The study of amplitude-dependent acoustical phenomena. The amplitude dependence is due to the nonlinear response of the medium in which the sound propagates, and not to the nonlinear behavior of the sound source. According to the linear theory of acoustics, increasing the level of a source by 10 dB results in precisely the same sound field as before, just 10 dB more intense. Linear theory also predicts that only frequency components radiated directly by the source can be present in the sound field. These principles do not hold in nonlinear acoustics.