Index of content:
Volume 121, Issue 1, January 2007
- NOISE: ITS EFFECTS AND CONTROL 
121(2007); http://dx.doi.org/10.1121/1.2359233View Description Hide Description
Modeling a porous layer mounted on a vibrating structure using acoustic impedance is investigated in this paper. It is shown that the use of surface impedance usually measured with the impedance tube method can provide an inaccurate estimation of the acoustic pressure radiated by the covered structure. The paper focuses on the derivation of an impedance, denoted the “transfer impedance,” which describes accurately the dynamic movement of the porous layer. Biot’s theory is used in the model to account for deformations in the thickness of the layer. Experimental validation is performed using a circular piston covered by a foam or a fibrous layer, radiating in an infinite half space. The radiation model including the transfer impedance shows close agreement with experimental data.
121(2007); http://dx.doi.org/10.1121/1.2395916View Description Hide Description
Sound absorption and optical transparency are among the most useful properties of noise barriers. While the latter is required to reduce visual impact and for aesthetical reasons, the former is required whenever conditions of multiple reflections and presence of close, high receivers occur. The technical feasibility of a transparent, sound-absorbing panel for outdoor antinoise devices is investigated in this paper. An analysis of acoustical performance of multiple perforated plates is performed employing an existing theory for microperforated absorbers under normal incidence and diffused sound field. An optimization of the geometrical parameters is carried out on the basis of the European classification criteria of noise barriers for roadways. An optimized three-layer configuration can achieve sound-absorptionproperties similar to nontransparent products with only a limited loss of visual transparency and appropriate mechanical strength. Experimental data obtained with an impedance tube on small test samples made of transparent polycarbonate and in a reverberation room on full-scale prototypes are reported, showing a rather good agreement with the theoretical predictions. The optical performance of a multilayered configuration is evaluated also.
121(2007); http://dx.doi.org/10.1121/1.2384843View Description Hide Description
This paper summarizes theoretical and experimental work on the feedback control of sound radiation from honeycomb panels using piezoceramic actuators. It is motivated by the problem of sound transmission in aircraft, specifically the active control of trim panels. Trim panels are generally honeycomb structures designed to meet the design requirement of low weight and high stiffness. They are resiliently mounted to the fuselage for the passive reduction of noise transmission. Local coupling of the closely spaced sensor and actuator was observed experimentally and modeled using a single degree of freedom system. The effect of the local coupling was to roll off the response between the actuator and sensor at high frequencies, so that a feedback control system can have high gain margins. Unfortunately, only relatively poor global performance is then achieved because of localization of reduction around the actuator. This localization prompts the investigation of a multichannel active control system. Globalized reduction was predicted using a model of 12-channel direct velocity feedback control. The multichannel system, however, does not appear to yield a significant improvement in the performance because of decreased gain margin.