In this chapter the main acoustic equation (2.1) will be determined in the momentum representation. Homogeneous media and non-homogeneous media of particular types will be considered in a more detailed way than in previous chapters. Similarly to previous chapters we will discuss here the Lie (Ovsjannikov 1978, Ibragimov 1983, Poluyanov and Voronjn 1984, Meinhardt 1981) and operator (Miller 1981, Barut and Ronchka 1980, Osherov et al. 1983) symmetries. We will obtain concrete Lie algebras and Lie groups of symmetry. The relations between the symmetries in coordinate and momentum representations will also be considered.