Fig. 1.

(A) An acoustic pulse is emitted from a point source (red circle). The solid blue lines represent wavefronts. The red dashed lines indicate the acoustic propagation paths, each with a corresponding time of flight, t_i. The black outlined rectangles schematically represent the array elements. The recorded waveforms for each element are shown as a single-cycle pulse arriving at different times. (B) When $\overrightarrow{r}$ corresponds to the source location and is used to compute the time delays to shift the waveforms, the waveforms will sum constructively. (C) When $\overrightarrow{r}$ is a location away from the source, the time-delayed waveforms do not add constructively and the summed waveform has a lower amplitude and less energy.