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Reflecting targets

Reflecting target image and schematic

Cone-shaped reflectors are normally used in the fabrication of totally reflecting targets. These are manufactured from air-backed thin metal membranes that provide a close approximation to an ideal water-air interface, having a reflection coefficient very close to unity. The most common configuration, that of a convex conical reflector of cone-angle 90°, has been depicted in the diagram. In use, the treatment head is positioned facing the apex of the cone and is aligned perpendicularly to the base of the cone, such that the incident ultrasound is reflected away from the acoustic beam axis. For a perfectly reflecting target, the value of P and the resultant force are related as follows:

P = cF / 2cos²θ

where θ is the angle between the propagation direction of the ultrasonic wave and the normal to the reflecting surface. For a 90° cone angle, θ = 45°, the expression simplifies to

P = cF

The alignment is critical due to the angular factor. A level of angular variation has been provided in the table below. For an angle of 50° to the reflecting surface shows a reduction in the measured power of 17%.

A requirement of the reflecting target is that its buoyancy must be stable and this means that it must be incompressible to changes in hydrostatic pressure caused by the atmosphere or water temperature. In practice, this can be realised by using a venting pipe. The ultrasound reflected away from the cone must be absorbed at the walls of the vessel containing the water medium. If this does not occur then ultrasound returning from the walls may impinge on the target, leading to an over-reading of the power and travel back to the face of the transducer, which can change its response through acoustic loading. The requirements for the acoustic properties of materials that may be used as tank linings are less stringent than in the absorbing target case.

One property of the convex type of target is that it will tend to de-centre when an acoustic radiation force is applied which, due to misalignment, is not coincident with the central axis of the target. In practice, this can be prevented by attaching the reflecting target to an associated rigid gantry arrangement that also interfaces to the weighing mechanism of the balance being used. Clearly, the accuracy of the radiation force measured using either type of target is dependent on the geometry of the field under test.

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