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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. 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.
A modern acoustic radiometer, used to measure the total power of an ultrasonic sound beam in water and other liquids, employs a vane suspended in the fluid in such a manner that its displacement in a direction normal to its face is proportional to the net force pushing on its front face. The vane is ideally of dimensions somewhat larger than the incident beam's diameter, so that the encountered force is associated with the entire incident beam. To eliminate the possibility of sound being reflected toward the transmitting transducer, the vane is oriented at 45° to the incident sound beam. The vane’s horizontal displacement is made to be proportional to the imposed force by fastening the vane at one end of a long pendulum whose rotation from the vertical is opposed by the effect of gravity, such that the apparent spring constant for displacement in a direction at 45° to the face is approximately Mg/L, where M is the apparent mass of the vane (corrected for the presence of water), g is the acceleration of gravity, and L is the length of the pendulum. A nonlinear acoustics theory for such a circumstance yields a proportionality relation between the net horizontal radiation force on the vane and the acoustic power associated with the incident beam. Be cause the deflection of the vane is proportional to the radiation force, the acoustic power can be determined.
The concept of the vane device evolved from that of the Rayleigh disk, which was a circular disk that could rotate about its diameter and whose deflection from a nominal 45° orientation was opposed by a torsional spring. The Rayleigh disk was taken to have a radius much smaller than the wavelength, and its use ideally yielded a measurement of the local acoustic intensity that would have existed at the center of the disk were the disk not present.
