Explore the vast range of titles available.

“Schroeder diffuser” is a classical design, proposed over 40 years ago, for artificially creating optimal and predictable sound diffuse reflection. It has been widely adopted in architectural acoustics, and it has also shown substantial potential in noise control, ultrasound imaging, microparticle manipulation et al. The conventional Schroeder diffuser, however, has a considerable thickness on the order of one wavelength, severely impeding its applications for low-frequency sound. In this paper, a new class of ultrathin and planar Schroeder diffusers are proposed based on the concept of an acoustic metasurface. Both numerical and experimental results demonstrate satisfactory sound diffuse reflection produced from the metasurface-based Schroeder diffuser despite it being approximately 1 order of magnitude thinner than the conventional one. The proposed design not only offers promising building blocks with great potential to profoundly impact architectural acoustics and related fields, but it also constitutes a major step towards real-world applications of acoustic metasurfaces.

When sound interacts with geometrically asymmetric structures, it experiences coupling between pressure and particle velocity, known as Willis coupling. While in most instances this phenomenon is perturbative in nature, tailored asymmetries combined with resonances can largely enhance it, enabling exotic acoustic phenomena. In these systems, Willis coupling obeys reciprocity, imposing an even symmetry of the Willis coefficients with respect to time reversal and the impinging wave vector, which translates into stringent constraints on the overall scattering response. In this work, we introduce and experimentally observe a dual form of acoustic Willis coupling, arising in geometrically symmetric structures when time-reversal symmetry is broken, for which the pressure-velocity coupling is purely odd-symmetric. We derive the conditions to maximize this effect, we experimentally verify it in a symmetric subwavelength scatterer biased by angular momentum, and we demonstrate the opportunities for sound scattering enabled by odd Willis coupling. Our study opens directions for acoustic metamaterials, with direct implications for sound control, non-reciprocal scattering, wavefront shaping and signal routing, of broad interest also for nano-optics, photonics, elasto-dynamics, and mechanics. Exploiting Willis coupling in acoustic metamaterials with designed geometrical asymmetries opens up new opportunities related to sound control and manipulation. Here, the authors report a dual form of Willis coupling in geometrically symmetric acoustic scatterers.

Invisibility cloaks that can suppress wave scattering by objects have attracted a tremendous amount of interest in the past two decades. In comparison to prior methods that were severely limited by narrow bandwidths, here we present a practical strategy to suppress sound scattering across an ultra-broad spectrum by leveraging illusion metamaterials. Consisting of a collection of subwavelength tunnels with precisely crafted internal structures, this illusion metamaterial has the ability to guide acoustic waves around the obstacles and accurately recreate the incoming wavefront on the exit surface. Remarkably, two ultra-broadband illusionary effects are produced, disappearing space and time shift. Sound scatterings are removed at all frequencies below a limit determined by the tunnel width, as confirmed by full-wave simulations and acoustic experiments. Our strategy represents a universal approach to solve the key bottleneck of bandwidth limitation in the field of cloaking in transmission, and establishes a metamaterial platform that enables the long-desired ultra-broadband sound manipulation such as acoustic camouflage and reverberation control, opening up exciting new possibilities in practical applications. Invisibility cloaks have attracted a tremendous amount of interest. Here, the authors present a strategy to realize ultra-broadband disappearing-space and time-shift camouflages by leveraging illusion metamaterials, thereby removing the longstanding bottleneck of limited bandwidth in invisibility.

Based on the study of the static sound scattering characteristics of propellers, the dynamic analysis of the propeller static sound scattering characteristics at different moments is carried out during the modulation period. A method for predicting the sound scattering characteristics of curved thin plate targets based on the combination of sound propagation theory and discrete surface element thickness information is established using the slab element algorithm. The reflection coefficients of all surface elements are solved based on the thickness and material attribute information of the surface elements, and the total scattered sound field of the target is obtained. Finally, statistical analysis is carried out on the simulation data to obtain the typical scattering characteristics of propellers under different speeds and frequencies.

Ocean dynamics initiate the structure of nutrient income driving primary producers, and these, in turn, shape the distribution of subsequent trophic levels until the whole pelagic community reflects the physicochemical structure of the ocean. Despite the importance of bottom-up structuring in pelagic ecosystems, fine-scale studies of biophysical interactions along depth are scarce and challenging. To improve our understanding of such relationships, we analyzed the vertical structure of key oceanographic variables along with the distribution of acoustic biomass from multi-frequency acoustic data (38, 70, and 120 kHz) as a reference for pelagic fauna. In addition, we took advantage of species distribution databases collected at the same time to provide further interpretation. The study was performed in the Southwestern Tropical Atlantic of northeast Brazil in spring 2015 and autumn 2017, periods representative of canonical spring and autumn conditions in terms of thermohaline structure and current dynamics. We show that chlorophyll- a , oxygen, current, and stratification are important drivers for the distribution of sound scattering biota but that their relative importance depends on the area, the depth range, and the diel cycle. Prominent sound scattering layers (SSLs) in the epipelagic layer were associated with strong stratification and subsurface chlorophyll- a maximum. In areas where chlorophyll- a maxima were deeper than the peak of stratifications, SSLs were more correlated with stratification than subsurface chlorophyll maxima. Dissolved oxygen seems to be a driver in locations where lower oxygen concentration occurs in the subsurface. Finally, our results suggest that organisms seem to avoid strong currents core. However, future works are needed to better understand the role of currents on the vertical distribution of organisms.

Pelagic fauna is expected to be impacted under climate change according to ecosystem simulations. However, the direction and magnitude of the impact is still uncertain and still not corroborated by observation-based statistical studies. Here we compile a global underwater sonar database and 20 ocean climate projections to predict the future distribution of sound-scattering fauna around the world’s oceans. We show that global pelagic fauna will be seriously compromised by the end of the twenty-first century if we continue under the current greenhouse emission scenario. Low and mid latitudes are expected to lose from 3% to 22% of animal biomass due to the expansion of low-productive systems, while higher latitudes would be populated by present-day temperate fauna, supporting results from ecosystem simulations. We further show that strong mitigation measures to contain global warming below 2 °C would reduce these impacts to less than half.The authors compile an underwater sonar database to understand the current and future distribution of pelagic fauna in the world’s oceans. They show loss of 3–22% of these fauna in low and mid latitudes under high-emissions scenarios, with impact reduced to less than half if global warming is contained below 2 °C.

The acoustic scattering characteristics of spheres are the basis for studying the sound scattering law of the target. The directivity of acoustic scattering is one of the most important characteristics of the target sound scattering law. In this paper, the acoustic scattering characteristics of rigid spheres and elastic spherical shells are analyzed from the two typical basic models of the rigid sphere and elastic spherical shell. The sound scattering directivity corresponding to different ka values under different models is analyzed by COMSOL finite element simulation analysis. The change law of spheroid scattering directivity of spheres under different elastic backgrounds of the rigid sphere and elastic spherical shell is compared and studied. It is found that the scattering directivity of both rigid spheres and elastic spherical shells will change with the change of ka value. The larger the ka, the stronger the forward directivity, and the more side lobes for rigid spheres. The backward directivity is greater than the forward directivity at ka≤1, while the forward directivity of the elastic spherical shell is always greater than the backward direction. And it is not difficult to find that the directivity of rigid spheres is relatively uniform in backward scattering when ka=1. When ka>1, the directivity increases with ka, the forward directivity becomes stronger and stronger, and the side lobes gradually increase. The elastic spherical shell uses air as the internal filler, and its directivity has always been forward than backward. The forward directivity also increases with the increase of ka, and the side lobes also increase with the increase of frequency.

Geometrical acoustics is well suited for real-time room acoustics simulation and is often implemented using the image source model (ISM). One drawback of the ISM is its limitation to specular reflections, while sound scattering plays an important role in real environments. Here, computationally-efficient, digital-filter approximations are proposed to account for effects of non-specular scattered reflections in the ISM. For scattering at large surfaces such as room boundaries, each reflection is energetically split into a specular and a scattered part, based on the scattering coefficient. The scattered sound is coupled into a diffuse reverberation model. Temporal effects of the underlying surface scattering for an infinite ideal diffuse (Lambertian) reflector are derived and the resulting monotonic decay is simulated using cascaded all-pass filters. Effects of scattering and multiple (inter-) reflections caused by larger geometric structures at walls, and by objects in the room are accounted for in a highly simplified manner. A single parameter is used to quantify deviations from an empty shoebox room. The cumulated temporal effect of scattering along a reflection path is mimicked using cascaded all-pass filters adjusted to obtain a gamma-distribution-shaped envelope. The proposed method was perceptually evaluated with both music and pulse stimuli against dummy head recordings of real rooms. The results show a better agreement between the recording and the simulation for transient stimuli. In a technical evaluation, the temporal evolution of echo density showed a comparable profile for the suggested method and real rooms.

In order to study the sound scattering characteristics of array transducers on different shell structures, based on the COMSOL finite element simulation calculation method, this paper analyzes the scattered sound field characteristics of the array transducer in the single-shell structure, the single-double transition shell structure, and the double-shell structure. A quantitative comparison between the radiated sound field and the scattered sound field is made, the law of the scattered sound field is analyzed, the scattering characteristics of the array transducer under different shells are explained, and the simulation basis for the engineering application of the array transducer is provided.

This work represents the results of a study of the acoustic characteristics of a room, which is intended (with appropriate additional equipment), according to the project, for the studies of the characteristics of various types of machines of small dimensions. The obtained results show us, that the investigated room, in case the sound-scattering boards with sound-reflecting elements are installed inside, could be attributed to the category of «Echonic» rooms, with the parameters close to reverberation chambers. Therefore, this allow one to carry out practical works to reduce the noise of machines and equipment that do not have high directivity indicators. At the same time, works aimed at reducing the noise of machines in such a «live» room are less laborious compared to similar work in echo-free chambers. Besides, such rooms are less fire hazardous.