A device to measure the acoustic power or intensity of a sound beam by means of the force or torque that the beam exerts on an inserted object or interface. The underlying theory involves the concept of radiation pressure, which is the time-independent part of the pressure associated with a nominally sinusoidal acoustic disturbance. Such pressure occurs, for example, when a plane sound wave is partially reflected at an interface between two materials, with the nonlinear interaction between the incident and reflected waves giving rise to a steady pressure on the interface. If a narrow beam is incident on the interface and the transmitted wave is fully absorbed by the second material, the magnitude of the radiation force F (area integral of radiation pressure) equals a constant times W/c, where W is the power of the sound beam and c is the sound speed. The multiplicative constant is of the order of unity and depends on the thermodynamic properties of the fluid. The force acts in the vector direction of propagation of the sound wave. When the inserted object is of small size, the force is not given by as simple an expression, but it can nevertheless be predicted from basic principles. Thus, acoustic radiometers do not require external calibration and are themselves sometimes used in the calibration of transducers.