IEC 61161:2013 specifies a method of determining the total emitted acoustic power of ultrasonic transducers based on the use of a radiation force balance; establishes general principles for the use of radiation force balances in which an obstacle (target) intercepts the sound field to be measured; establishes limitations of the radiation force method related to cavitation and temperature rise; establishes quantitative limitations of the radiation force method in relation to diverging and focused beams; provides information on estimating the acoustic power for diverging and focused beams using the radiation force method; and provides information on assessment of overall measurement uncertainties. This International Standard is applicable to the measurement of ultrasonic power up to 1 W based on the use of a radiation force balance in the frequency range from 0,5 MHz to 25 MHz; the measurement of ultrasonic power up to 20 W based on the use of a radiation force balance in the frequency range 0,75 MHz to 5 MHz; the measurement of total ultrasonic power in well-collimated, diverging and focused ultrasonic fields; and to the use of radiation force balances of the gravimetric type or force feedback type. This third edition cancels and replaces the second edition published in 2006. It constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition: - whereas the second edition tacitly dealt with circular transducers only, the present edition as far as possible deals with both circular and rectangular transducers, including a number of symbols for rectangular transducers; - attention is paid to focused cases and the influence of scanning has been added; - the method of calibrating the radiation force balance now depends on whether the set-up is used as a primary or as secondary measurement tool; - and Annex B (basic formulae) has been updated and in Annex C the buoyancy change method is mentioned (see also future IEC 62555).