Acoustic radiation force

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Acoustic radiation force is a physical phenomenon resulting from the interaction of an acoustic wave with an obstacle placed along its path. Generally, the force exerted on the obstacle is evaluated by integrating the acoustic radiation pressure (due to the presence of the sonic wave) over its time-varying surface.

Acoustic waves are a type of longitudinal waves that propagate by means of adiabatic compression and decompression. Longitudinal waves are waves that have the same direction of vibration as their direction of travel. Important quantities for describing acoustic waves are sound pressure, particle velocity, particle displacement and sound intensity. Acoustic waves travel with the speed of sound which depends on the medium they're passing through.

Acoustic radiation pressure is the apparent pressure difference between the average pressure at a surface moving with the displacement of the wave propagation and the pressure that would have existed in the fluid of the same mean density when at rest. Numerous authors make a distinction between the phenomena of Rayleigh radiation pressure and Langevin radiation pressure.

The magnitude of the force exerted by an acoustic plane wave at any given location can be calculated as: [1] [2]

where

The neper is a logarithmic unit for ratios of measurements of physical field and power quantities, such as gain and loss of electronic signals. The unit's name is derived from the name of John Napier, the inventor of logarithms. As is the case for the decibel and bel, the neper is a unit defined in the international standard ISO 80000. It is not part of the International System of Units (SI), but is accepted for use alongside the SI.

The effect of frequency on acoustic radiation force is taken into account via intensity (higher pressures are more difficult to attain at higher frequencies) and absorption (higher frequencies have a higher absorption rate). As a reference water has an acoustic absorption of 0.002 dB/(MHz2cm). [3]

Acoustic radiation forces on compressible particles such as bubbles are also known as Bjerknes forces, and are generated through a different mechanism, which does not require sound absorption or reflection. [4]

Bubble (physics) globule of one substance in another, usually gas in a liquid

A bubble is a globule of one substance in another, usually gas in a liquid. Due to the Marangoni effect, bubbles may remain intact when they reach the surface of the immersive substance.

Bjerknes forces are translational forces on bubbles in a sound wave. The phenomenon is a type of acoustic radiation force. Primary Bjerknes forces are caused by an external sound field; secondary Bjerknes forces are between pairs of bubbles in the same sound field. They were first described by Vilhelm Bjerknes in his 1906 Fields of Force.

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References

  1. "A finite-element method model of soft tissue response to impulsive acoustic radiation force". IEEE Trans Ultrason Ferroelectr Freq Control. 52 (10): 1699–712. October 2005. PMC   2818996 Lock-green.svg. PMID   16382621.
  2. "Estimates of echo correlation and measurement bias in acoustic radiation force impulse imaging". IEEE Trans Ultrason Ferroelectr Freq Control. 50 (6): 631–41. June 2003. PMID   12839175.
  3. Diagnostic ultrasound imaging : inside out. ISBN   9780126801453.
  4. Leighton, T.G., Walton, A.J. and Pickworth, M.J.W., 1990. Primary bjerknes forces. European Journal of Physics, 11(1), p.47.