This paper discusses active noise control in ducts via side-branch resonators. A general analysis of the problem is given together with a detailed study of the case of an active straight pipe resonator. The analysis focuses on how to minimize the secondary source strength (volume velocity) necessary for active control. The main conclusion is that, by using a resonator with an end-mounted high impedance secondary source, it is possible to reduce the required source strength compared to the case with the secondary source directly mounted on the duct wall. This is important for applications on machines that generate very high sound levels, e.g., compressors and IC engines. The result from an experimental test of an ‘‘active quarter wavelength resonator’’ is also presented. This test verifies that a large reduction in the necessary secondary source strength is achieved at and around the resonators resonance frequencies.
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