Aperture apodization to reduce the off-axis intensity of the pulsed-mode directivity function of linear arrays

There is considerable interest in aperture apodization for enhancing image quality. With continuous wave excitation, the influence of apodization can quite easily be predicted: the directivity function in the far-field corresponds to the Fourier transform of the aperture function. We, however, analyse a practical situation — short acoustic pulse emission focused in the near-field. Nine types of aperture function (for example, Tukey, Parzen, Hamming) are compared.It appears that with the pulsed mode a considerable reduction in off-axis intensity can be obtained by judicious choice of the aperture function. Side-lobes can be completely eliminated.As a tool, an approximate relationship is established between the Fourier transform of the aperture function and the main-lobe width of the directivity function. Further, an optimum balance is sought between the main-lobe width and the width at the ?25 dB level of the directivity function.