Apparatus for calculating the delay coefficients to be used for the transducer of a linear array at successive focal points along each radial line of a sector along which ultrasonic pulses are transmitted comprising a plurality of clocked accumulators connected in series, the accumulators being preloaded with appropriate combination of the coefficients of the successive terms of a series expressing an approximation of the formula, D=R-.sqroot.(X-Xo).sup.2 +Yo.sup.2 where R is the distance of a focal point from a given point in the sector, X is the number of the accumulator from the origin and Xo, Yo are the coordinates of the focal point.