Sound absorption and transmission through microperforated-panel structures

 

Teresa Bravo María

 

The prediction of the isolating properties of absorbing materials is a subject that has been intensively studied due to their important applications in a wide range of areas, such as building acoustics and the aeronautic, astronautic and automotive industries. Micro-Perforated Panels (MPP) constitute a new type of fibreless absorbers tagged as “next-generation” absorbing materials due to their huge potential in comparison with conventional porous materials. Indeed, as they are normally made of steel or plastic, they constitute ideal options in environments where special hygienic conditions are needed, such as hospitals or food industries. Typical MPP Absorbers (MPPA) are composed of a panel with sub-millimetric holes backed by a cavity. The physical parameters defining the acoustic properties of the screen are the thickness, the size of the perforation and the perforation ratio (or porosity). After the configuration parameters have been selected, the backing cavity depth has to be chosen to build the Helmholtz-type resonance. To avoid limitations concerning the size of the whole device, multi-layered MPPAs, combining several parallel MPPs and cavities can be used.

In all the previous analyses whether for simple or multi-layer devices, MPPs have been considered as rigid structures, accounting only for inertia and neglecting any vibrating effects. However, measurements show that the absorption performance of thin MPPAs generates extra absorption peaks or dips that cannot be understood assuming a rigid MPPA. We have established a theoretical model that exactly accounts for structural-acoustic interaction between the micro-perforated panel and the backing cavity without restriction on the absorber cross-sectional shape or on the panel boundary conditions. This model has been verified experimentally against impedance tube measurements and laser vibrometric scans of the cavity-backed panel response. It has been shown analytically and experimentally that the air-frame relative velocity is a key factor that alters the input acoustic impedance of thin MPPAs. Coupled mode analysis reveals that the two first resonances of an elastic MPPA are either panel-cavity, hole-cavity or panel-controlled resonances, depending on whether the effective air mass of the perforations is greater or lower than the first panel modal mass. A critical value of the perforation ratio is found through which the MPPA resonances experience a frequency "jump" and that determines two absorption mechanisms operating out of the transitional region.

Measured vibrating response of the MPPA disk in relation with local maxima of the sound absorption coefficient. Comparison with the velocity predicted at the resonance frequencies of the panel-cavity volume displacing modes

Another limitation regarding detailed analysis of MPPs is related to the fact that the majority of the studies are only concerned about the MPP absorption properties, as they consider that the microperforated structure is backed by a rigid panel that does not allow any transmission of the incident sound wave. This situation is not representative of real operating conditions, where the panels are vibrating surfaces, but experimental evidence of the influence of the vibrating response of thin components has been found. The aim of this work has been to complete those previous studies by including an elastic transmitting back wall and analyzing both absorption and transmission properties of a fully coupled finite MPP system. A fully-coupled modal approach has been proposed to calculate the absorption coefficient and the transmission loss of MPP structures with general boundary conditions. It has been validated against infinite partition models and experimental data. A practical methodology has been introduced to better characterize the absorption properties of conventional MPP structures, based on the measurement of the injected power, and which is of particular interest for MPPs thinner than twice the holes diameter. Coupled mode analysis is also performed and analytical approximations are derived of the resonance frequencies and mode shapes of a flexible MPP structure. It is found that the Helmholtz-type resonance frequency is deduced from the one associated to the rigid MPP absorber shifted up by the mass-air mass resonance of the non-perforated double-panel.

Experimental set-up for the determination of the absorption and transmission properties of the insulating MPP partition

The proposed fully-coupled modal formulation has been applied for evaluating the absorption and transmission performances of multi-layer microperforated structures whose facings are excited by different noise sources, optimized both in absorption and transmission. In particular three layouts obtained from a typical aircraft partition by micro-perforating the trim panel (MPP–Porous–Panel), removing the fiberglass material (MPP–Cavity–Panel) and adding a second MPP inside the separating cavity (MPP–MPP–Panel) have been optimized and validated against full-scale measurements performed with a pressure-velocity probe and a laser vibrometer to estimate the absorption and transmission coefficients.