The dynamic behaviour of an original double anode plasma torch which provides a long lifetime and highly stable supersonic low-pressure nitrogen plasma jet is analyzed by means of classical tools such as FFT, correlation function and Wigner distribution. At both arc chamber stages, the plasma is investigated by means of spectroscopic diagnostics. They reveal that the anode attachments are diffuse rather than local. Electron temperature and number density are comparable to those derived from a simple LTE model. Voltage, arc current and light intensity temporal series exhibit two main characteristic frequencies : the electric generators yield very stable and reproducible 150 Hz oscillations, whereas the firing of the second arc gives birth to a sharp 6.7 kHz peak which is ascribed to the generation of acoustic waves in the region of the first anode attachment. The value of this frequency that depends neither on the electrical injected power nor on the mass flow rate is interpreted by assuming that the arc chamber acts as a Helmholtz oscillator.