The fine manipulation of sound fields is critical in acoustics yet is restricted by the coupled amplitude and phase modulations in existing wave-steering metamaterials. Commonly, unavoidable losses make it difficult to control coupling, thereby limiting device performance. Here we show the possibility of tailoring the loss in metamaterials to realize fine control of sound in three-dimensional (3D) space. Quantitative studies on the parameter dependence of reflection amplitude and phase identify quasi-decoupled points in the structural parameter space, allowing arbitrary amplitude-phase combinations for reflected sound. We further demonstrate the significance of our approach for sound manipulation by producing self-bending beams, multifocal focusing, and a single-plane two-dimensional hologram, as well as a multi-plane 3D hologram with quality better than the previous phase-controlled approach. Our work provides a route for harnessing sound via engineering the loss, enabling promising device applications in acoustics and related fields. The formation of true holograms requires control of both amplitude and phase; however, acoustic metamaterials are generally limited to phase control only. Here, Zhu et al. tailor lossy metamaterials to independently control the amplitude and phase of acoustic wavefronts.
Underwater radiated noise is part of the anthropogenic emissions into the environment and as such a pressing problem for the preservation of the marine ecosystem. In order to direct attention to the most relevant noise sources associated with ships it is crucial to precisely determine the local origins of the acoustic emissions. As acoustics are by nature perceived through a very subjective auditory perception, visual post-processing support is required in engineering applications to assess the impact on structures and to create an understanding of the overall noise field geometrically, topologically, and directionally. In the context of CFD simulations, this may be achieved by considering the pressure pulses on domain boundary surfaces or passive surfaces, or by evaluating various volumetric information, such as Proudman acoustic sources or the Lighthill stress tensor, which is the fundamental input for many acoustic analogies including the Ffowcs-Williams-Hawkings method. For a propeller-hull configuration, the acoustic emissions from modeled and scale-resolved turbulence two-phase CFD analyses are evaluated in detail with different visualization methods. It is shown that the spatial distribution information of frequency domain pressure pulses, and the corresponding complex phase angles on specific passive geometries, as well as the Lighthill stress tensor may be utilized to create a better understanding of underwater acoustics. This allows the identification of source types and their respective excitation of the hull and emission characteristics of the hydrodynamic sources into the fluid domain, as well as the effect of the CFD simulation domain geometry extent.
The acoustic properties of seafloor sediments have always been important parameters in sound field analyses and exploration for marine resources, and the accurate acquisition of the acoustic properties of sediments is one of the difficulties in the study of underwater acoustics. In this study, sediment cores were taken from the northern South China Sea, and the acoustic properties were analyzed. Since traditional methods (such as regression equations or theoretical models) are difficult to apply in practical engineering applications, we applied remote sensing data to sound velocity prediction models for the first time. Based on the influencing mechanism of the acoustic properties of seafloor sediments, the sediments’ source, type and physical properties have a great influence on the acoustic properties. Therefore, we replaced these influencing factors with easily accessible data and remote sensing data, such as parameters of granularity, distance to the nearest coast, decadal average sea surface productivity, water depth, etc., using deep neural networks (DNN) to develop a sound velocity prediction model. Compared with traditional mathematical analyses, the DNN model improved the accuracy of prediction and can be applied to practical engineering applications.
Abstract Broadband sound absorption at low frequency with minimal space consumption and material cost is a trending research problem in engineering acoustics. Conventional sound absorbers like porous materials and microperforated panels (MPP) are not good enough to absorb sound in the low frequency regime. To enhance the acoustic performance, Helmholtz resonator with inserted neck (HRIN) is integrated with MPP in this investigation. The sound absorption characteristics of series and parallel arrangements of both MPP and multiple HRINs are theoretically modelled using electro-acoustic analogy and the results are compared with full field finite element simulations. Most importantly, it is observed that the parallel arrangement of MPP and multiple HRINs considerably reduced the thickness of the absorber. In addition, a single unit comprising four parallel Helmholtz resonators when arranged in conjunction with MPP absorbed more than 65% sound over a band width of 318 Hz to 880 Hz whereas the successive arrangement of the same unit with MPP is affected with high anti-resonance effect. It is demonstrated that, the proposed modelling strategy very well predicts the acoustic performance of HR integrated MPP absorbers on par with the computational intensive numerical simulations. Moreover, these configurations exhibited efficient low frequency sound absorption capability even with reduced absorber thickness.
The motto of the paper concerning physics and nature is a quotation by Eugene Wigner: ‘The miracle of the appropriateness of the language of mathematics for the formulation of the laws of physics is a wonderful gift which we neither understand nor deserve’. The first case in this paper is based on an original paper by this author, and it uses Fletcher’s scale of speech intelligibility, for analysis of specific noise effect in the presence of background noise. The idea is based on the assumption that the brain uses the same scale for estimating both intelligibility and nuisance by specific noise. This notion was confirmed by experiments. The second case below is the use of the ‘simplex theory’, which is a mathematical model used in computer sciences, and was published originally by this author as a means for design of environmental sound barriers. The third application involves sophisticated use of specular reflections and scattering in acoustics, with application of acoustic quadratic residues in interiors acoustics, based on the mathematical theory of numbers, where prime numbers are involved. Leading mathematicians in this development are Euclid (325 BC–265 BC) and Gauss (1777–1855), who discovered in the 18th century quadratic residues. In the context of diffraction physics, the main contribution was by Röntgen (1845–1923), von Laue (1879–1960), W.H. Bragg (1862–1942), and W.L. Bragg (1890–1971). This long way towards sophisticated acoustic diffusers that turn specular reflections into a uniform diffusion needed one more leading physicist to make the breakthrough. Manfred Schroeder (1926–2009) published a seminal paper in 1975, adding the number theory to room acoustics as a legitimate part. He has proved mathematically that specific panels with a sequence of one-dimensional or two-dimensional grooves result in a diffusive-phase grating of wide band, instead of a specular reflection panel. This result was directly applicable from the theory of x ray diffraction. D’Antonio and Cox continued improving the results, leading to special quadratic residue diffuser (QRD) shapes. Many of the panels resemble shapes that exist in nature.
This study investigated the acoustical conditions in high-school Technology-Education Shops (TES) and the conflict that may exist due to their use at the same time as classrooms for learning and as industrial workshops for fabrication. This was done by measuring relevant acoustical characteristics in twenty TES in British Columbia when unoccupied with building services operating, and in ten of them when occupied and in normal operation with shop equipment running. The results were compared to existing acceptability/design criteria for classrooms and industrial workshops, related to background-noise level, teacher noise exposure, reverberation time, speech-intelligibility index (SII) and sound-level reduction with distance doubling (DL2), and pass/fail ratings were assigned. Noise levels and reverberation times in most unoccupied TES were higher than the acceptability/design criteria for core learning spaces. Teacher daily noise exposures in occupied TES were high and often exceeded regulatory limits. SII values often indicated ‘poor’ speech intelligibility for ‘normal’ and ‘raised’ vocal outputs, and ‘good’ speech intelligibility only for ‘loud’ and ‘shout’ vocal outputs. DL2 values were unacceptable in most of the TES. Most TES received a failing grade with respect to most evaluation criteria. The results confirm the existence of an acoustical conflict; the acoustical conditions in the TES as industrial rooms are generally unsatisfactory for their use as classrooms. In general, the results suggest that TES provide poor acoustical conditions, overexposing teachers and students to noise, and are in need of effective sound-design and noise-control measures.