A method, comprising: providing a fundamental frequency corresponding to the RPM of an engine of a vehicle; determining a plurality of harmonics of the fundamental frequency; determining a plurality of first gain values, the plurality of first gain values comprising, for each harmonic of the plurality of harmonics, a first gain value as a function of RPM; determining a plurality of first phase values, the plurality of first phase values comprising, for each harmonic of the plurality of harmonics, a first phase value as a function of RPM; combining the plurality of harmonics, the plurality of first gain values, and the plurality of first phase values to produce a first set of harmonic enhancement signals; determining a plurality of second gain values, the plurality of second gain values comprising, for each harmonic of the plurality of harmonics, a second gain value as a function of RPM; determining a plurality of second phase values, the plurality of second phase values comprising, for each harmonic of the plurality of harmonics, a second phase value as a function of RPM; combining the plurality of harmonics, the plurality of second gain values, and the plurality of second phase values to produce a second set of harmonic enhancement signals; characterized by combining the harmonic enhancement signals of the first set to provide a first channel harmonic enhancement signal; and combining the harmonic enhancement signals of the second set to provide a second channel harmonic enhancement signal, wherein a per harmonic phase difference between the plurality of first phase values and the plurality of second phase values varies as a function of the engine RPM. providing a fundamental frequency corresponding to the RPM of an engine of a vehicle; determining a plurality of harmonics of the fundamental frequency; determining a plurality of first gain values, the plurality of first gain values comprising, for each harmonic of the plurality of harmonics, a first gain value as a function of RPM; determining a plurality of first phase values, the plurality of first phase values comprising, for each harmonic of the plurality of harmonics, a first phase value as a function of RPM; combining the plurality of harmonics, the plurality of first gain values, and the plurality of first phase values to produce a first set of harmonic enhancement signals; determining a plurality of second gain values, the plurality of second gain values comprising, for each harmonic of the plurality of harmonics, a second gain value as a function of RPM; determining a plurality of second phase values, the plurality of second phase values comprising, for each harmonic of the plurality of harmonics, a second phase value as a function of RPM; combining the plurality of harmonics, the plurality of second gain values, and the plurality of second phase values to produce a second set of harmonic enhancement signals; characterized by combining the harmonic enhancement signals of the first set to provide a first channel harmonic enhancement signal; and combining the harmonic enhancement signals of the second set to provide a second channel harmonic enhancement signal, wherein a per harmonic phase difference between the plurality of first phase values and the plurality of second phase values varies as a function of the engine RPM. <EOS>