A new, self‐modulated mode of operation of the marginal oscillator ultrasonic spectrometer, in which no external modulation is required, is described and analyzed. In this mode, the system consisting of an oscillator‐coupling network‐acoustic resonator is induced to oscillate simultaneously at two frequencies. Small changes in acoustic attenuation are reflected as changes in the amplitude or frequency of the detected beat frequency signal resulting from the interference of the two oscillations. Applications to magnetoacoustic phenomena are reported.