Experimental demonstration of Fourier synthesis of an annular ultrasonic intensity distribution

Ultrasonic source velocity distributions can be manipulated to provide beam shaping, as demonstrated by much previous work on focussing, side-lobe reduction and the production of Gaussian beam profiles. In principle, other beam profiles can be specified to order using inverse source calculations. We have presented a Fourier optics calculation for the source velocity distribution required to produce a desired distribution in intensity at a given axial distance. In this Paper, we present experimental verification of this approach using a 3.8 cm radius lead metaniobate transducer divided into 10 annuli of equal width, operated at 850 kHz continuous wave. The radiofrequency (RF) voltages supplied to each annulus were adjusted to approximate a source distribution calculated to produce an annular intensity distribution with inner and outer radii of 1.5 and 2.5 cm, respectively, at an axial distance of 30 cm in water. Good experimental verification of predicted intensities was achieved.