Location of Focal Points of Focus CW Ultrasonic Transducers

摘要

Because of diffractive effects at the limited aperture of ultrasonic transducers, the effective focus of spherically curved-element or lensed transducers lies in front of the geometric focus defined by the radius of curvature of the element or lens, and is a function of ultrasonic frequency. This focal-point shift can be quite significant in many practical cases, and can lead to serious degradation of system performance, if not accounted for. In this work, the focal-point shift was calculated as a function of frequency and of transducer diameter. It is shown that the focal-point shift can be described in terms of the ratio d/lambda and of the transducer, lambda is the wavelength, and F is the conventional ratio of geometric focal length to effective diameter of the transducer. A nomogram relating these quantities is developed and presented. (ERA citation 08:027071)