Application of acoustically tuned resonators for the improvement of sound insulation in aircraft

One of the aims of the EU project FACE (Friendly Aircraft Cabin Environment) is to reduce aircraft interior noise. For modern aircraft flying at cruise conditions, the turbulent boundary layer is the main source for cabin noise. Normally, the turbulent boundary layer causes the trim panels to vibrate, and hence to radiate sound into the aircraft cabin. The purpose of the present work is to reduce this kind of noise by means of sound insulating trim panels with tuned acoustic resonators. The length and the radius of these resonators are tuned in such a way that the volume velocities at the vibrating panel surface and at the entrance of the resonators are equal in magnitude but opposite in phase. In this way, maximum reduction of the radiated sound can be achieved for a specified frequency range. Because of the repetitive pattern of the resonators in the panel, the influence of the resonators on the sound radiated in normal direction by the panel is studied with a one-dimensional model. The so-called low reduced frequency model is extended to describe the viscothermal wave propagation in the vibrating resonators. An advantage of the viscothermal effects is that, in the low frequency range, more sound reduction is obtained than if these effects are not present or very small. Calculations show that a large reduction of the radiated sound can be achieved. The model is also validated by experiments in an impedance tube. Good agreement is found between theory and measurements.