We report on Westerbork Synthesis Radio Telescope observations of the Crab pulsar at 350 \mathrm { MHz } from 2012 November 24 until 2015 June 21 .
During this period we consistently observe variations in the pulse profile of the Crab .
Both variations in the scattering width of the pulse profile as well as delayed copies , also known as echoes , are seen regularly .
These observations support the classification of two types of echoes : those that follow the truncated exponential shape expected for the thin-screen scattering approximation , and echoes that show a smoother , more Gaussian shape .
During a sequence of high-cadence observations in 2015 , we find that these non-exponential echoes evolve in time by approaching the main pulse and interpulse in phase , overlapping the main pulse and interpulse , and later receding .
We find a pulse scatter-broadening time scale , \tau , scaling with frequency as \nu ^ { \alpha } , with \alpha = -3.9 \pm 0.5 , which is consistent with expected values for thin-screen scattering models .