Influence of non-ideal diffuse sound field excitations on the control performance of active panel structures

The sound transmission loss of lightweight structures can be increased by the application of facing formworks. In the aircraft industry this task is accomplished by means of sidewall panels (linings) mounted on the primary fuselage structure of an aircraft. At low frequencies (< 1000Hz), however, the sound transmission loss is dominated by the so-called mass-law which prescribes an inverse relationship between mass and transmitted sound power. This behavior is worsened by the fact that around the so-called mass-air-massresonance-frequency the transmission loss of a double panel structure (e.g., an aircraft fuselage structure with linings) falls below that of a single panel (of equal mass). Furthermore, the effectiveness of passive damping methods is limited due to constraints on mass and volume. On the other hand, the interior acoustics in the aircraft cabin is an important issue regarding passenger comfort, which raises the demand for alternative solutions. Active structural acoustic control (ASAC) provides a lightweight-compliant solution to the problem of low-frequency sound transmission through single or double panel structures. Prior to wind-tunnel or flight tests, the acoustic performance of active lightweight structures is usually tested in sound transmission loss facilities. A reverberation room, equipped with one or a number of independent sound sources, is used to generate a diffuse sound field excitation. Although it is well-known that the statistical properties of such a non-ideal diffuse sound field will deviate from the ideal case, potential implications on the performance of active feedforward control systems have not been discussed so far. This is why this work evaluates the spatial coherence of ideal and non-ideal diffuse sound fields and considers the implications on feedforward control performance. The system under consideration is an aircraft-typical double panel system, equipped with an active sidewall panel, which is realized in a transmission loss facility. Experimental results for different numbers of sound sources in the reverberation room are compared to simulation results of a generic double panel system excited by an ideal diffuse sound field. It is shown that the number of statistically independent noise sources acting on the primary structure depends not only on the type of diffuse sound field but also on the sample lengths of the processed signals. The experimental results show that the number of reference sensors required for a defined control performance has an inverse relationship to the control filter length. prim. structure (P) sec. structure (S) mech. link mech. link