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"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 metasurfacebased 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. DOI: 10.1103/PhysRevX.7.021034 Subject Areas: Acoustics, MetamaterialsIn the 1970s, Schroeder published two seminal papers on sound scattering from maximum-length-sequence and quadratic-residue-sequence diffusers [1,2]. For the first time, a simple recipe was proposed to design sound-phase grating diffusers with defined acoustic performance. These two papers opened a brand-new field of sound diffusers with applications in architectural acoustics [3][4][5], noise control [6][7][8], ultrasound imaging [9], and microparticle separation [10] and have inspired other disciplines such as energyharvesting photodiodes [11]. D'Antonio and Konnert [12] presented one of the most accessible review papers examining the theory behind Schroeder's diffusers (SDs). Most importantly, they commercialized SDs and promoted them to be widely adopted in architectural acoustics, where the diffusers can be used to spread the reflections into all directions, reducing the strength of the undesired specular reflection and echo, as well as preserving the sound energy in space [3]. In contrast to diffusers, sound absorbers reduce the energy in the room, which can be problematic for unamplified performances in concert halls, opera houses, and auditoria. Sound diffusers are also used to promote desired reflections in order to enhance spaciousness in auditoria, to improve speech intelligibility, and to reduce the noise on urban streets [3,13,14]. Instead of using a surface with random or geometric reflectors, Schroeder innovatively designed a family of diffusers based on numbertheory sequences, with the ultimate goal to produce predicable and optimal scattering (i.e., the sound is scattered evenly in all directions regardless of the angle of incidence). In spite of the great success that SDs have achieved, they are conventionally designed to have a grating structure with a thickness that can be as large as half of the wavelength at the design frequency in order to achieve...

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.

Free controls of optic/acoustic waves for bending, focusing or steering the energy of wavefronts are highly desirable in many practical scenarios. However, the dispersive nature of the existing metamaterials/metasurfaces for wavefront manipulation necessarily results in limited bandwidth. Here, we propose the concept of dispersionless wavefront manipulation and report a theoretical, numerical and experimental work on the design of a reflective surface capable of controlling the acoustic wavefront arbitrarily without bandwidth limitation. Analytical analysis predicts the possibility to completely eliminate the frequency dependence with a specific gradient surface which can be implemented by designing a subwavelength corrugated surface. Experimental and numerical results, well consistent with the theoretical predictions, have validated the proposed scheme by demonstrating a distinct phenomenon of extraordinary acoustic reflection within an ultra-broad band. For acquiring a deeper insight into the underlying physics, a simple physical model is developed which helps to interpret this extraordinary phenomenon and predict the upper cutoff frequency precisely. Generations of planar focusing and non-diffractive beam have also been exemplified. With the dispersionless wave-steering capability and deep discrete resolution, our designed structure may open new avenue to fully steer classical waves and offer design possibilities for broadband optical/acoustical devices.

We design and experimentally demonstrate an acoustic tunnel completely open for substances like fluids or other energy fluxes to exchange while allowing sound to pass only in one direction. This significant feature is based on a distinctive mechanism using metasurface pairs to yield asymmetric extraordinary reflections along opposite directions. Theoretical analysis is presented to analytically predict the trajectory of the wave. The experimental results agree well with the numerical results and the theoretical predictions. Our design may pave the way to more versatile acoustic one-way devices with potential applications in many scenarios like duct noise control and ultrasonic therapy.

We introduce a multi-coiled acoustic metasurface providing a quasi-perfect absorption (reaching 99.99% in experiments) at extremely low-frequency of 50 Hz, and simultaneously featuring an ultrathin thickness down to λ/527 (1.3 cm). In contrast to the state of the art, this original conceived multi-coiled metasurface offers additional degrees of freedom capable to tune the acoustic impedance effectively without increasing the total thickness. We provide analytical derivation, numerical simulation and experimental demonstrations for this unique absorber concept, and discuss its physical mechanism which breaks the quarter-wavelength resonator theory. Furthermore, based on the same conceptual approach, we propose a broadband lowfrequency metasurface absorber by coupling unit cells exhibiting different properties.Due to the weak intrinsic dissipation of conventional materials in the low-frequency region, the perfect absorption of sound at low frequency (<100Hz) is still a scientific challenge [1]. To enhance the dissipation, it is necessary to increase the energy density inside the relevant material by some means like through resonances [2]. In the past two decades, the explosion of interest in developing artificial resonant structures like acoustic metamaterials [3][4][5][6] and metasurfaces [7][8][9][10][11][12] was drastically increased owing to their exotic capabilities of manipulating sound waves and their deep subwavelength thickness [13, 14]. The remarkable characteristics of acoustic metamaterials such as negative mass density, refractive index, double negativity, and controlled anisotropy have attracted massive research in the field of developing deep subwavelength sized acoustic devices for low-frequency applications such as negative refraction [13], deep subwavelength focusing/imaging [14], cloaking [15], perfect absorbers with ultrathin thickness [12,[14][15][16][17][18][19][20][21][22] etc. In related researches, perfect metasurface absorbers have received considerable attention, and variety of acoustic metasurfaces designs have been designed for the applications in noise control at low-frequency regime, which could be used in aircraft, locomotives, automobiles, machines, and buildings [13,[17][18][19][20][21][22][23][24].Ultra-thin acoustic absorber metasurface is generally based on a single or hybrid resonant system [5-12; 16-23]. The hybrid resonant system [25] usually has a broader bandwidth than a single resonant one. One interesting way to design the perfect acoustic absorber is to use an ultrathin decorated membrane with thin air chamber. Such a system was reported by Ma et al. [7] to form hybrid resonance capable of to realizing complete absorption at low-frequency range [8, 9] with a thickness of about ~1/133 of the operating wavelength. However, this design requires

BackgroundMale infertility due to multiple morphological abnormalities of the sperm flagella (MMAF) is a genetically heterogeneous disorder. Previous studies revealed several MMAF-associated genes, which account for approximately 60% of human MMAF cases. The pathogenic mechanisms of MMAF remain to be illuminated.Methods and resultsWe conducted genetic analyses using whole-exome sequencing in 50 Han Chinese probands with MMAF. Two homozygous stop-gain variants (c.910C>T (p.Arg304*) and c.3400delA (p.Ile1134Serfs*13)) of the SPEF2 (sperm flagellar 2) gene were identified in two unrelated consanguineous families. Consistently, an Iranian subject from another cohort also carried a homozygous SPEF2 stop-gain variant (c.3240delT (p.Phe1080Leufs*2)). All these variants affected the long SPEF2 transcripts that are expressed in the testis and encode the IFT20 (intraflagellar transport 20) binding domain, important for sperm tail development. Notably, previous animal studies reported spontaneous mutations of SPEF2 causing sperm tail defects in bulls and pigs. Our further functional studies using immunofluorescence assays showed the absence or a remarkably reduced staining of SPEF2 and of the MMAF-associated CFAP69 protein in the spermatozoa from SPEF2-affected subjects.ConclusionsWe identified SPEF2 as a novel gene for human MMAF across the populations. Functional analyses suggested that the deficiency of SPEF2 in the mutated subjects could alter the localisation of other axonemal proteins.

We demonstrate multifunctional acoustic metasurfaces (MAMs) that can simultaneously realize the same functionality or multiple different functionalities at multiple tunable frequencies. The fundamental physical mechanism is based on designing supercell of Helmholtz resonators (HRs) with multiple resonances operating at different frequencies. We theoretically, numerically and experimentally demonstrate the achievement of multiple functionalities by the proposed designed metastructure, which produces extraordinary reflection at different angles and different acoustic focusing localizations under the same normal incidence. Our finding paves the way towards multifunctional compact acoustic devices and can lead to pragmatic contemporary applications such as multiple beam shaping, or functional device with special dispersion property.