Ultrasonic power measurement
The time-average ultrasonic power emitted by an ultrasonic transducer is a characteristic quantity which is important under safety aspects and whose declaration is required for medical ultrasonic equipment by IEC 61157 (diagnostic instruments) and by IEC 61689 (therapeutic instruments). The recommended measurement method according to IEC 61161 is the radiation force method. The acoustic radiation force is the time-average force exerted by the field on an obstacle ("target"). The ultrasonic beam is directed on an appropriately chosen target having clearly defined acoustic properties and a sufficient cross-sectional area, and the time-average value of the acting force is measured. The ultrasonic power is calculated from the radiation force by means of theoretical expressions.
The measurement set-up described in IEC 61161 is the radiation force balance in various versions. The most sensitive is the so-called arrangement A where the target is suspended from a micro- or semi-micro-balance which measures the radiation force, and the suspension is by means of a thin wire penetrating the surface of the sound-propagating liquid. In addition, there are other balance arrangements which are particularly suitable for the higher power range. Several of these measurement set-up types are used in the Section 1.62 for determining the ultrasonic power in water emitted by medical ultrasonic equipment as well as that emitted by ultrasonic reference transducers which are available on loan to interested parties as an external calibration service.
The section 1.62 has contributed largely to the development of the measurement methods described. The standard IEC 61161 mentioned above was drafted here. The section 1.62 has participated successfully in three international intercomparisons of ultrasonic power measurements. It was the pilot laboratory of the world-wide CIPM key comparison of ultrasonic power measurement.
A theoretical understanding of the acoustic radiation force is not only of practical importance for the above-mentioned calculation formulas but also of general physical interest. Whereas in former times the radiation force was regarded as being caused by an acoustic radiation pressure, the more recent literature deals increasingly with the concept of an acoustic radiation stress tensor, this development being largely influenced by the scientific work of the section (cf. the list of references).