  Traditional approaches to combination tones based on Helmholtz theory
encounter essential interpreting difficulties, which the most known example is
the anomalous behaviour of the combination tone 2f1-f2. Without doubt the
phenomenon of combination tones is connected with nonlinear phenomena in the
human ear. Therefore, it is generally assumed that the explanation of this
phenomenon is possible on the level of the ear. This paper proposes different
approach, which assumes the existence of an additional mechanism beyond the
ear. The aim of this mechanism is to diminish sensitivity of the auditory
process to nonlinear distortions by appropriate decoding of sound information.
The general rule of construction and the mathematical model of such a mechanism
is proposed, which allows for the explanation of many problems associated with
combination tones. As a consequence an unexpected result is obtained with
regard to a combination tone 2f1-f2 that it can originate already from the
nonlinear distortion of the second order. This mathematical model is formulated
in terms of finite groups.
