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Low frequency noise breakout through duct walls of the ventilation system is a serious problem. The addition of sound absorbing materials such as fiberglass inside the duct wall to improve sound absorption performance at high frequency is a well-known technique. The experiment presented in this paper is to investigate and analyze the duct noise reduction at low frequency. The range of effective sound absorption frequency band and maximum sound absorption are also predicted in terms of the thickness of face sheet absorption material inside the cavity and the lateral dimensions. The feasibility of simultaneous optimization of acoustic properties of panel absorber for such applications is discussed. It is found that the new design of duct with panel absorber is more effective than conventional fiberglass blanket.

This paper considers acoustic treatment of honeycomb sandwich panels in close-fitting enclosures. Honeycomb sandwich panels are efficient radiators of sound because they are generally optimized for high stiffness and low weight. Results confirm that adding absorption at the mass-spring-mass resonance of a honeycomb panel with a thin plate has practical benefit. The absorption minimizes the effect of the 'mass-air-mass' resonance which usually reduces the low-frequency transmission loss, and offers increased performance at higher frequencies. The potential of composite honeycomb sandwich panels for reducing noise below 500 Hz is also demonstrated.

Traditional single-layer panels are typically made of a thin metal or plastic and are often unsuitable for an interior finish because thin limp panels do not have enough resistance to combined acoustic-thermal loading. For medium and low frequencies, panels usually require a relatively large absorber thickness and weight. The sound absorber presented is a composite sandwich panel with an air gap between a comparatively stiff thick panel and a flexible thin plate. Under thermal loading, the flexible thin plate is buckled. When suitably adjusted to a particular curvature, an increase in stiffness is obtained which can increase the sound insulation by up to 27 dB between 250 Hz and 500 Hz.