acoustic pressure (the local deviation from the ambient pressure), and where is the speed of sound.

A similar looking wave equation but for the vector field particle velocity is given by


In some situations, it is more convenient to solve the wave equation for an abstract scalar field velocity potential which has the form

and then derive the physical quantities particle velocity and acoustic pressure by the equations (or definition, in the case of particle velocity):



The following solutions are obtained by separation of variables in different coordinate systems. They are phasor solutions, that is they have an implicit time-dependence factor of where is the angular frequency. The explicit time dependence is given by

Here is the wave number.

Cartesian coordinates[edit]


Cylindrical coordinates[edit]


where the asymptotic approximations to the Hankel functions, when , are


Spherical coordinates[edit]


Depending on the chosen Fourier convention, one of these represents an outward travelling wave and the other an nonphysical inward travelling wave. The inward travelling solution wave is only nonphysical because of the singularity that occurs at r=0; inward travelling waves do exist.

See also[edit]


  1. ^ William Thomson, Lord Kelvin (1904) Lecture notes taken by A. S. Hathaway, Molecular dynamics and the wave theory of light, page 80, Cambridge University Press, link from Internet Archive
  2. ^ S. P. Näsholm and S. Holm, "On a Fractional Zener Elastic Wave Equation," Fract. Calc. Appl. Anal. Vol. 16, No 1 (2013), pp. 26-50, DOI: 10.2478/s13540-013--0003-1 Link to e-print
  3. ^ a b Richard Feynman, Lectures in Physics, Volume 1, 1969, Addison Publishing Company, Addison
  4. ^ Richard Feynman, Lectures in Physics, Volume 1, Chapter 47: Sound. The wave equation, Caltech 1963, 2006, 2013