When size limitations of loudspeakers prevent the reproduction of low-frequency sounds, nonlinear devices can be used to create the illusion of the missing bass. Such virtual bass systems generate harmonics of the missing fundamental. However, they also can generate unwanted intermodulation distortion, which appears to be dependent on the particular audio sample and the selected nonlinearity. A detailed analysis showed that the ideal nonlinearity should not be even-symmetric, and its second derivative should be less than zero on the input interval 0 to 1.
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