Influence of the resonator damping on its coupling with a slot-tone

In ventilation systems, disturbing whistlings can occur at outlets. This whistling
phenomenon belongs to the self-sustained tones class which can be produced when a low
velocity flow impinges on a downstream obstacle. The coupling of the tones with acoustic
resonances (so-called indirect feedback path) can promote high-level harmonic noises. This
experimental study deals with the coupling of a jet-slot oscillator, modelling a ventilation
outlet, with the flow supply duct resonances. The flow supply duct is damped by using
various amounts of acoustic damping foam, and the effect of this damping on the sound
production is studied.
It is shown that for a jet exit/obstacle distance higher than 2.5 jet height (L/H>2.5), the
tone’s frequency matches satisfactorily with frequencies predicted by a model describing
favourable phase relationships for the occurrence of tones along the feedback loop. When the
flow supply duct is resonant, it is noticeable that the tone’s frequency occurs at a duct
resonance frequency which is close to the frequency described by this model. For lower
values of L/H (slot closer to the jet exit), a better knowledge of the convection velocity
behaviour along the vortices path could improve the tone’s frequency prediction. The use of
damping foam within the flow supply duct prevents the occurrence of the tones’ coupling
with a duct resonance, especially with high amounts of damping material, and dramatically
reduces the emitted acoustic levels. For moderate amounts of damping material, the
suppression of coupling occurs only for higher flow speeds, as the tone’s frequencies are
higher and the damping material is then more efficient.