A sufficiently strong sound source generates in a thermoviscous fluid, due to nonlinearity, a frequency spectrum consisting of all multiples of the original frequencies and the sums and differences of these multiples. After a certain distance, a shock front is formed because of the energy transfer from lower to higher frequencies. In the case of two original frequencies as a source (the biharmonic case), the damping of high frequencies leaves us at a large distance from the source with primarily the difference frequency. The propagation of plane waves is described by the Burgers’ equation whose solution in the regions before and after the shock formation exhibits significantly different approximate analytical expressions. In this work, an analytical description of the total amplitude in the region after formation of shock in the case of a biharmonic sound source is found. This is a generalization of the well‐known Khokhlov solution for a monochromatic (single frequency) source. This description is turned into a Fourier series which can be specialized into the classical Fay solution for a monochromatic source. From this Fourier series the behavior of the individual frequencies is obtained, in particular the difference frequency which is also examined.