Periodic Variations of Pitch in Sound Reproduction by Phonographs

Causes of recurrent variations of pitch frequently encountered in the reproduction of phonograph records, commonly called "wow," are discussed. Considerations for the design of an instrument to measure the magnitude of these variations are given, minimum requirements and limitations of the design of such equipment are presented, and an instrument for use with 78-revolutions-per-minute turntables built to meet these requirements is described.