# Acoustic wave equation

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 $p$ or particle velocity u as a function of position x and time $t$ . A simplified form of the equation describes acoustic waves in only one spatial dimension, while a more general form describes waves in three dimensions.

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.

## In one dimension

### Equation

The wave equation describing sound in one dimension (position $x$ ) is

${\partial ^{2}p \over \partial x^{2}}-{1 \over c^{2}}{\partial ^{2}p \over \partial t^{2}}=0,$ where $p$ is the acoustic pressure (the local deviation from the ambient pressure), and where $c$ is the speed of sound.

### Solution

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

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

$p=p_{0}\sin(\omega t\mp kx)$ .

where $\omega$ is the angular frequency of the wave and $k$ is its wave number.

### Derivation

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)

$PV=nRT$ In an adiabatic process, pressure P as a function of density $\rho$ can be linearized to

$P=C\rho \,$ where C is some constant. Breaking the pressure and density into their mean and total components and noting that $C={\frac {\partial P}{\partial \rho }}$ :

$P-P_{0}=\left({\frac {\partial P}{\partial \rho }}\right)(\rho -\rho _{0})$ .

The adiabatic bulk modulus for a fluid is defined as

$B=\rho _{0}\left({\frac {\partial P}{\partial \rho }}\right)_{adiabatic}$ which gives the result

$P-P_{0}=B{\frac {\rho -\rho _{0}}{\rho _{0}}}$ .

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

$s={\frac {\rho -\rho _{0}}{\rho _{0}}}$ The linearized equation of state becomes

$p=Bs\,$ where p is the acoustic pressure ($P-P_{0}$ ).

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

${\frac {\partial \rho }{\partial t}}+{\frac {\partial }{\partial x}}(\rho u)=0$ .

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

${\frac {\partial }{\partial t}}(\rho _{0}+\rho _{0}s)+{\frac {\partial }{\partial x}}(\rho _{0}u+\rho _{0}su)=0$ 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:

${\frac {\partial s}{\partial t}}+{\frac {\partial }{\partial x}}u=0$ Euler's Force equation (conservation of momentum) is the last needed component. In one dimension the equation is:

$\rho {\frac {Du}{Dt}}+{\frac {\partial P}{\partial x}}=0$ ,

where $D/Dt$ 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:

$(\rho _{0}+\rho _{0}s)\left({\frac {\partial }{\partial t}}+u{\frac {\partial }{\partial x}}\right)u+{\frac {\partial }{\partial x}}(P_{0}+p)=0$ .

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

$\rho _{0}{\frac {\partial u}{\partial t}}+{\frac {\partial p}{\partial x}}=0$ .

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

${\frac {\partial ^{2}s}{\partial t^{2}}}+{\frac {\partial ^{2}u}{\partial x\partial t}}=0$ $\rho _{0}{\frac {\partial ^{2}u}{\partial x\partial t}}+{\frac {\partial ^{2}p}{\partial x^{2}}}=0$ .

Multiplying the first by $\rho _{0}$ , subtracting the two, and substituting the linearized equation of state,

$-{\frac {\rho _{0}}{B}}{\frac {\partial ^{2}p}{\partial t^{2}}}+{\frac {\partial ^{2}p}{\partial x^{2}}}=0$ .

The final result is

${\partial ^{2}p \over \partial x^{2}}-{1 \over c^{2}}{\partial ^{2}p \over \partial t^{2}}=0$ where $c={\sqrt {\frac {B}{\rho _{0}}}}$ is the speed of propagation.

## In three dimensions

### Equation

Feynman provides a derivation of the wave equation for sound in three dimensions as

$\nabla ^{2}p-{1 \over c^{2}}{\partial ^{2}p \over \partial t^{2}}=0,$ where $\nabla ^{2}$ is the Laplace operator, $p$ is the acoustic pressure (the local deviation from the ambient pressure), and where $c$ is the speed of sound.

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

$\nabla ^{2}\mathbf {u} \;-{1 \over c^{2}}{\partial ^{2}\mathbf {u} \; \over \partial t^{2}}=0$ .

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

$\nabla ^{2}\Phi -{1 \over c^{2}}{\partial ^{2}\Phi \over \partial t^{2}}=0$ and then derive the physical quantities particle velocity and acoustic pressure by the equations (or definition, in the case of particle velocity):

$\mathbf {u} =\nabla \Phi \;$ ,
$p=-\rho {\partial \over \partial t}\Phi$ .

### 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 $e^{i\omega t}$ where $\omega =2\pi f$ is the angular frequency. The explicit time dependence is given by

$p(r,t,k)=\operatorname {Real} \left[p(r,k)e^{i\omega t}\right]$ Here $k=\omega /c\$ is the wave number.

#### Cartesian coordinates

$p(r,k)=Ae^{\pm ikr}$ .

#### Cylindrical coordinates

$p(r,k)=AH_{0}^{(1)}(kr)+\ BH_{0}^{(2)}(kr)$ .

where the asymptotic approximations to the Hankel functions, when $kr\rightarrow \infty$ , are

$H_{0}^{(1)}(kr)\simeq {\sqrt {\frac {2}{\pi kr}}}e^{i(kr-\pi /4)}$ $H_{0}^{(2)}(kr)\simeq {\sqrt {\frac {2}{\pi kr}}}e^{-i(kr-\pi /4)}$ .

#### Spherical coordinates

$p(r,k)={\frac {A}{r}}e^{\pm ikr}$ .

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.