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Although the term "intermodulation" has been used for a long time to describe a form of distortion, only recently has simplified testing equipment been made available which permits the commercial testing of audio, radio, and general sound-recording equipment by this method. In this paper there is described a compact piece of apparatus which is being used for routine maintenance tests on the sound recording and reproducing systems. A comparison is made between the harmonic and intermodulation methods of making such tests to show the advantage of the latter method. Circuits and descriptions of the apparatus for generating and analyzing the intermodulation test tone needed in making the tests are given. Tests on radio transmission systems, sound disk and film recording apparatus, iron-core coupling devices, and general sound systems are described which show how the intermodulation method of measuring distortion is in many cases a more adequate indication of equipment performance than other methods commonly used, as well as being less laborious.