Aortic valve area calculation

2013-06-30T03:14:10Z

98.236.186.100: /* The continuity equation */

{{Expert-subject|Physics|date=November 2008}}
{{Refimprove|date=January 2007}}

'''Non-linear acoustics''' is a branch of physics dealing with sound waves being distorted as they travel.

==Introduction==
A sound [[wave]] propagates through a material as a localized [[pressure]] change. Increasing the pressure of a gas, or fluid, increases its local temperature and also increases the local [[speed of sound]] in a compressible material increases with temperature; as a result, the wave travels faster during the high pressure phase of the oscillation than during the lower pressure phase. This affects the wave's frequency structure; for example, in an initially plane [[sinusoidal]] wave of a single frequency, the peaks of the wave travel faster than the troughs, and the pulse becomes cumulatively more like a sawtooth wave. In other words, the wave self-distorts. In doing so, other [[frequency]] components are introduced, which can be described by the Fourier series. This phenomenon is characteristic of a [[non-linear system]], since a linear acoustic system responds only to the driving frequency. This always occurs but the affects of geometric spreading and of absorption usually overcome the self distortion, so linear behavior usually prevails and nonlinear acoustic propagation occurs only for very large amplitudes and only near the source.

Additionally, waves of different amplitudes will generate different pressure gradients, contributing to the non-linear effect.

==Physical analysis==

The pressure changes within a medium cause the wave energy to transfer to higher harmonics. Since [[attenuation]] generally increases with frequency, a counter effect exists that changes the nature of the nonlinear effect over distance. To describe their level of nonlinearity, materials can be given a nonlinearity parameter, <math>B/A</math>. The values of <math>A</math> and <math>B</math> are the coefficients of the first and second order terms of the [[Taylor series]] expansion of the equation relating the material's pressure to its density. The Taylor series has more terms, and hence more coefficients (C, D, .. etc) but they are seldom used. Typical values for the nonlinearity parameter in biological mediums are shown in the following table.<ref>{{Cite doi|10.1088/0034-4885/62/5/201}}</ref>

{| class="wikitable"
|-
! Material
! <math>B/A</math>
|-
| Blood
| 6.1
|-
| Brain
| 6.6
|-
| Fat
| 10
|-
| Liver
| 6.8
|-
| Muscle
| 7.4
|-
| Water
| 5.2
|}

In a liquid usually a modified coefficient is used known as <math>\beta = 1 + \frac{B}{2A}</math>.

==Mathematical model==
===Westervelt equation===
The general wave equation that accounts for nonlinearity up to the second-order is given by the Westervelt equation<ref>{{cite book | first1=M.F. | last1=Hamilton | first2=D.T. | last2=Blackstock | title=Nonlinear Acoustics | publisher=Academic Press | year=1998 | isbn=0-12-321860-8 | page=55}}</ref>

<math>\, \nabla^{2} p - \frac{1}{c_{0}^{2}} \frac{\partial^{2} p}{\partial t^{2}} + \frac{\delta}{c_{0}^{4}} \frac{\partial^{3} p}{\partial t^{3}} = - \frac{\beta}{\rho_{0} c_{0}^{4}} \frac{\partial^{2} p^{2}}{\partial t^{2}}</math>

where <math>p</math> is the sound pressure, <math>c_0</math> is the small signal sound speed, <math>\delta</math> is the sound diffusivity, <math>\beta</math> is the non-linearity coefficient and <math>\rho_0</math> is the ambient density.

The sound diffusivity is given by

<math>\, \delta = \frac{1}{\rho_{0}} (\frac{4}{3}\mu+\mu_{B}) + \frac{k}{\rho_{0}} (\frac{1}{c_{v}} - \frac{1}{c_{p}})</math>

where <math>\mu</math> is the shear viscosity, <math>\mu_{B}</math> the bulk viscosity, <math>k</math> the thermal conductivity, <math>c_{v}</math> and <math>c_{p}</math> the specific heat at constant volume and pressure respectively.

===Burgers equation===
The Westervelt equation can be simplified to take a one-dimensional form with an assumption of strictly forward propagating waves and the use of a coordinate transformation to a retarded time frame. This is known as the Burgers equation<ref>{{cite book | first1=M.F. | last1=Hamilton | first2=D.T. | last2=Blackstock | title=Nonlinear Acoustics | publisher=Academic Press | year=1998 | isbn=0-12-321860-8 | page=57}}</ref>

<math>\frac{\partial p}{\partial z} = \frac{\beta p}{\rho_{0} c_{0}^{3}}\frac{\partial p}{\partial \tau} + \frac{\delta}{2 c_{0}^{3}}\frac{\partial^{2} p}{\partial \tau^{2}}</math>

where <math>\tau = t-z/c_0</math> is [[retarded time]]. The Burgers equation is the simplest model that describes the combined effects of nonlinearity and losses on the propagation of plane progressive waves.

===KZK equation===
An augmentation to the Burgers equation that accounts for the combined effects of non-linearity, diffraction and absorption in directional sound beams is described by the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation.<ref>{{cite journal
| author = Anna Rozanova-Pierrat
| title = Mathematical analysis of Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation
| publisher = Laboratoire Jacques-Louis Lions, Université Pierre et Marie Curie
| url = http://hal.archives-ouvertes.fr/docs/00/11/21/47/PDF/R06022.pdf
| format = [[PDF]]
| accessdate = 2008-11-10}}</ref> Solutions to this equation are generally used to model non-linear acoustics.

If the <math>z</math> axis is in the direction of the sound beam path and the <math>(x,y)</math> plane is perpendicular to that, the KZK equation can be written<ref>{{cite journal
| author = V. F. Humphrey
| title = Non-linear propagation for medical imaging
| publisher = Department of Physics, University of Bath, Bath, UK
| url = http://www.sfa.asso.fr/wcu2003/procs/cd1/articles/000383.pdf
| format = [[PDF]]
| accessdate = 2008-11-10}}</ref>

<math>\, \frac{\partial^2 p}{\partial z \partial \tau} = \frac{c_0}{2}\nabla^2_{\perp}p + \frac{\delta}{2c^3_0}\frac{\partial^3 p}{\partial \tau^3} + \frac{\beta}{2\rho_0 c^3_0}\frac{\partial^2 p^2}{\partial \tau^2}</math>

The equation can be solved for a particular system using a [[finite difference]] scheme. Such solutions show how the sound beam distorts as it passes through a non-linear medium.

==Common occurrences==
===Sonic boom===
The nonlinear behavior of the atmosphere leads to change of the wave shape in a [[sonic boom]]. Generally, this makes the boom more 'sharp' or sudden, as the high-amplitude peak moves to the wavefront.

===Acoustic levitation===
The practice of [[acoustic levitation]] would not be possible without understanding nonlinear acoustic phenomena.<ref>http://science.howstuffworks.com/acoustic-levitation.htm</ref> The nonlinear effects are particularly evident due to the high-powered acoustic waves involved.

===Ultrasonic waves===
Because of their relatively high [[amplitude]] to [[wavelength]] ratio, [[ultrasound|ultrasonic waves]] commonly display nonlinear propagation behavior. For example, nonlinear acoustics is a field of interest for [[medical ultrasonography]] because it can be exploited to produce a better image quality.

==References==
<references/>

[[Category:Acoustics]]
[[Category:Nonlinear systems]]

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Aortic valve area calculation