A method and apparatus for determining the percent nodularity of a workpiece as a function of the speed of ultrasonic sound through the workpiece. According to a first embodiment this is achieved by positioning the workpiece at a predetermined distance from an electroacoustic transducer and in a second embodiment by positioning the casting between two electroacoustic transducers which are spaced at a predetermined distance. DC voltage signals relating the time between the generation of the ultrasonic impulses and the detection of back reflection impulses by the electroacoustic transducers are combined by a signal-generating circuit in accordance with an empirical relationship so as to generate a DC voltage output signal proportional to the percent nodularity of the workpiece.