Active systems for sound attenuation in ducts

Passive attenuation of sound in ducts is related to wavelength, and therefore requires large silencers at low frequencies. Active methods using auxiliary sound sources placed in the duct wall provide an economical solution to the problem. To achieve high attenuation, adaptive electronic control has to be applied. The microphone in the upper portion of the duct provides the time function of the controlled signal. The `zero´ microphone has to be strategically located to eliminate the adverse effects of evanescent waves, radiated by the secondary sources. Reflections from the duct termination or the duct discontinuities can cause instability of the system. While suitable attenuations under the first mode cutoff frequency have been obtained, the cross-modes in the duct require different microphone configurations than the low-frequency plane wave field