James S. Martin
Peter H. Rogers
School of Mech. Eng., Georgia Inst. of Technol., Atlanta, GA 30332-0405
The extreme near field of several targets that incorporated abrupt surface impedance discontinuities was measured in the frequency range of 5 to 40 kHz using a medical ultrasound probe as a miniature hydrophone. The probe was mounted in an automated positioning system designed specifically for this application. Although it incorporated a relatively insensitive hydrophone and a weak source, the measurement system exhibited adequate dynamic range (on the order of 80 dB) for this experiment. Targets were acoustically large with ka in the range of 1 to 10 in the measurement frequency range. Insonifying signals were narrow band and virtually plane wave. Cylindrical, spherical, and planar targets were studied. Measurements were made of total surface pressure and pressure gradient on both uniform and mixed boundary objects. High wave-number standing waves were observed in the region of the discontinuities. Good agreement with both three-dimensional boundary element and two-dimensional collocation theory was observed at the low end of the frequency range. Issues of hardware design, signal processing, hydrophone--surface separation, accuracy of positioning, and spatial resolution will be discussed in addition to a qualitative analysis of data in the light of previous modeling efforts. [Work supported by ONR.]