The authors present the results of studies on compressor-duct systems which are characterized by unstable operation over some portion of the performance curves. The principles of the Helmholtz resonator are explained, and the prediction of pulsations in a duct system may be developed by considering that the system acts as a simple Helmholtz resonator. For certain cases electrical analogy is used to explain acoustical conditions in the duct. Methods of preventing pulsations are described.