  We have recently discovered stochastic resonance (SR) in chaotic inertia-less
viscoelastic channel flow. SR appears just above a pure elastic instability at
a critical Weissenberg number, $Wi_c=150$, of a transition regime. In this
lower sub-region up to $Wi\sim 300$, only the streamwise velocity, $u$,
exhibits a chaotic spectrum, $E_u$, while the spanwise velocity continues to
exhibit white noise, verified by its flat spectrum, $E_w$, accompanied by weak
intensity elastic waves. However, SR vanishes at the upper limit at $Wi\sim
300$, when $E_w$ becomes chaotic, indicating $Wi$ as the control parameter.
Here we clarify the mechanism of SR emergence by validating the control
parameters, namely $Wi$ and the rms velocity fluctuations, $u_{rms}$, measured
at multiple channel locations, which determine the range of SR existence. Our
experiments verify three key ingredients of the SR mechanism: chaotic $E_u$,
white noise $E_w$, and weak elastic waves, which are consistent with three
constituents of autonomous dynamical systems exhibiting SR.
