HANDBOOK OF ACOUSTICS - online book

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COMBINATION TONES.                          135
Summary. A Combination or Resultant Tone is a third sound, which may be heard, when two tones of different pitch are simultaneously sounded, and which is not heard, when either of these two tones is sounded alone.
The two tones which give rise to a Combination Tone are termed its generators. There are two kinds of Combination Toneó
(1). The Differential Tone: the vibration number of which is the difference of the vibration numbers of its generators; (2) the Summation Tone: the vibration number of which is the sum of these vibration numbers. Differential Tones may be of various orders. A Differential of the 1st order is that which is produced by two independent tones or generators.
A Differential of the 2nd order is that which is produced by the Differential of the 1st order, and either of the generators.
A Differential of the 3rd order is that which is produced by the Differential of the 2nd order, and either of the foregoing tones; i.e., either the Differential of the 1st order, or one of the generators.
A Differential of the \th order is that which is produced by the Differential of the 3rd order and either of the foregoing tones ; and so on. Differential Tones are not the result of the coalescence of beats.