Experimental estimation of the transmission loss contributions of a sound package placed in a double wall structure

Abstract

The objective of this paper is to propose a practical impedance tube method to optimize the sound transmission loss of double wall structure by concentrating on the sound package placed inside the structure. In a previous work, the authors derived an expression that breakdown the transmission loss of a double wall structure containing a sound absorbing blanket separated from the panels by air layers in terms of three main contributions; (i) sound transmission loss of the panels, (ii) sound transmission loss of the blanket and (iii) sound absorption due to multiple reflections inside the cavity. The sound transmission loss contributions of the blanket can thus be estimated from three acoustic measurements using impedance tube techniques: two reflection coefficients at the front face and the rear face of the blanket placed in specific positions characteristic of its position inside the double wall structure and its sound transmission coefficient. The method is first validated in the case of a double wall structure filled with a 2 inch foam material. Next, it is applied to investigate (i) the effect of frame compression of a 2 inch fibre glass in an aeronautic-type double wall structure and (ii) the effect of double porosity with or without porous inclusions in a building-type double wall structure.

Tailoring broadband acoustic energy suppression characteristics of double porosity metamaterials with compression constraints and mass inclusions

“…This outcome agrees with the influence of compression on the TL of glass fiber based materials that were considered on practical grounds. 11 In the experimental results of Figs. 6(c) and 6(d), the narrowband reduction of TL for the MM-A and MM-S is also increased in frequency due to compression, in agreement with the FE model results.…”

“…Interestingly, introducing a second manifestation of porosity into a porous material may improve absorption coefficient under certain circumstances by the removal of mass, particularly when the pores are parallel to the wave propagation direction. [11][12][13][14] Detailed studies of double porosity poroelastic layers have revealed the roles of multi-layering and variation of the porosity in order to modify sound attenuation properties, such as by leveraging periodic void patterns. 15 Indeed, similar periodic void patterns are shown to be critical toward the collapse of elastic metamaterials under compressive loads or constraints, [16][17][18] even potentially yielding large elastic 19 and vibroacoustic 20 energy dissipation characteristics.…”

“…15 Indeed, similar periodic void patterns are shown to be critical toward the collapse of elastic metamaterials under compressive loads or constraints, [16][17][18] even potentially yielding large elastic 19 and vibroacoustic 20 energy dissipation characteristics. Geometric constraints and compression are regularly applied to poroelastic materials in practice, 11 which suggests that double porositybased metamaterials may exhibit compression-dependent acoustic properties, possibly resulting in a means to tailor the significance of acoustic energy dissipation. Moreover, the numerous observations that mass inclusions in bulk poroelastic materials can manipulate the acoustic wave attenuation characteristics indicate that the incorporation of mass inclusions in double porosity materials may be another avenue for controlling wave energy transfer.…”

An acoustic trade-off chart for the design of multilayer acoustic packages

“…Absorption coefficient or transmission loss are usually plotted against the frequency, for different configurations of materials. This allows direct visual comparison between configurations ( [6], [10] or [11] for instance).…”

“…The transfer matrix method (TMM) [5] was developed to predict both absorption and transmission loss of multilayer acoustic panels. Since it is easily translated into a code and automatable, the method is used to investigate new acoustic materials and their acoustic performance in a wide range of studies and applications ( [6], [7] for instance). Statistical energy analysis [8] can be used to predict acoustic performance by an energy approach.…”