Assessment of asymmetry in broad-band focussed ultrasonic fields

Abstract

Measurements are reported on the short-pulse acoustic field radiated by a weakly focussed transducer in water. An index of asymmetry based on the lateral distances at which the peak-to-peak pressure amplitude dropped to a given level below the maximum axial value was determined for four levels in two perpendicular planes. The index was very variable due to its sensitivity to small positional errors (≲ 0.05 mm), particularly for closely symmetrical beams. Spectral analysis of the signals received revealed an approximately linear decrement with frequency compared to the axial spectra. An asymmetry index based on these spectral difference gradients appears to be more consistent, to be less sensitive to small errors and to be relatively easy to automate. A need for detailed analysis of the spectral structure of the focal zones of pulse excited focussed transducers is identified.