AMMs based on the concept of slow sound propagation have been used to design broadband acoustic perfect absorbers, leading to deep-subwavelength structure thicknesses. Their performance is driven by the balancing of intrinsic losses of the resonators and the leakage rate of energy of the open system, resulting in so-called critical coupling. Until now, all structures are analysed in controlled conditions of acoustic plane wave incidence. However, in practice, the acoustic field is more complex and in this case the absorption capabilities of these structures are reduced
DC1 will design deep-subwavelength acoustic absorbers for diffuse fields. These are of high scientific and technological interest, since the acoustic field to be absorbed in most of the practical situations is neither plane wave nor spherical, but rather diffuse. A synergetic approach will be adopted, combining theoretical developments, numerical simulations and experimental validation at every step.
Innovative aspects: Deep-subwavelength anisotropic AMMs will be developed as well as new theoretical models to analyse and exploit the anisotropy of the system.
VAMM design is typically based on regular periodic arrangements of identical resonators on or in a regular host structure, and relies on periodic unit cell models. In practice, however, material variations and manufacturing imperfections introduce disorder. Recently, intentional disorder has attracted attention as a means to improve or broaden performance. Although wave trapping and Anderson localization analogies were reported, more insight is needed to understand the effects and to control the attenuation.
DC2 will model, analyse and validate the effects of disorder in the resonators, their arrangement, the host structure and combinations on the performance of VAMMs. This will be achieved by developing full structure, unit cell and homogenization based modelling for disorder in the metamaterial constituents: e.g. quasi-periodicity, spatial correlations, hyperuniformity and random disorder.
Innovative aspects: Including disorder in VAMM modelling and analysis will lead to fundamental insights in the impact of disorder and their governing mechanisms, which will enable designing intentionally disordered VAMMs with improved, broadband attenuation.
The potential of PCs for audible sound can be boosted by combining Bragg stop bands, directly acting on transmission, with classical sound absorbing materials. Another way to broaden the high transmission loss frequency range of such structures consists of including (periodic) resonant elements in PCs, leading to hybridization of Bragg and resonance stop bands. Recently, subwavelength resonators have been designed to absorb low-frequency sound both in reflection and transmission problems. Hence, combining PCs with such low-frequency deep-subwavelength resonant absorbers is a promising technique to design efficient sound barriers with enhanced transmission and absorption properties.
DC3 will design a new type of PC combining Bragg band gaps with deep-subwavelength resonators which either act as sub-wavelength absorbers or are coupled to lower their resonance frequencies. This unique combination is expected to enable designing broadband sound barriers, which not only act on transmission but also on reflection of sound. A synergetic approach will be adopted, combining theoretical developments, numerical simulations and experimental validation at every step.
Innovative aspects: The design of resonant and absorbent PCs is envisaged, using unique combinations of Bragg and resonance stop bands and deep-subwavelength absorbers, which are expected to be highly efficient to control low and broadband frequency sound.
Alternative to using additive manufacturing, separate resonators can be produced with faster and cheaper production methods such as laser cutting, but their addition to a host structure is tedious and time consuming. Thermoforming was recently considered as an efficient method to produce VAMMs, though still leads to quite some variation on the tuned resonance frequencies, depending on their position on the structure.
DC4 will investigate two common production techniques for large-series production of metamaterials: injection moulding and thermoforming. For both technologies, mould design will be crucial: numerical simulations will support the design and predict filling ratios, warpage, etc. The geometrical accuracy of the resonators and dependency on the location in the full structure will be investigated. Variations and uncertainties will be propagated towards the resonance frequencies of the resonators as to quantify the effect on the final structure.
Innovative aspects: Thermoforming and injection moulding will be investigated as manufacturing methods to produce VAMMs. Local geometry variations will be investigated along with their effect on the resulting resonance frequencies. This track will allow faster, large series scale production of VAMMs.
Laser sintering enables producing freeform complex structures. Accuracy and material properties can be controlled by tailoring the slicing process (into process-layers) and process (especially laser) parameters. The link between these parameters and the accuracy and material properties remains a challenging research question, since both are influenced by the local processing conditions, which strongly depend on feature size, print orientation, scanning strategy and position on the build platform. To evaluate geometrical accuracy, CT-scanning is reliable, but slow and expensive.
DC5 will focus on the laser sintering quality of geometrically detailed shapes encountered in metamaterials. As the evaluation of the resonance frequency of small resonant samples allows (local) evaluation of geometrical accuracy and/or material properties, small resonators in the build may allow for print quality control. Resonators shapes will be investigated and a fast monitoring setup will be devised.
Innovative aspects: Evaluating the resonance frequency of small resonant VAMM samples allows (local) evaluation of geometrical accuracy and/or material properties and can hence be used as (cheap and fast) quality sensor to relate process parameters to product quality and performance. This can enable implementing a fast quality control loop, to achieve a faster optimization of the process parameters and to meet quality requirements.
The use of PCs as noise barriers is still not widespread. Some technical solutions have been explored, such as thin resonant cylinders with elastic shells, hollow cylinders or a combination of both, which may lead to barriers with improved insulation resulting from both Bragg interference and the scatterers’ resonances. Locally resonant PCs made of wood or PC barriers coated with porous materials have been explored. Porous concrete, on itself or with internal resonant structures, was shown to enable excellent sound absorption, allied with intrinsic durability. Hence, it is an interesting host to develop high performance PCs.
DC6 will address the possibility of producing a modular concrete-based PC noise barrier solution for traffic noise. Production possibilities of a high-performance solution will be studied, which is quickly and efficiently mountable on-site after production in a factory-based, hence more controlled, environment. The use of an external porous concrete layer is envisaged in order to maximize performance.
Innovative aspects: Development of feasible and affordable production strategies for PC barriers, based on a modular concept, which can be quickly assembled on-site. The use of porous concrete based on residues forms a sustainable solution with high performance and lower cost and will be a significant leap from metal-based solutions, while additional incorporation of internal resonant structures will allow maximization of performance.
The sound attenuation provided by metamaterials is accompanied by spectral content changes, including phase distortions due to strong dispersion. Current psychoacoustic models do not account for annoyance related to such distortions. A recent DENORMS study investigated the annoyance perceived for transmitted noise through metamaterial partitions and correlations with classical metrics e.g., loudness, sharpness, and articulation index. The impact of dispersion and transient effects on the sound quality of metamaterial treatments remains unclear, especially for speech intelligibility, comfort, or music. Recent developments in resonant media characterisation could enable understanding the sensitivity of the psycho-acoustic behaviour of metamaterial treatments to design parameters.
DC7 will model the sound quality of metamaterial-based sound absorbers or insulators in industrial applications, replacing case-by-case subjective tests. Three aspects will be considered: (i) developing representative sound quality metrics, (ii) application to the prediction of psychoacoustic annoyance characteristics of metamaterials and validation through listening tests, (iii) developing a framework for metamaterial testing and design exploration based on perceptual requirements, using deterministic and statistical optimisation.
Innovative aspects: Development of a new psychoacoustic annoyance model, incorporating transient effects and spectral features of the 3 metamaterial classes. A framework for metamaterial testing and design optimisation including sound quality as well as energy dissipation targets will be proposed for industrial applications.
The acoustic performance of partition elements is usually based either on their mass, on impedance mismatching or on using layered structures including materials with different properties. Recently, increased sound insulation in plate-like elements has been achieved by using different classes of acoustic and vibro-acoustic metamaterials. However, one major problem in building partition elements is related with their critical frequency, for which large insulation dips occur, greatly reducing their insulation performance. Some recent studies indicate the possibility of significantly alleviating this problem using metamaterial solutions.
DC8 will design novel partitions for room acoustic applications comprising acoustic metamaterials, targeting the critical frequency of the panel. The performance of the panel will be maximised without compromising their weight. Main focus is also to look at appropriate manufacturing processes allowing the embedment of the structures within the panel.
Innovative aspects: The development of a simple strategy to incorporate acoustic resonant structures in wall panels to mitigate critical frequency effects. The incorporation of periodic acoustic resonators in the design of partitions is sought, maximizing their performance without increasing their mass. Appropriate manufacturing processes will be considered to allow embedding of the resonant structures within the panel, rendering competitive and useful practical solutions.
PCs for audible sound have shown great potential for use as acoustic barriers. Recently, industrial applications such as acoustic barriers for infrastructures or industrial machines have been studied which show the capabilities of reducing noise while allowing for ventilation. Furthermore, comparisons with existing solutions have been performed, showing favourable performance gains. There is, however, still a lack of thorough and readily usable industrial insights as well as demonstrated performance for these types of applications.
DC9 will develop PC based acoustic solutions enabling high sound transmission loss and allowing airflow for ventilation. Novel designs will be explored via numerical simulation and optimization. Validation on existing machines and benchmarking against current solutions will be done. To come to a viable, competitive, industrial application, appropriate materials and manufacturing will be considered.
Innovative aspects: Via numerical optimization, testing on existing machines and comparison with existing solutions, the potential of PCs for applications requiring ventilation will be shown. Appropriate material uses and manufacturing methods will be considered to derive and define competitive solutions.
Elastic and locally resonant metamaterials take advantage of the strong sound attenuation in the low frequency properties linked to gratings or local resonances. Recent studies also show interesting features when the vibration of the porous skeleton is considered. Although these aspects are well known for absorption applications, there also is strong industrial interest in acoustic insulation, for which acoustic metamaterials are also suitable, especially at low frequencies.
DC10 will develop new types of AMMs for sound insulation. AMM performance will be improved by combining mechanical and elastic effects in multilayered structures composed of impervious skins and rigid/flexible internal structures with local resonances and periodicity to come to subwavelength (<λ/50) broadband, thin and reconfigurable AMMs. Designs will be manufactured in an industrially representative way (1m2 sized sample for a diffuse field) to experimentally validate the acoustic potential of AMMs in realistic applications.
Innovative aspects: Vibrational and acoustic phenomena will be coupled to discover configurations with strong, low-frequency insulation. The main goal is to come up with and realize products of only few centimetres thickness, supported by simulations.
The concept of VAMMs has been successfully applied to reduce sound and vibration transmission. Standard metamaterial modelling uses infinite periodic structure theory, considering a single unit cell model. Practical applications require addressing 2 additional questions: (i) how do edges of finite samples affect the dynamics and (ii) can transmission reduction be achieved by aperiodic structures. In both cases, full-scale models lead to long calculation times. Fast optimization schemes such as Bayesian techniques will be used to design finite structures with optimized properties. This can be done within the stringent limitations defined by the end user.
DC11 will investigate the dynamics of finite VAMM plates, focussing on (i) the influence of edge effects of finite samples and (ii) transmission reduction by aperiodic design. Model reduction will be used for efficient and accurate finite VAMM performance predictions and energy flow quantification and visualization. Fast (e.g. Bayesian) optimization schemes will be used to optimize VAMM performance under different mounting scenarios. Fundamental principles are combined with direct application for industrial partners.
Innovative aspects: The developed methodology will allow efficient and accurate sound and/or vibration insulation predictions for finite VAMM panels, and optimizations to reach maximum insulation potential, including specific mounting scenarios. A fast prediction tool will open the promising capabilities of these metamaterials to the user community (i.e., lightweight transport and buildings).