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Diverging fields

A diverging beam is most likely to occur with a small diameter active element transducer operating at low megahertz frequencies typically 1MHz or below. A good indication of divergence can be gauged from the ka value (see target dimensions) with a limit of 17.4 when using a reflecting target.

A correction factor as a function of the ka for an absorbing target has been derived as follows:

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The factor is determined from calculated values for the ratio P/cF as a function of ka for a circular plane piston source in comparison to plane wave. The method and calculations have been described in IEC standard 61161.

It is known that due to the angular dependence of a reflecting target an underestimation of the measured power can occur (see reflecting targets). It is advisable that for beams where the pressure distribution has not been assessed or where it is known that the beam is divergent that an absorbing target should be used.

References

• Ultrasonic power measurement in liquids in the frequency range 0.5 MHz to 25 MHz International Electrotechnical Commission (1992), IEC Publication 61161, IEC, Geneva