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Acoustic theory is a scientific field that relates to the description of sound waves. It derives from fluid dynamics. See acoustics for the engineering approach.

Propagation of sound waves in a fluid (such as water) can be modeled by an equation of continuity (conservation of mass) and an equation of motion (conservation of momentum) . With some simplifications, in particular constant density, they can be given as follows:

where is the acoustic pressure and is the flow velocity vector, is the vector of spatial coordinates , is the time, is the static mass density of the medium and is the bulk modulus of the medium. The bulk modulus can be expressed in terms of the density and the speed of sound in the medium () as

If the flow velocity field is irrotational, , then the acoustic wave equation is a combination of these two sets of balance equations and can be expressed as[1]

where we have used the vector Laplacian, . The acoustic wave equation (and the mass and momentum balance equations) are often expressed in terms of a scalar potential where . In that case the acoustic wave equation is written as

and the momentum balance and mass balance are expressed as

Contents

Derivation of the governing equationsEdit

The derivations of the above equations for waves in an acoustic medium are given below.

Conservation of momentumEdit

The equations for the conservation of linear momentum for a fluid medium are

 

where   is the body force per unit mass,   is the pressure, and   is the deviatoric stress. If   is the Cauchy stress, then

 

where   is the rank-2 identity tensor.

We make several assumptions to derive the momentum balance equation for an acoustic medium. These assumptions and the resulting forms of the momentum equations are outlined below.

Assumption 1: Newtonian fluidEdit

In acoustics, the fluid medium is assumed to be Newtonian. For a Newtonian fluid, the deviatoric stress tensor is related to the flow velocity by

 

where   is the shear viscosity and   is the bulk viscosity.

Therefore, the divergence of   is given by

 

Using the identity  , we have

 

The equations for the conservation of momentum may then be written as

 

Assumption 2: Irrotational flowEdit

For most acoustics problems we assume that the flow is irrotational, that is, the vorticity is zero. In that case

 

and the momentum equation reduces to

 

Assumption 3: No body forcesEdit

Another frequently made assumption is that effect of body forces on the fluid medium is negligible. The momentum equation then further simplifies to

 

Assumption 4: No viscous forcesEdit

Additionally, if we assume that there are no viscous forces in the medium (the bulk and shear viscosities are zero), the momentum equation takes the form

 

Assumption 5: Small disturbancesEdit

An important simplifying assumption for acoustic waves is that the amplitude of the disturbance of the field quantities is small. This assumption leads to the linear or small signal acoustic wave equation. Then we can express the variables as the sum of the (time averaged) mean field ( ) that varies in space and a small fluctuating field ( ) that varies in space and time. That is

 

and

 

Then the momentum equation can be expressed as

 

Since the fluctuations are assumed to be small, products of the fluctuation terms can be neglected (to first order) and we have

 

Assumption 6: Homogeneous mediumEdit

Next we assume that the medium is homogeneous; in the sense that the time averaged variables   and   have zero gradients, i.e.,

 

The momentum equation then becomes

 

Assumption 7: Medium at restEdit

At this stage we assume that the medium is at rest, which implies that the mean flow velocity is zero, i.e.,  . Then the balance of momentum reduces to

 

Dropping the tildes and using  , we get the commonly used form of the acoustic momentum equation

 

Conservation of massEdit

The equation for the conservation of mass in a fluid volume (without any mass sources or sinks) is given by

 

where   is the mass density of the fluid and   is the flow velocity.

The equation for the conservation of mass for an acoustic medium can also be derived in a manner similar to that used for the conservation of momentum.

Assumption 1: Small disturbancesEdit

From the assumption of small disturbances we have

 

and

 

Then the mass balance equation can be written as

 

If we neglect higher than first order terms in the fluctuations, the mass balance equation becomes

 

Assumption 2: Homogeneous mediumEdit

Next we assume that the medium is homogeneous, i.e.,

 

Then the mass balance equation takes the form

 

Assumption 3: Medium at restEdit

At this stage we assume that the medium is at rest, i.e.,  . Then the mass balance equation can be expressed as

 

Assumption 4: Ideal gas, adiabatic, reversibleEdit

To close the system of equations we need an equation of state for the pressure. To do that we assume that the medium is an ideal gas and all acoustic waves compress the medium in an adiabatic and reversible manner. The equation of state can then be expressed in the form of the differential equation:

 

where   is the specific heat at constant pressure,   is the specific heat at constant volume, and   is the wave speed. The value of   is 1.4 if the acoustic medium is air.

For small disturbances

 

where   is the speed of sound in the medium.

Therefore,

 

The balance of mass can then be written as

 

Dropping the tildes and defining   gives us the commonly used expression for the balance of mass in an acoustic medium:

 

Governing equations in cylindrical coordinatesEdit

If we use a cylindrical coordinate system   with basis vectors  , then the gradient of   and the divergence of   are given by

 

where the flow velocity has been expressed as  .

The equations for the conservation of momentum may then be written as

 

In terms of components, these three equations for the conservation of momentum in cylindrical coordinates are

 

The equation for the conservation of mass can similarly be written in cylindrical coordinates as

 

Time harmonic acoustic equations in cylindrical coordinatesEdit

The acoustic equations for the conservation of momentum and the conservation of mass are often expressed in time harmonic form (at fixed frequency). In that case, the pressures and the flow velocity are assumed to be time harmonic functions of the form

 

where   is the frequency. Substitution of these expressions into the governing equations in cylindrical coordinates gives us the fixed frequency form of the conservation of momentum

 

and the fixed frequency form of the conservation of mass

 

Special case: No z-dependenceEdit

In the special case where the field quantities are independent of the z-coordinate we can eliminate   to get

 

Assuming that the solution of this equation can be written as

 

we can write the partial differential equation as

 

The left hand side is not a function of   while the right hand side is not a function of  . Hence,

 

where   is a constant. Using the substitution

 

we have

 

The equation on the left is the Bessel equation, which has the general solution

 

where   is the cylindrical Bessel function of the first kind and   are undetermined constants. The equation on the right has the general solution

 

where   are undetermined constants. Then the solution of the acoustic wave equation is

 

Boundary conditions are needed at this stage to determine   and the other undetermined constants.

ReferencesEdit

  1. ^ Douglas D. Reynolds. (1981). Engineering Principles in Acoustics, Allyn and Bacon Inc., Boston.

See alsoEdit