Active control of higher-order acoustic modes in ducts

Abstract

The active control of higher-order acoustic modes within a semi-infinite hard-walled rectangular duct is examined analytically for harmonic constant volume velocity sound sources. The effect of source location, size, strength, and relative phase upon the total mean acoustic power flow is investigated for both single and dual control source configurations, at a number of excitation frequencies. For the analysis, the primary source is mounted in the plane of the duct cross section, which may be a uniform surface of arbitrary finite impedance. It was found that the reduction in total acoustic power is dependent upon the relative location of the primary and control sources, both laterally across the wall of the duct and axially along the duct. The reduction in total acoustic power was found to be a maximum at axial source separation distances corresponding to multiples of half-wavelengths of the propagating modes, and at relative lateral locations such that the primary and control source mode shape function values were equivalent.