Active Sound Transmission Control of an Experimental Double-panel Partition Using Decoupled Analog Feedback Control

This paper addresses the construction, measurement, and analysis of a double panel active partition (DPAP) and its accompanying analog feedback controllers. The DPAP was constructed by attaching an aluminum cone loudspeaker at each end of a short segment of a circular duct. Two analog feedback controllers were designed and built using the measured frequency response function of each panel. Two independent (decoupled) feedback controllers were then used to minimize the vibration amplitude of each panel in the presence of an acoustic disturbance. A normal-incidence transmission loss measurement system was used to assess the performance of the DPAP and of a single panel passive partition. Error signal attenuations show that it is both feasible and effective to simultaneously control both panels with decoupled feedback controllers, and that simultaneously controlling both panels of the DPAP has a distinct advantage over controlling a single panel. The reduction in vibration amplitude across the surface of the transmitting panel was confirmed with scanning laser vibrometer measurements. Transmission loss results were obtained for two passive and three active configurations. The average normal incidence transmission loss over the active measurement bandwidth (501,000 Hz) for the active double panel was 60 dB. This is an average of 39 dB more transmission loss than a passive single panel partition. INTRODUCTION There has long been interest in the use of partitions to reduce sound transmission into noise-sensitive environments. There is particular need for the improvement of partitions at frequencies where the passive transmission loss is inadequate. This is the case in single and double panel partitions where the transmission loss is severely degraded at low frequencies due to resonance effects. The primary passive method to reduce low frequency sound transmission is to add mass to the partition. However, this solution is not feasible for situations where extra weight cannot be tolerated, such as in aerospace vehicles, large ceiling structures, high rise buildings, etc. A solution to this problem is active structural control. Active control strategies have been utilized to improve low frequency sound transmission performance. Active structural acoustic control (ASAC) methods have been explored thoroughly [1-12]. This approach involves actuating a continuous transmitting panel in such a way as to minimize the acoustic radiation into the receiving space. ASAC is typically implemented by distributed sensor and actuator pairs that change the radiating mode shapes of the panel in order to reduce the radiated acoustic power. In general, receiving side attenuations with ASAC methods have been small. Additionally, comparing the performance of one ASAC implementation to another is difficult because the measurement techniques have been inconsistent. Finally, the major drawbacks to the ASAC approach are the large number of fully-coupled actuator/sensor pairs, the need for microphones in the receiving space, and the spatial control spillover that inevitably results when using a continuous transmitting panel. Effective active sound transmission control (ASTC) has been implemented by Leishman [13-16] wherein the partition is broken into an array of discrete modules that are acoustically and mechanically segmented. The active segmented partition (ASP) allows for localized control of each module, thus eliminating the impracticality of a large number of fullycoupled actuator/sensor pairs that exists for ASAC control. Furthermore, the ASTC approach integrates the error signal sensors inside the cavity of the double panel partition and does not require the placement of microphones in the receiving space. Using digital feed forward active noise control, Leishman achieved single frequency transmission loss results near 60 dB over a band of 30-290 Hz for an array of 4 modules [16]. The primary limitations of Leishman’s configuration were its unidirectional performance, need for an advanced reference signal, and lack of broadband control capabilities. A practical ASTC partition should be bidirectional, stopping sound transmission in both directions through the partition. It is proposed that direct, decoupled actuation of a two panel system might lend itself to bidirectional control. One concern with this actuation scheme is that a fully-coupled MIMO controller would be necessary to counteract the strong acoustic coupling associated with the cavity mode between the two panels. A practical ASTC partition should also be able to control both tonal and broadband noise. An analog feedback controller should more effectively attenuate broadband noise than a digital feed forward scheme. The purpose of this paper is to explore the performance of analog feedback controllers and panel control on an experimental DPAP module. Furthermore, it will be shown experimentally that effective control can be implemented with two decoupled controllers.