Adaptive model-based control of boundary impedance in an acoustic impedance tube

Impedance tubes are employed for measurements of acoustic properties in various fluids. The ability to manipulate and control the frequency-dependent boundary impedance of the tube improves the accuracy of the estimation. Passive solutions, which use composite materials to change the boundary impedance, have been used in the past, and these enable one to realize a finite combination of boundary impedances. In this paper, the tube boundary impedance is being tuned at will by using two loudspeakers, one at each tube’s end. The suggested method operates in the presence of dispersion by estimating, in real-time, a parametric reduced-order model using a multichannel least mean square algorithm. The identified model is fed to a nonlinear, adaptive control algorithm to realize modal traveling wave ratio control. It has been noted that the traveling wave ratio is smooth and parabolic with respect to a second actuator’s complex amplitude across closed regions in the parameter space, thus assuring the convergence of the nonlinear control. An experimental case study utilizing an air-filled impedance tube with two loudspeakers is presented. The results demonstrate the ability to control the dynamics of the principal acoustic mode at will. Thus, enabling one to set the desired tube’s boundary impedance.