A new simple scheme of an acoustical laser with mechanical pumping is suggested. In accordance with this scheme the pumping is created by a piezoelectric emitter of the piston type. The liquid with dispersed particles (gas bubbles) plays the role of an active medium. Phase bunching of the particles is realized by acoustical radiation forces. Generation conditions for this acoustical laser are evaluated. It is shown that two types of losses must be overcome for the beginning of the generation. The first type results from the energy dissipation in the active medium and the second one is caused by radiation losses at the boundaries of the resonator.