A sound source including a first gas filled underwater resonator, a second gas filled underwater resonator connected to the first resonator, where the second resonator comprises a Helmholtz resonator, and at least one excitation member configured to excite the first resonator and the second resonator is disclosed. The first resonator is permanently tuned to produce a first resonant frequency upon excitation by the at least one excitation member and the second resonator is permanently tuned to produce a second resonant frequency upon excitation by the at least one excitation member. The first resonant frequency is different from the second resonant frequency.