Conical type ultrasonic transducer with apodised vibration area for generation of limited diffraction beams

Conical transducers with a velocity distribution varying in accordance with the envelope function of the Bessel function J₀ can easily be used to create Bessel-type limited diffraction beams. The authors present a simple and practical method of realising weighted conical transducers. The method uses apodisation on the vibration area by forming thin electrode strips of a uniform width spreading radially on the transducer. An almost uniform -6 dB beamwidth of 6-9 wavelengths has been obtained in the axial range of z=10 to 180 mm using a fabricated transducer of 25 mm in diameter operating at 3.6 MHz