Air Cavity Depth Research Articles

Impact of design variations of a micro-perforated panel on psychoacoustic metrics of transmitted sound

Recent work reveals that sound travelling through metamaterials can lead to changes in sound perception [L. De Ryck et al., Proceedings of ISMA and USD, pp. 1147–1162 (2018)]. As a starting point unravelling the impact of intricate vibro-acoustic design on perception, this study investigates the relationship between the intrinsic physical properties of micro-perforated panels (MPP) and several psychoacoustic metrics (loudness, sharpness, prominence ratio, and speech intelligibility index) in sound transmission conditions. A Sobol global sensitivity analysis is performed to quantify the impact of variations in MPP design parameters—panel thickness, perforation rate, hole diameter, and air cavity depth between panels—on these sound quality metrics [I. M. Sobol, MATCOM 55, 271–280 (2001)]. The analysis is conducted using both broadband and narrowband noise stimuli. The data trends suggest that the psychoacoustic effects of MPP acoustic treatments exhibit a non-trivial dependence on specific parameters of the panel setup. This also provides a framework towards innovative material design, where desired psychoacoustic metrics can help guide the manufacturing of metamaterials in order to meet specific sound quality targets. [The European Commission is gratefully acknowledged for its support of the Marie Sklodowska Curie program through the Horizon Europe DN METAVISION project (GA 101072415).]

Dosimetric impact of VMAT delivery angles for early glottic cancer treatment

This study investigates the dosimetric effects of different gantry rotation angles used in volumetric modulated arc therapy (VMAT) for early glottic carcinoma. VMAT treatment plans using full-arc, half-arc, and partial-arc gantry rotation angles were generated from 22 computed tomography datasets of early-stage (T1-2N0) glottic laryngeal cancer. Dosimetric parameters associated with the planning target volume (PTV) and organs at risk (OARs), specifically the carotid arteries and thyroid, were compared. To assess the robustness of the VMAT plans, dose variations were analyzed by introducing positional shifts of 1, 3, and 5 mm from the isocenter of each plan along the superior-inferior, left-right, and anterior-posterior axes. Furthermore, we examined the size of the PTV, the air cavity volume within the PTV, and the variability of the beam path length through the gantry angles to investigate their correlations with PTV dose variations in the presence of positioning errors. Compared to full-arc and half-arc plans, the dosimetric parameters of partial-arc plans were found to be higher in PTV (D2%, D5%, D50%, and Dmean) and lower in OARs, while their dose variations of OAR parameters were greater for positioning errors. In addition, a correlation was observed between PTV size and PTV dose variations. Air cavity volume and depth variability were also correlated with some PTV parameters, depending on the arc plan. The results presented in this study suggest that the partial-arc gantry angles can allow higher PTV doses while minimizing OAR doses in VMAT treatment planning for early glottic cancer. However, the small delivery angles may lead to greater dose variations in the OARs when positioning errors occur.

Acoustic Attenuation Performance Analysis and Optimisation of Expansion Chamber Coupled Micro-perforated Cylindrical Panel Using Response Surface Method

This paper describes boundary element method (BEM), experimental and optimization studies conducted to understand the potential of expansion tube coupled micro-perforated cylindrical panel (MPCP) to enhance the acoustic attenuation for in-duct noise control issues. Due to complex structure of the MPCP and for the correct prediction of acoustic attenuation, BEM is adopted on the basis of PLM Simcenter 3D software to compute the sound transmission loss (TL). As the MPCP is cylindrical in-shape with numbers of sub-milimeter holes, additive manufacturing based 3D printing was utilized for the model prototyping to reduce current design limitation and enabled fast fabrication. The TL measurement based two-load method is adopted for modal validation. Subsequently, a parametric studies of the MPCP concerning the perforation hole diameter, perforation ratio and depth of air space are carried out to investigate the acoustical performance. Optimization via response surface method (RSM) is used as it allows evaluating the effects of multiple parameters as required in this study. The model validation result shows that the error between the BEM and and measured values is relatively small and show a good agreement. The R-square value is 0.89. The finding from parametric study shows that a widen peak attenuation can be achieve by reducing the perforation hole diameter and one way to increase the transmission loss amplitude is by increasing the air cavity depth. Finally, the optimized MPCP model was adopted to the commercial vacuum cleaner for the verification. The sound pressure level (SPL) of the vacuum cleaner is significantly attenuated within the objective frequency of 1.7 kHz and its A-weighted SPL is reduced by 1.8 dB.

Multi-chamber micro-perforated panel absorbers optimised for high amplitude broadband absorption using a two-point impedance method

An optimised, multi-chamber, micro-perforated panel absorber (MC-MPPA) with micro-perforated adjoining panels is presented in this paper. An MPPA is a clean and efficient noise absorber with a simple structure making it an increasingly popular choice over porous material-based solutions. For the basic configuration, the bandwidth is typically limited by the perforation size and the frequency range by the backing cavity depth. In order to improve these parameters, innovative variations of the MPPA have been proposed in the literature, but the development of a solution which delivers both a broadband frequency response at low frequencies, with a shallow cavity has yet to appear. This paper proposes such a technological breakthrough by employing a graph-theory-based method to the MPPA problem. By using the two-point impedance method (TpIM) based on graph theory, a model for multi-chamber MPPAs which are coupled with each other in two dimensions through perforated side panels can be developed, an onerous task for standard equivalent circuit analysis due to the generation of non-planar circuits. The resulting model lends itself to optimisation and thus allows the differing geometric parameters of a tessellated network of chambers to be combined to maximise the sound absorption in a frequency range for a particular depth. An overall absorption coefficient of 0.83 can be achieved experimentally in the frequency range of 660 Hz to 2 kHz with an MC-MPPA of only 22 mm thick. In addition, when the air cavity depth is slightly increased to 50 mm, an overall absorption coefficient of greater than 0.82 can be achieved in the 281 Hz to 1 kHz frequency range. Experimentally, an absorption coefficient of approximately 0.82 is achieved at 340 Hz at an air cavity depth of 50 mm, which is a depth-to-wavelength ratio of 20, making the MC-MPPA a deeply subwavelength absorber. Results have been verified theoretically, numerically and experimentally.

Evaluation of acoustic performance of cascaded cylindrical micro-perforated panel

A cylindrical micro-perforated panel (MPP) can be used to absorb the sound of a flow system in a circular duct of a vacuum cleaner. A cascaded cylindrical MPP is a special class type where two cylindrical MPPs are arranged in a series to improve sound attenuation. The manufacturing of MPP primarily involves the machining of micro-perforations, because small holes are not readily made using injection moulding due to the complexity of the die, flow control of the molten polymer through the small orifices and dimensional stability, making it unsuitable for mass production. This limitation can be overcome with the use of additive manufacturing (AM) technology, where the micro-perforations can be designed and manufactured, with relatively larger tolerances. Experimental validation ensures that the manufactured prototype in this study is performed according to design. Results show that the transmission loss of the model and the experimental outcomes agree. The cascaded arrangement of the cylindrical MPP results in a wider effective frequency range and an increased transmission loss. Parametric studies of the combined effects of the perforation diameter, perforation ratio and the depth of air cavity on the diameter of the duct and length ratio are conducted using a transfer matrix method. A case study is demonstrated here in the design. Moreover, an AM of cascaded cylindrical MPP is performed to attenuate peak noise at 1650 Hz, where the optimum parameters of the cascaded cylindrical MPP are obtained using a genetic algorithm. The manufactured cascaded cylindrical MPP is installed on a vacuum cleaner duct, and the measurement of sound power level shows a reduction of 4 dB(A).

Experimental and numerical analysis of naturally ventilated PV-DSF in a humid subtropical climate

Photovoltaic double-skin façades (PV-DSFs) have great potential to simultaneously achieve power generation, thermal insulation and natural lighting when applied in buildings. Among them, crystalline silicon PV-DSF is one of the common types due to its high efficiency and mature technology. However, the alternate arrangement of opaque crystalline silicon and transparent glass in crystalline silicon PV-DSF complicates its heat transfer characteristics. Furthermore, the ventilation air cavity in crystalline silicon PV-DSF could induce a vertical convective heat transfer process. It is difficult to accurately predict the complex multidimensional heat transfer process in crystalline silicon PV-DSF by simplified 1-D or 2-D thermal models. Therefore, a comprehensive numerical model of PV-DSF including the optics, electricity and 3-D heat transfer sub-models is developed in this paper to estimate the annual electrical and thermal properties of PV-DSF in a humid subtropical climate. The effects of structure, orientation, air cavity depth and solar cell coverage on the overall performance of PV-DSF are analyzed. In comparison with single-glazed semi-transparent photovoltaic, PV-DSF provides a 30.4% reduction in heat gain and a 50.3% reduction in heat loss. The preferred installation orientation is due south for PV-DSF in a humid subtropical climate.

Water entry dynamics of rough microstructured spheres

In this work, we proposed a facile underwater air cavity generation strategy based on rough microstructured spheres and explored its water entry dynamics and drag reduction characteristics. Under the assistance of microstructures, the three-phase contact line is pinned near the sphere equator and inhibits the wetting of the liquid film along the sphere surface, so that leading the formation of air cavity. The water entry process is mainly divided into four stages: flow formation, cavity opening and stretching, cavity closure and entrapment, and cavity collapse. With the Froude number Fr, the pinch-off depth of air cavity obviously increases, and the pinch-off time is also delayed, which contributes to the formation of a longer bottom air cavity. In addition, the spheres with a larger impact velocity would fall faster in water during the initial falling period, while the terminal velocities are nearly the same for all the spheres when they are in a stable falling period. It is worth noting that for a same sphere, the larger impact velocity could not only contribute to the formation of a longer air cavity but also makes the generated air cavity keep in a stable and streamlined shape at different underwater depth, which is vitally important for achieving continuous drag reduction. Finally, we demonstrated numerically that the stable streamlined sphere-in-cavity structure could reduce the hydrodynamic resistance levels up to 91.3% at Re ∼ 3.12 × 104, which is related to the boundary slip caused by an air layer trapped in the microstructures.

Performance prediction of a novel double-glazing PV curtain wall system combined with an air handling unit using exhaust cooling and heat recovery technology

In solar buildings, the problems associated with indoor comfort and energy demand have garnered considerable attention, including overheating of BIPV systems, excessive cooling load, and cold-heat offset of HVAC systems. To address these problems, this study proposes a novel exhaust ventilation double-glazing PV curtain wall system (EVPV) combined with an air handling unit (AHU) based on waste heat recovery (HR). This hybrid system cools the PV curtain wall by utilizing exhaust air as a coolant. Additionally, the exhaust air is recycled to preheat the dew-point supply air and then precool the fresh air. Thus, solar energy is captured and converted into electricity and heat, while thermal energy is efficiently recovered. The system realizes superior electricity output, reduced cooling energy consumption, and remarkable reheat savings. For comprehensive performance prediction, a case study is conducted based on a restaurant building with a south-facing EVPV in Hefei, compared to one with a non-ventilation BIPV facade system without HR (NVPV). Based on the validated model, the results of a typical summer week operation indicate that the EVPV system can achieve (1) 7.87 kWh/day power generation and 7.14% PV efficiency, with 0.35% enhancement; (2) 7.68 kWh/day heat recovery from the PV double-glazing façade; (3) 8.41 kWh/day (62.31%) mitigation in reheat demand, 2.97 kWh/day (26.60%) reduction in room cooling load, and 39.94 kWh/day fresh air cooling capacity; (4) additional fan power of 16.29 kWh/day. In total, integrating the PV curtain wall with AHU using HR reduces overall energy consumption by 63.12 kWh/day (19.26%). Furthermore, the effects of air cavity depth and PV coverage ratio on the electrical and thermal behavior of EVPV are investigated. The results suggest that 0.08 m air cavity depth and a higher PV coverage ratio provide the building with a more promising energy-saving potential.

Transmission Loss analysis of a simple expansion chamber muffler with extended inlet and outlet combined with inhomogeneous micro-perforated panel (iMPP)

This paper studies the acoustic performance of a simple expansion chamber muffler combined with an inhomogeneous microperforated panel (iMPP). Three different shaped mufflers, i.e., circular, rectangular, and elliptical, with extended inlet and outlet pipes, have been studied. The thermo-acoustic module of COMSOL Multiphysics 5.5a software is used for finite element method (FEM) simulation. Then the 3D printing (stereolithography) technology is used to manufacture the samples. The transmission loss (TL) of the muffler given by the FEM simulation model is verified and measured by the Two-load method experiment, and the muffler with and without iMPP is tested. Different configurations have been tested to detect the effect of resonator absorbers based on iMPP in the expansion chambers. To observe the effectiveness of using iMPP, it is compared with other homogeneous MPPs. The results show that the TL of the low to medium frequency region increases by 25∼30 dB after adding iMPPs into the expansion chamber. Finally, the air-cavity depth behind the iMPP and the length of the expansion chamber is varied to study its effect on TL performance. The results show that the larger the cavity depth, the lower the TL peak. At the same time, with the decrease of the expansion chamber length, the TL peak gradually decreases and narrows. By carefully controlling and selecting the iMPP and expansion chamber parameters, we can obtain the required TL. Therefore, when the reactive effect of the muffler becomes invalid, iMPP can be used as an alternative further to improve the TL from low frequency to medium frequency.

Sound absorption characteristics of DL-MPP with non-circular perforations using electro-acoustical model

The analytical studies on the sound absorption characteristics of a homogeneous Double Leaf Micro Perforate Panel (DL-MPP) absorber with non-circular perforations are presented in this study. The DL-MPP's sound absorption coefficient is calculated using an equivalent electro-acoustical model. The performance of the DL-MPP with Non-Circular perforations was investigated by varying factors such as hole topology, aspect ratio, hole size, perforation ratio, air cavity depth, and panel thickness. The obtained results indicate that rectangular holes have higher absorptivity. The results also show that the DL-MPP absorptivity increases with increasing the aspect ratio of rectangular perforations and decreasing panel thickness.

Ratios of Siding Depth to Cavity Depth in Mixed Convection of Air Cavity behind Vinyl Siding for Building Envelopes

Providing an air cavity behind vinyl siding within a building envelope is an approach to mitigating building moisture-related issues, which also improves the building’s thermal performance. Previous studies commonly assume the cavity air as still and thus neglect the influence of mixed convection on the performance of the building envelope. Additionally, the air cavity is usually considered uniformly rectangular, neglecting the effects of its geometric irregularity and complexity. Therefore, to comprehensively investigate the effects of air cavities, this study analyzes ratios of siding depth to cavity depth in the mixed convection of air cavities, through rigorously formulating the problem and solving it using the perturbation method. The results show that increasing the ratio of the vinyl siding depth to the air cavity depth amplifies the metrics directly related to the energy performance of the building envelope, including the cavity air’s velocity, temperature, and mass fraction; the skin friction at cavity walls; as well as heat and mass transfer across the cavity. The effects of other critical parameters are also investigated, including the Reynolds number and temperature of the irregular cavity wall. This study provides guidance for comprehensively evaluating the thermal performance of an air cavity behind vinyl siding and improving the design of vinyl siding.

The study of a double-skin ventilated window integrated with CdTe cells in a rural building

CdTe cells have many advantages such as high electrical efficiency, and low cost. Few studies on PV windows combined with CdTe cells exist. The PV windows investigated in the past were mainly designed for cooling needs. In this work, a double-skin ventilated window integrated with CdTe cells (CdTe-DSV) was proposed, which could meet seasonal thermal demands. A mathematical model of the proposed window was developed and validated against experimental data. Based on the model, firstly, the thermal comfort in a building with the proposed window was discussed. Secondly, the energy performance (including heat, daylight, PV output) of the proposed window was analyzed and compared with that of the a-Si-DSV window. Finally, the effects of cavity depth, and PV coverage ratio on energy performance were evaluated. The results indicated that the CdTe-DSV window could ensure a relatively neutral thermal sensation. The energy saving of heat transmission in winter and summer was 205.76kwh and 333.09kwh respectively. The CdTe-DSV window had a higher PV output than the a-Si-DSV window by 62%. The increase of PV coverage can decrease the cooling load and increase the heating load. Increasing the air cavity depth is always beneficial to energy saving.

The effect from different level of Melinjo (Gnetum gnemon Linn) leaf extract and storage duration on the quality of duck eggs

Eggs are one of perishable food products and a very short shelf life. If the eggs are left in the opened air (room temperature), it is lasting only for about 10-14 days. After these days, the eggs undergo alteration of the damage such as evaporation of water content, foul smelling and changing taste. Efforts to overcome the damage, it is necessary to perform preservation. Preservation can be done by marinating. Marinating eggs can be conducted by soaking with various solutions such as lime water or vegetable tanners containing tannins. Melinjo leaves contain high levels of tannins 4.55% which are expected to be one of the vegetable tanning materials. The experiment was conducted in the Laboratory of Animal Technology. This study was used 176 duck eggs that were arranged using completely randomized design (CRD) of factorial pattern 4 x 4 with 3 replications, and consisting of 2 factors. The first factor was the levels of melinjo extract ( 0%, 30%, 40% and 50% ) and the second factor was storage duration (1, 14, 21 and 28 days, respectively ). The results of this study showed that higher levels of leaf melinjo extract, the Haugh unit (HU) value was increased and the depth of the air cavity was decreased. The longer of the storage duration, the lower of HU value of yolk index. On the other hand, the depth of air cavity, depreciation, were increased. In brief, Melinjo leaf extract (30%) can be used as a preservative of duck eggs.

Transmission Loss Analysis of Simple Expansion Tube with Micro – perforated Cylindrical Panel

This study presents a development of simple expansion tube attach with micro-perforated cylindrical panel (MPCP) to improve the acoustic performance. A simulation based boundary element method (BEM) was carried out using PLM Simcenter 3D. The model was constructed in three dimensional in CAD NX Nastran 12 application and then meshing into small elements. The acoustic properties and boundary condition were defined for the simulation purpose. The model was then fabricated by 3D printer material and verified with the transmission loss measurement utilized the two-load method. In comparison, the transmission loss of simple expansion tube with and without MPCP show a good agreement of BEM analysis and experimental result. The addition of the MPCP inside the expansion tube improved the transmission loss by 2-10 dB in wider frequency band compared to the simple expansion tube. Finally, the air cavity depth of the expansion tube is varied to study it effect. Its showed that, the larger air cavity depth caused the transmission loss peak shift to a lower frequency range.

Sound absorption properties of multi-layer structural composite materials based on waste corn husk fibers

In order to find a reasonable way to use the waste corn husk, waste degummed corn husk fibers were used as reinforcing material in one type of composite material. And polylactic acid particles were used as matrix material. The composite materials were prepared by mixing and hot-pressing process, and they were processed into the micro-slit panel. Then, the multi-layer structural sound absorption composite materials were prepared sequentially by micro-slit panel, air cavity, and flax felt. Finally, the sound absorption properties of the multi-layer structural composite materials were studied by changing flax felt thickness, air cavity depth, slit rate, and thickness of micro-slit panel. As the flax felt thickness varied from 0 to 10 mm in 5 mm increments, the peak of sound absorption coefficient shifted to low frequency. The sound absorption coefficient in the low frequency was improved with the air cavity depth varied from 0 to 10 mm in 5 mm increments. With the slit rate increased from 3% to 7% in 2% increments, the peak of sound absorption coefficient shifted to high frequency. With the thickness of micro-slit panel increased from 2 to 6 mm in 2 mm increments, the sound absorption bandwidth was broaden, and the peak of sound absorption coefficient was increased and shifted to low frequency. Results showed that the highest sound absorption coefficient of the multi-layer structural composite materials was about 1 under the optimal process conditions.

Optimization of micro-perforated sound absorber using Particle Swarm Optimization (PSO)

This study investigates the acoustic properties of micro-perforated panel (MPP) as an alternative to passive noise control. In order to obtain high sound absorption over broad frequency band, the analysis of sound absorption performance of MPP sound absorber should be conducted based on 3 major design parameters, such as perforation diameter, d, air cavity depth, D, and distance between perforations, b, of MPP. Next, the optimization of sound absorption coefficient for MPP sound absorber need to be done by using the Particle Swarm Optimization (PSO) algorithm and the optimum parameters of a single layer MPP sound absorber should be obtained for achieving higher sound absorption which the sound absorption coefficient, α + 0.5 with a wider frequency band. The PSO is used in order to optimize the panel construction to provide maximum sound absorption coefficient in a determined wide band frequency range. Experiments are carried out at normal sound incidence and plane waves. A good agreement is achieved between the theory and the experiments.

Optimization and Evaluation of Naturally Ventilated BIPV Façade Design

To improve the photovoltaic (PV) power generation, the temperature control measures and optimization of BIPV systems are critical, particularly in tropical weather. There are two categories of cooling mechanisms, i.e. passive and active cooling. In the present study, a simplified numerical model is set up to evaluate the effect of passive cooling of BIPV façades with various configurations, including the PV periphery openings (i.e. area ratio), behind air cavity depth, wind velocity and attack angle (0 to 90 degrees) on the cell temperature of BIPV module. A parametric study is performed. Three-dimensional computational fluid dynamics (CFD) simulation is applied to predict the cell temperatures under various BIPV façade configurations and environmental conditions. The results of the CFD model show that in the case where the areas of the top and bottom openings are equal, there is limited air ventilation at the behind air cavity. However, the BIPV surface temperature has a significant drop when the inlet opening at the bottom is enlarged, compared with that of the same area of inlet and outlet. Changing the wind attack angle from 0º to 60º has limited effect on the BIPV cell temperature, which results in a variation of less than 5 ºC. However, a dramatic temperature drop of around 15ºC is observed when the attack angle is larger than 60º. The influence of air cavity depth is studied with the fixed top and bottom opening size ratio. By varying the air cavity depth from 30 to 200 mm, a lower PV cell temperature can be achieved.

Frequency Tuning of Graphene Nanoelectromechanical Resonators via Electrostatic Gating.

In this article, we report on a comprehensive modeling study of frequency tuning of graphene resonant nanoelectromechanical systems (NEMS) via electrostatic coupling forces induced by controlling the voltage of a capacitive gate. The model applies to both doubly clamped graphene membranes and circumference-clamped circular drumhead device structures. Frequency tuning of these devices can be predicted by considering both capacitive softening and elastic stiffening. It is shown that the built-in strain in the device strongly dictates the frequency tuning behavior and tuning range. We also find that doubly clamped graphene resonators can have a wider frequency tuning range, while circular drumhead devices have higher initial resonance frequency with same device characteristic parameters. Further, the parametric study in this work clearly shows that a smaller built-in strain, smaller depth of air gap or cavity, and larger device size or characteristic length (e.g., length for doubly clamped devices, and diameter for circular drumheads) help achieve a wider range of electrostatic frequency tunability. This study builds a solid foundation that can offer important device fabrication and design guidelines for achieving radio frequency components (e.g., voltage controlled oscillators and filters) with the desired frequencies and tuning ranges.

Study on optimizing design of solar chimney for natural ventilation and smoke exhaustion

Solar chimney has been primarily utilized for natural ventilation, but its application to smoke exhaustion was rarely explored. A 1:3 reduced-scale test platform with a dimension of 1.5 m × 1.5 m × 0.9 m (height) was used to optimize solar chimney under natural ventilation and smoke exhaustion, considering four influencing factors, including height of cavity inlet from the floor (0.2–0.8 m), cavity depth (2.5–17.5 cm), solar radiation (400–1200 W/m2) and fire size (6.8–15.8 kW). Both natural ventilation and smoke exhaustion follow the same trend along the air inlet height and cavity depth, which confirms its viability on smoke exhaustion under fire condition without compromising the performance of natural ventilation. Experimental results suggested a chimney configuration of 0.5 m high air inlet and 12.5 cm cavity depth with optimized functions. External radiation shows obvious benefit on enhancing natural ventilation, while its influence on smoke exhaustion is limited. An empirical model was developed to predict the flow rate under normal and fire conditions. The outcomes of this study provide a technical guidance for the design of solar chimney under scenarios of both natural ventilation and smoke exhaustion.

Lightweight absorption and barrier systems comprising N-layer microperforates

Since the concept of microperforated panels (MPPs) was introduced by Maa, there have been continuing efforts to apply MPPs, primarily as fiber-free sound absorbing materials, typically wall-mounted. The objective of the present work was to demonstrate that multi-layer MPPs can also be effective functional absorbers and lightweight barrier systems. The acoustical properties of lightweight MPPs depend on hole diameter, thickness, porosity, mass per unit area, and air cavity depth. In the case of a single layer, it is possible to find a combination of these parameters that results in good performance over one or two octaves. However, to be effective for noise control over a broader range of frequencies, it is necessary to design multi-layer MPPs. Thus here the focus was on the optimal design of multi-layer MPPs in the speech interference range, 500 to 4000 kHz. In the case of functional absorbers, the total absorption of the system was optimized, while in the case of barriers, a high transmission loss was desired, without necessarily sacrificing the absorption of the system. In the latter case, in particular, it was possible to create systems having transmission losses well in excess of the mass law over a broad range of frequencies.