The ECK is described as an audible current flowing out from SUGMAD and back, perceptible as sound and as light. The sounds are catalogued by plane — a flute on one, the buzzing of bees on another, thunder, a woodwind. This is the family resemblance that gives the movement's history its difficulty: the surat shabd yoga of the Radhasoami and Sant Mat lineages teaches the same audible current under the same descriptions. Eckankar's position is that the correspondence reflects a common source older than either. Its critics' position is that the correspondence reflects Twitchell's reading list.