How are harmonic generated?
As a low pressure sound wave passes thru soft tissue a sinusoidal wave is generated ina relatively pure wave. As the pressure is increased, differences in a progagation speed in various parts of the sound wave are introduced. In areas of high pressure, the sound speed increases, while in low pressure regions it decreases. This changes the shape of the once-sinusoidal wave, and also gives rise to the harmonic frequencies.