Introduction | Radiation Force Balances | Measurement | Additional Information | Further Reading

Target dimensions

Target size can affect the force that is measured as some of the beam may miss the target. An important parameter of a transducer defining the distribution of generated pressure is its ka value, calculated from the wave number, k, and the radius, a, of the transducer. The lowest values result from transducers operating at low frequency with a small radius. As a general requirement, an absorbing target should be at least 1.5 times the diameter of the ultrasonic transducer under test. A minimum value for the diameter of a circular target can be calculated which produces an underestimate of power of less than 2% for a circular, plane piston transducer. Typical values of commonly available physiotherapy transducers have been given in the following table:

It can be seen that, if a treatment head is kept within a small distance of the target surface (<10 mm) then a target diameter of 50 mm should be adequate for low ka transducers. The minimum size can also be extended to the reflecting target case. For a 90º convex-conical reflecting target there is limiting ka value below which the requirements of these formulas contained in the standard can never be fulfilled, which is ka = 17.4. This limitation arises from the closest approach between the transducer output face and the widest part of the reflecting target (i.e. the base).

References

• IEC61161 Ultrasonic power measurement in liquids in the frequency range 0,5 MHz to 25 MHz
• Beissner K 1984 'Minimum target size in radiation force measurements' Journal of the Acoustical .Society of America 76 (5)