Nonlinear control of boundary impedance in an acoustic waveguide

Wavetubes 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 estimation accuracy. Passive solutions, which use composite materials to change the boundary impedance, enable one to realize a finite combination of boundary impedances. In this paper, the tube boundary impedance is tuned at will by using two loudspeakers. 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 (TWR) control. It has been noted that the TWR is smooth and parabolic across closed regions in the parameter space, thus assuring the convergence of the nonlinear control. Several methods to estimate the TWR gradient are considered and compared based on an analytical model of a rigid impedance tube. 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. (c) 2021 Acoustical Society of America.

Automated summary

This paper presents a method for measuring acoustic properties in fluids using wavetubes, and proposes a new approach to manipulate the boundary impedance of the tube at will using loudspeakers. By estimating a parametric reduced-order model in real-time and implementing nonlinear control algorithms, modal traveling wave ratio control can be achieved, ensuring smooth and convergent control of the system.