Random Incidence Transmission Loss Research Articles

Sound transmission loss through triple-walled cylindrical shells with porous layers.

Sound transmission loss through triple-walled sandwich cylindrical shells in the presence of an external mean flow is analytically examined. Love's theory and a simplified method based on Biot's theory are considered to describe the motions of thin isotropic shells and wave propagation in the porous material cores, respectively. The random incidence transmission loss in a diffuse field is calculated numerically by considering the limiting incidence angle due to the total internal reflection. The analytical model and numerical code are validated against both experimental and analytical results reported by previous studies. The transmission loss of triple-walled structure in the diffuse sound field is compared with its double-walled counterpart at the same weight. The results generally show a superior performance in the sound insulation for the triple-walled shell, considerably at mid-high and high frequencies, in comparison with its double-walled counterpart at the same weight. The effects of sandwich shell configuration and air gap depth are also investigated on the sound transmission loss.

Analytical modelling of acoustic transmission across double-wall sandwich shells: Effect of an air gap flow

This paper presents an analytical modelling on the effect of an internal air gap flow on acoustic transmission across double-wall sandwich shells lined with poroelastic materials. The shell motions are described by Love’s theory, and the poroelastic lining is modelled as an equivalent fluid by considering only the energetically most dominant wave. The vibroacoustic problem with the boundary conditions is formulated in a transfer matrix equation and solved simultaneously. The random incidence transmission loss in a diffuse field and the limiting angle of incidence due to total internal reflection are calculated numerically and analysed in detail. It is shown that the air gap flow enhances the transmission loss in the mid-high frequency range near the critical frequency but has no influence on the thin-shell characteristic frequencies. The gap flow effect is dependent on the flow velocity, flow direction, gap depth, air pressure and shell configuration. Despite the marginal effect in the presence of an external flow, the air gap flow has great potential for tuning the sound insulation performance of double-wall sandwich shells through the enlarged design space.

Effects of external and gap mean flows on sound transmission through a double-wall sandwich panel

This paper studies analytically the effects of an external mean flow and an internal gap mean flow on sound transmission through a double-wall sandwich panel lined with poroelastic materials. Biot׳s theory is employed to describe wave propagation in poroelastic materials, and the transfer matrix method with three types of boundary conditions is applied to solve the system simultaneously. The random incidence transmission loss in a diffuse field is calculated numerically, and the limiting angle of incidence due to total internal reflection is discussed in detail. The numerical predictions suggest that the sound insulation performance of such a double-wall panel is enhanced considerably by both external and gap mean flows particularly in the high-frequency range. Similar effects on transmission loss are observed for the two mean flows. It is shown that the effect of the gap mean flow depends on flow velocity, flow direction, gap depth and fluid properties and also that the fluid properties within the gap appear to influence the transmission loss more effectively than the gap flow. Despite the implementation difficulty in practice, an internal gap flow provides more design space for tuning the sound insulation performance of a double-wall sandwich panel and has great potential for active/passive noise control.

SOUND TRANSMISSION THROUGH MULTI-PANEL STRUCTURES LINED WITH ELASTIC POROUS MATERIALS

Theory and measurements related to sound transmission through double panels lined with elastic porous media are presented. The information has application to the design of noise control barriers and to the optimization of aircraft fuselage transmission loss, for example. The major difference between the work described here and earlier research in this field relates to the treatment of the porous material that is used to line the cavity between the two panels of the double panel structure. Here we have used the porous material theory proposed by Biot since it takes explicit account of all the wave types known to propagate in elastic porous materials. As a result, it is possible to use the theory presented here to calculate the transmission loss of lined double panels at arbitrary angles of incidence; results calculated over a range of incidence angles may then be combined to yield the random incidence transmission loss. In this paper, the equations governing wave propagation in an elastic porous material are first considered briefly and then the general forms for the stresses and displacements within the porous material are given. Those solutions are expressed in terms of a number of constants that can be determined by application of appropriate boundary conditions. The boundary conditions required to model double panels having linings that are either directly attached to the facing panels or separated?!from them by air gaps are presented and discussed. Measurements of the random incidence transmission loss of aluminium double-panel structures lined with polyurethane foam are presented and have been found to be in good agreement with theoretical predictions. Both the theoretical predictions and the measured results have shown that the method by which an elastic porous lining material is attached to the facing panels can have a profound influence on the transmission loss of the panel system. It has been found, for example, that treatments in which the lining material is not directly attached to the facing panels are generally to be preferred to those in which the lining is directly bonded to the panels. These effects may be explained by considering the degree to which the various wave types within the elastic porous material are excited, which in turn can be related to the method by which the lining is mounted to the facing panels.

The transmission loss of finite sized double panels in a random incidence sound field

The extension of the multiple-reflection theory to take account of absorption at the edges of the cavity between the septa of a double panel is described, and an expression is obtained for the random incidence transmission loss of a double panel with edge absorption, given values of the panel dimensions and edge absorption coefficient. A curve is given of the measured random incidence transmission loss of a thin double aluminium panel with glass fibre absorbent around the edge of the cavity and theoretical curves given for the same panel with different values of the edge absorption coefficient. From the curves it is inferred that using a value of the edge absorption coefficient of unity in the theory, gives the best agreement between theory and experiment. A curve of the measured transmission loss of a thin polythene panel with glass fibre absorbent around the cavity edges and a theoretical curve with the absorption coefficient equal to unity are given. Good agreement between experiment and theory is obtained without the somewhat unrealistic assumptions of our previous paper. The effect of truncation of multiple reflections between the septa of a double panel upon the “mass-spring-mass” resonance is considered, and the finite panel theory is compared to modal theory.

The transmission loss of double panels

Existing theories relating to the transmission loss of double panels are studied. It is shown that it is not possible to obtain agreement between theoretical and measured curves for random incidence transmission loss without the use of terms other than the mass of the panels arid their separation. These extra terms are often of doubtful origin. A new theory is discussed which, in its simplest form, agrees with the prediction of the other theories. With this new theory it is possible to introduce as an extra term the addition of sound-absorbing material in the cavity between two impervious septa. Using this extra factor, the absorption coefficient of the cavity walls, it is possible to obtain good agreement between theoretical predictions and observed values of transmission loss. When applied to cavity brick walls the theory predicts values of transmission loss far in excess of most of the measured values. This implies that the insulation of a brick wall is determined solely by flanking paths and vibration bridging and not by the inherent insulating properties of the wall as is the case with simple “mass law” panels.