An acoustic fluid machine such as an air compressor comprises an acoustic resonator, a valve device and a piston. Air is sucked into the resonator through the valve device at one end of the resonator. The piston at the other end of the resonator is reciprocated by an actuator to compress the air in the resonator to cause resonance to increase pressure of the air significantly. The inner surface of the resonator is suitably curved to comply with the formula of a half-period cosine function.