Noise reduction in a flow duct: Implementation of a hybrid passive/active solution

This paper deals with the design of a hybrid acoustic treatment combining porous material properties and active control techniques. Such an acoustic system was studied with a view to reducing broadband noise spectra in flow duct applications. Special attention was paid to the selection of the passive layer. The main objective was to achieve target impedance at the front absorber face, so as to attain maximum sound-attenuation over a wide frequency bandwidth. This investigation was carried out for a specific laboratory flow duct. Different porous layers were studied to reproduce optimum impedance at the hybrid liner surface. Results showed the difficulty of simultaneously achieving optimum resistance and reactance. Thus, a compromise was struck by applying a criterion of maximum attenuation. A wire mesh with a resistance close to a third of the characteristic impedance of air was selected as the optimal passive layer. Experiments were carried out in the flow duct under grazing acoustic incidence and with flow velocities up to 50 m/s. The experimental transmission loss was in agreement with predictions. Significant noise reduction levels were achieved throughout a large frequency range from 0.7 to 2.5 kHz, with a cut-off frequency between active and passive mode set at 1.8 kHz.