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Absorbing targets

In the simplest case, assuming plane waves, and using a target that totally absorbs the incident momentum, the relationship between the ultrasonic power, P, and the radiation force, F, is:

P = cF

where c is the propagation speed of the ultrasound in the medium, usually water. Whilst a totally absorbing target clearly cannot be realised in practice, reasonably close approximations can be achieved using acoustically anechoic materials. Specifications of a suitable absorbing target material are as follows:

• an amplitude reflection factor less than 5% (an echo reduction less than -26 dB re: an ideal reflector, defined by a water-air interface)
• absorption of at least 99% of the incident ultrasonic energy (equivalent to a transmission loss of 40 dB).

Such specifications can be met using elastic rubber materials. Ideally the material would employ textured front surfaces to decrease specular reflections arising from the water-rubber interface travelling back to the face of the treatment head.

Although conceptually simple, the use of absorbing targets introduces some problems whilst measurements are being made. The absorbed ultrasound will cause the target to heat up resulting in thermal expansion and a change in buoyancy, which will be seen as a change in target weight during the On time of the transducer under test, and whilst the target cools after switch off (See Measurement effects). The transfer of the heat to the water medium will also cause a rise in temperature - On times should be limited such that any local increases are 2 °C or less. The advantages of an absorbing target are that there is negligible angular dependence on the incident ultrasound and is a preferred target when measuring the output from diverging or focussed beams.

References

• A new anechoic material for ultrasonic applications Zeqiri, B, Bickley, C J. Ultrasound in Medicine and Biology (2000), 26, No.3, 481-485
• IEC 61161 Ultrasonic power measurement in liquids in the frequency range 0,5 MHz to 25 MHz

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