  Synthetic Jet (SJ) actuators are an intrinsically complex combination of
electronics, electric and mechanical systems. When studied theoretically, these
elements are often simplified to coupled damped harmonic oscillators (DHO) that
induce a pressure field within the cavity and drive momentum exchange. Thus,
the performance of an SJ actuator results from coupling these DHOs, naturally
leading to a few resonant modes. There is good evidence in the specialized
literature of two resonant modes developing in SJ actuators: the
membrane/piezoelectric mode and the Helmholtz resonance. In this work, we
report on the effect of a third resonant mode that develops at very low
frequencies due to a cavity much larger than the volume displaced by the
actuator. We present evidence that the large-cavity dynamics determine the SJ
performance in combination with the well-described formation criteria. We
compare the intensity of this resonant mode with the first room modes using
standard frequency analysis. Unlike typical room modes, the distribution of
this resonant mode is very biased to lower frequencies. We also show that the
resonant mode may be dimmed and focused by adding an obstacle in different
cavity positions for the lower sound intensities. This mode overcomes the
Helmholtz resonance, dominating the dynamics for higher sound intensities. We
show that jet and vortex velocities mimic the sound pressure curve for the
low-frequency range. Its effect mitigates for the higher range due to a delve
through smaller stroke lengths, characterized as a fixed relation between the
Reynolds and the Stokes numbers. We consider that the large-cavity dynamics is
an additional element that, if integrated as design criteria, could extend the
optimum frequency response of SJs.
