Discrimination of coupled structural/acoustic duct modes by active control: principles and experimental results

The coupling of acoustic waves in an elastic-walled duct with structural waves in the wall(s), at low frequencies, gives rise to dispersive coupled structural/acoustic modes. Propagating coupled modes consist of a “fast wave”, which propagates at all frequencies, together with “slow waves”, which propagate above their “cut-on” frequencies. The fast wave is well known and has been experimentally observed many years ago. The lowest order slow wave has also been—indirectly—observed previously, though not independently of the fast wave. In this investigation, an identification technique involving a genetic algorithm and active control has permitted the separate visualization of the lowest order slow wave and the fast wave, in the case where these two modes coexist.