Fig. 4

Average focal radial positive (red) and negative (blue) pressure amplitude profiles for the three transducers, obtained by nonlinear transient acoustic holography. The positive (FWHM⁺) and negative (FWHM⁻) full-width half-maxima, and the distance between the first side-lobe maxima (DSLM) are shown. FWHM⁺, FWHM⁻, and DSLM decrease with increasing frequency due to reduced diffraction at higher frequencies. The maximum pressure, and the ratio between the maximum pressure and side-lobe maxima, is highest for 11.8 MHz, indicating a higher focusing gain.