Index of content:
Volume 128, Issue 4, October 2010
- ARCHITECTURAL ACOUSTICS 
128(2010); http://dx.doi.org/10.1121/1.3474235View Description Hide Description
On concert hall stages the sound traveling between players consists of the direct sound, a floor reflection and early reflections off players and objects on stage such as instruments and music stands. In smaller music ensembles, the acoustic communication between players is normally good. In larger ensembles, there is a similar situation for short distances between players. However for ensembles like a symphony orchestra, the number of players on stage results in large distances between some players with many other players sitting in between, which block the direct sound and floor reflection paths. This study investigates the sound levels on stage with and without a large orchestra present, in the absence of any stage enclosure. Sound levels within the octave bands 63–2000 Hz on an empty stage were studied analytically, while sound levels over the same frequency range with players present were investigated in a 1:25 scalemodel, both without and with risers on stage. The main results are presented in terms of the attenuation introduced by the orchestra, with linear models developed to describe behavior for the octave bands 500–2000 Hz.
Microstructure based model for sound absorption predictions of perforated closed-cell metallic foams128(2010); http://dx.doi.org/10.1121/1.3473696View Description Hide Description
Closed-cell metallic foams are known for their rigidity, lightness, thermal conductivity as well as their low production cost compared to open-cell metallic foams. However, they are also poor sound absorbers. Similarly to a rigid solid, a method to enhance their sound absorption is to perforate them. This method has shown good preliminary results but has not yet been analyzed from a microstructure point of view. The objective of this work is to better understand how perforations interact with closed-cell foam microstructure and how it modifies the sound absorption of the foam. A simple two-dimensional microstructural model of the perforated closed-cell metallic foam is presented and numerically solved. A rough three-dimensional conversion of the two-dimensional results is proposed. The results obtained with the calculation method show that the perforated closed-cell foam behaves similarly to a perforated solid; however, its sound absorption is modulated by the foam microstructure, and most particularly by the diameters of both perforation and pore. A comparison with measurements demonstrates that the proposed calculation method yields realistic trends. Some design guides are also proposed.
Solid-perforated panel layout optimization by topology optimization based on unified transfer matrix128(2010); http://dx.doi.org/10.1121/1.3483721View Description Hide Description
This paper presents a numerical method for the optimization of the sequencing of solid panels, perforated panels and air gaps and their respective thickness for maximizing sound transmission loss and/or absorption. For the optimization, a method based on the topologyoptimization formulation is proposed. It is difficult to employ only the commonly-used materialinterpolation technique because the involved layers exhibit fundamentally different acoustic behavior. Thus, an optimization method formulation using a so-called unified transfer matrix is newly proposed. The key idea is to form elements of the transfer matrix such that interpolated elements by the layer design variables can be those of air, perforated and solid panel layers. The problem related to the interpolation is addressed and bench mark-type problems such as sound transmission or absorption maximization problems are solved to check the efficiency of the developed method.
Analytical prediction of break-out noise from a reactive rectangular plenum with four flexible walls128(2010); http://dx.doi.org/10.1121/1.3463801View Description Hide Description
This paper describes an analytical calculation of break-out noise from a rectangular plenum with four flexible walls by incorporating three-dimensional effects along with the acoustical and structural wave coupling phenomena. The breakout noise from rectangular plenums is important and the coupling between acoustic waves within the plenum and structural waves in the flexible plenum walls plays a critical role in prediction of the transverse transmission loss. The first step in breakout noise prediction is to calculate the inside plenum pressure field and the normal flexible plenum wall vibration by using an impedance-mobility approach, which results in a compact matrix formulation. In the impedance-mobility compact matrix (IMCM) approach, it is presumed that the coupled response can be described in terms of finite sets of the uncoupled acoustic subsystem and the structural subsystem. The flexible walls of the plenum are modeled as an unfolded plate to calculate natural frequencies and mode shapes of the uncoupled structural subsystem. The second step is to calculate the radiated sound power from the flexible walls using Kirchhoff–Helmholtz (KH) integral formulation. Analytical results are validated with finite element and boundary element (FEM-BEM) numerical models.