ACTIVE NOISE-CONTROL IN DUCTS VIA SIDE-BRANCH RESONATORS

This paper discusses active noise control in ducts via side-branch res onators. A general analysis of the problem is given together with a de tailed study of the case of an active straight pipe resonator. The ana lysis 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 sour ce, 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 ex perimental 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 res onance frequencies.