Analytical analyses of active control of sound transmission through soft-core sandwich structures and double panel partitions

Abstract

Active structural acoustic control (ASAC) is a form of active noise control which focuses on the control of structural vibrations in a manner that minimizes acoustic radiation from a structure. The greatest difficulty ASAC suffers from is in finding an "optimal" error quantity, which can be easily implemented in a control algorithm. Volume velocity control (VVC) metric which is generally used in ASAC typically requires either a large number of sensors distributed across the entire structure, or a single distributed shaped sensor. A new parameter termed "weighted sum of spatial gradients (WSSG)" showed a great potential to significantly reduce noise transmission using four sensors only. This thesis carried out a comprehensive study on VVC and WSSG, and numerical calculations indicate that, by intelligent selection of modal indices for the calculation of WSSG and the frequency band of active control, WSSG is able to achieve comparable amount of sound attenuation with VVC.

Soft-core sandwich panels are increasingly used because of their high strength-to-weight ratios. However, acoustical properties of these are less desirable at low frequencies, which can lead to high noise levels. Since additional sound absorbing materials fail to provide satisfactory results at low frequencies, active control techniques have been researched. It is very well known that soft-core sandwich panels vibrate in flexural modes (occur at low frequencies) and dilatational modes (occur in high frequencies). Therefore, in this thesis, VVC and WSSG control metrics are used to control these modes in order to achieve control in a broader frequency band. Numerical studies illustrate that the control metrics are able to attenuate both modes and increase the sound transmission loss irrespective of isotropic core loss factors.

In buildings, windows are often the weak link in protecting the interior from outside noise. In particular, double panel windows have a poor sound insulation at low frequencies around the mass-air-mass resonance. Sound absorbing materials in the air gap fail to provide satisfactory results in the low-frequency region, therefore, an active controller is an attractive approach to reduce the noise level in buildings. This thesis used the WSSG control metric to drive piezoelectric actuators in order to attenuate the dipole-type of motion of double panel systems, a motion that commonly occurs at high frequencies. Thereby, sound attenuation in a large frequency band can be accomplished. Also, acoustic point source arrangements are considered between the panels. Numerical results indicate that the actuator should be placed on the incident panel to minimize the control quantities on the radiating panel, and combined WSSG and cavity control is able to provide better sound attenuation in a wider frequency band.