Acoustic propagation in plate‐shaped solid delay lines is analyzed using diffraction theory. The analysis yields the dependence of group velocity (time delay), phase velocity, pulse shape, pulse rise time, output amplitude ripple spacing, and ripple amplitude on the parameters of frequency, delay line length, free‐medium sound velocity, and plate thickness. Data are presented corroborating the theory quantitatively for time delay dependence on frequency and plate thickness, ripple spacing dependence on frequency and mode, and pulse rise‐time dependence on frequency. The diffraction theory is compared to the wave equation plane‐wave solutions for infinite plates. The results of the diffraction analysis are more appropriate for the description of sound propagation in plate‐shaped delay lines.