In
physics, the acoustic wave equation is a second-order
partial differential equation that governs the propagation of
acoustic waves through a
material medium resp. a
standing wavefield. The equation describes the evolution of
acoustic pressurep or
particle velocityu as a function of position x and time t. A simplified (scalar) form of the equation describes acoustic waves in only one spatial dimension, while a more general form describes waves in three dimensions. Propagating waves in a pre-defined direction can also be calculated using a first order
one-way wave equation.
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.[1]
In one dimension
Equation
The wave equation describing a standing wave field in one dimension (position ) is
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 () traveling up the x-axis and the other () down the x-axis at the speed . The particular case of a sinusoidal wave traveling in one direction is obtained by choosing either or to be a sinusoid, and the other to be zero, giving
The derivation of the wave equation involves three steps: derivation of the equation of state, the linearized one-dimensional continuity equation, and the linearized one-dimensional force equation.
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:
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):
,
.
Solution
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
where the asymptotic approximations to the
Hankel functions, when , are
.
Spherical coordinates
.
Depending on the chosen Fourier convention, one of these represents an outward travelling wave and the other a 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.
^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