In 1942 two R.C.A. engineers, G.L. Beers ahd H. Belar, drew attention to the presence of a form of distortion in loudspeaker reproduction not previously identified. It is the result of modulation of the frequency of one signal by the frequency of a second signal simultaneously applied. Because of the difficulties in obtaining reliable quantitative data on the extent of this distortion, its existence has been challenged by many writers and though its presence has been conclusively proved by Klipsch, More, Braun and others, its importance may still be in doubt. It is the purpose of the present contribution to describe some simple techniques for the segregation and measurement of this form of distortion and to provide data on its magnitude in some typical commercial speaker systems.
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