[Front cover]

The front cover highlights Acoustic propagation of a coded signal and a short pulse. Those images show the transmitted fields for a conventional short pulse excitation and for a linear FM excitation using the simulation program Field II, written by Prof. J. A. Jensen. The signals are applied to all elements of a 128-element array using a Hanning aperture apodization and a fixed transmit focus of 55 mm. The transducer has a 65% fractional bandwidth and a center frequency of 4 MHz. The pulse excitation (top) is a 3-cycle Hanning-apodized pulse at 4 MHz, and the coded signal (bottom) is a tapered linear FM signal 12 ??s long, designed to have the same bandwidth as the pulse excitation. See series of articles beginning on page 176.