Underwater Acoustic Materials Research Articles

Hydrostatic pressure effects in periodically structured polyurethane acoustic tiles

Traditional underwater acoustic materials commonly make use of periodic air voids to improve their ability to absorb or deflect sound through wave-mode conversion and scattering mechanisms. These air voids have the potential to deform or completely collapse under hydrostatic pressure loading, which may significantly affect the material’s acoustic performance. This study investigates the effects of hydrostatic pressure on the acoustic response of two different types of fabricated polyurethane samples. Developed numerical finite element simulations help identify the main factors associated with the acoustic pressure dependence of such tiles. The results of this work may aid in the development of pressure resistant materials that can better maintain acoustic performance at depth.

3D underwater acoustic Luneburg lens based on gradient face-centered-cubic phononic crystals

A Luneburg lens is a gradient refractive index lens that can focus plane waves on a point at the perimeter without aberration. Three-dimensional (3D) Luneburg lens for airborne sound has been well investigated in recent years. However, constructing a 3D Luneburg lens for underwater sound is a challenging task due to the difficulties in the designing and fabricating of the desired isotropic underwater acoustic materials. This work presents the practical implementation of a 3D Luneburg lens for underwater sound. Such a 3D Luneburg lens is designed based on 3D gradient face-centered-cubic phononic crystals, which have quasi-isotropic refractive index patterns and can be fabricated with photosensitive resin by 3D printing. The experimental results show that the lens can realize the omnidirectional imaging of underwater sound from 30 to 38 kHz. This 3D underwater acoustic Luneburg lens may prompt the potential applications in underwater acoustic wide-angle retroreflectors, sonars, and biomedical imaging devices.

A review of underwater acoustic metamaterials for underwater acoustic equipment

Researchers use underwater acoustic equipment to explore the unknown ocean environment, which is one of the important means to understand and utilize the ocean. For underwater acoustic equipment, the application of underwater acoustic metamaterials is the premise to ensure and improve the performance of underwater acoustic communication, acoustic stealth, and sonar detection. Due to the limitations of mass density law and high hydrostatic pressure, traditional underwater acoustic materials cannot effectively absorb low-frequency sound waves and have low efficiency of elastic energy conversion. The sound absorption effect is poor under low frequency and high hydrostatic pressure. In recent years, with the development of acoustic metamaterials technology, all kinds of underwater acoustic metamaterials have also been proposed. Compared with sound waves propagating in the air, underwater sound is more difficult to control than air sound with the same frequency, so the design of underwater acoustic metamaterials is more complicated. This paper reviews the basic characteristics, development history of sound absorption, sound insulation decoupling, and underwater acoustic guided metamaterials, then the existing problems and the future development direction of underwater acoustic metamaterials are discussed.

Review of underwater sound absorption materials

Underwater sound absorption is becoming more and more important for underwater military equipment. Underwater acoustic stealth technology is constantly developing, we have conducted a research, summary and classification of underwater sound stealth materials. There are mainly four kinds of underwater acoustic stealth materials: porous materials, resonance materials, piezoelectric materials and metamaterials. We also studied the sound stealth mechanism of these materials.

A review of underwater acoustic metamaterials

Underwater acoustic materials are the key to the noise and vibration reduction, stealth, reception and amplification of underwater acoustic signal. However, conventional underwater acoustic materials usually suffer from the weakness of great bulk, heavy weight and poor resistance to hydrodynamic pressure. The advancement of the acoustic metamaterials (AMMs) has provided new insights into fixing these problems, and a variety of underwater AMMs are proposed accordingly. This article reviews the history and the developments of underwater AMMs and the difficulties that existed in the developments of underwater AMMs as compared with the AMMs in air. The focuses are the three types of AMMs which are closely associated with the underwater acoustic stealth and signal reception & amplification of hull-mounted sonar, i.e., the AMMs for sound absorption, AMMs for decoupling and those for sound focusing. The classification, physical mechanisms and controlling effects of those three types of underwater AMMs are reviewed in details. The AMMs for underwater sound absorption can be divided into local-resonance (LR) and non-resonance types. The LR types take advantages of the local resonance principle with the controlling wavelengths orders of magnitude greater than the physical dimension of a unit. To overcome the intrinsic narrow-band limitations of LR and broaden the bandwidths, efforts have been made to utilize multilayer LR-AMMs or multi-size LRs, attach/embed elastic plate scatter on/into elastically coated plate, or introduce complex lattices with multiple (coupled) LRs, etc. The non-resonance types are based-on non-resonance mechanisms such as viscous loss, heat conduction, macromolecular relaxation or a combination of them. These AMMs mainly include porous metals and nonmetal materials, gradient AMMs and acoustic meta-surfaces, etc. The AMMs for underwater decoupling mainly use acoustically soft AMMs to decouple the vibration of the base structure from adjacent fluid. These AMMs contain the acoustic coating embedded with various cavities or local resonators, gradient coating, re-entrant honeycomb structure or structure with negative Poisson’s ratio and chiral porous coating, etc. The sound-focusing AMMs are categorized into diffractive, refractive and reflective types. The diffractive types focus underwater sound through constructive interference of diffracted waves, which mainly include all kinds of zone plates and fractal lens. The refractive types are mainly based on various acoustic lens and focus sound by controlling the refraction index of the materials. The reflective types focus sound by modulating the reflective waves, including the recently-proposed reflective acoustic meta-surfaces and acoustic coatings with attached/embedded periodically distributed signal conditioning plates, etc. Finally, the conclusions and drawbacks of the above AMMs are summarized. It is pointed out that although the present underwater AMMs have achieved some encouraging results, they are still confronted with several key problems such as the realizations of low-frequency and broadband, omnidirectional, subwavelength, lightweight and hydrostatic pressure resistant sound manipulations, the manufacture and testing of large AMM samples, and the environmental adaptations requirements. Therefore, there is still a long way for the current underwater AMM toward the real application. The trend and outlook toward future underwater AMMs are also presented. It is hoped that this review could provide guidelines to the design and realization of future materials demanded for underwater acoustic stealth and detection.

Measurement of the Acoustic Reflection Coefficient of Materials Based on Signal Subspace Decomposition Using a Single Vector Hydrophone

A method is presented for measuring the reflection coefficient of underwater acoustic materials in the free field with a single vector hydrophone. The main advantage of this method is that the incident and reflected signals are separated in the signal subspace without considering the aliasing of direct and reflected signals in the time domain. The postprocessing inverse filter technology is introduced into the measurement. The signal distortion caused by the transfer function of the measurement system is compensated for the postprocessing inverse filter, which is conducive to eliminating the interference of the edge diffraction. The basic mechanism is based on signal subspace decomposition and postprocessing inverse filter technology. Numerical simulations are presented to demonstrate the advantages of the method. The findings are confirmed by an underwater experiment conducted with a single vector hydrophone. The experimental results show that a satisfactory reflection coefficient can be obtained using this method, and the results agree well with the numerical predictions. The relative error of the measurement results is no more than 20%.

The Reflection Coefficients Measurement Method for the Underwater Acoustic Materials Based on the Single Vector Hydrophone

In order to obtain the normal acoustic reflection coefficients of underwater acoustic materials in the free field with the wideband pulse signal, a new measuring method with a single vector hydrophone and the post-processing inverse filtering is put forward. Numerical simulations had been conducted to comparatively analyze the measurement performances of the measuring method. The simulation results show that the present method should be established on the high ratio of signal to the noise and it is not sensitive to the system error. The experiment result shows that the post-processing inverse filtering measuring method is more effective comparing with the method without the post-processing inverse filtering, especially the measurement consequence of the post-processing inverse filtering measurement method tends to the theoretical value when the frequency is higher than 2.5 kHz, because limitation of the transducer.

A low frequency underwater meta structure composed by helix metal and viscoelastic damping rubber

Many underwater acoustic absorption materials have been proposed, but good low frequency acoustic absorption remains a challenge. We report an underwater metastructure with excellent acoustic absorption that is constructed by perforating helixes of metal into viscoelastic damping rubber. Finite element analysis shows that this metastructure can achieve an acoustic absorption coefficient of 0.75 at about 100 Hz with a thickness of only 1/68 wavelength at the same frequency. Compared with a homogeneous viscoelastic rubber material, the addition of the helix structure improves the acoustic absorption ability in the range of 0–1,000 Hz. An analysis of vibration displacement maps indicates that waveform transformation, multiple scattering, and reflection energy dissipation mechanisms are the critical factors affecting the absorption performance. Different geometries and materials can adjust the sound absorption characteristics below 1,000 Hz. The proposed metastructure has the advantages of high acoustic absorption ability, broader frequency bandwidth, and regular geometry, providing more possibilities for underwater acoustic manipulation applications.

Underwater acoustic properties of graphene nanoplatelet-modified rubber

A series of graphene nanoplatelet-modified acrylonitrile-butadiene rubber-based underwater acoustic absorbing materials were prepared. The dynamic mechanical properties, underwater sound absorption properties, differential scanning calorimetry, vulcanization property, and mechanical properties of graphene nanoplatelets/acrylonitrile-butadiene rubber nanocomposites were studied theoretically and experimentally. The results indicated that graphene nanoplatelet-modifiedacrylonitrile-butadiene rubber-based underwater acoustic absorbing materials exhibited excellent damping and underwater sound absorption properties. The storage modulus ([Formula: see text]) and loss modulus ([Formula: see text]) of graphene nanoplatelets/acrylonitrile-butadiene rubber nanocomposites were increased significantly with increasing graphene nanoplatelets content. At a graphene nanoplatelets content of 25 phr, the [Formula: see text] and [Formula: see text] at 15°C improved by 1201 and 603%, respectively. The obvious improvement in [Formula: see text] and [Formula: see text] were mainly attributed to the extremely high interfacial contact area between graphene nanoplatelets and acrylonitrile-butadiene rubber chains and the ultrahigh mechanical properties of graphene nanoplatelets. The underwater sound absorption coefficient ([Formula: see text]) was increased obviously as the graphene nanoplatelets were incorporated. The optimal [Formula: see text] of the nanocomposites was achieved as the graphene nanoplatelets content was 10 phr, and the average value of [Formula: see text] was improved from 0.35 to 0.73—an increase of nearly onefold. The notable improvement in [Formula: see text] was due to the marked increase in damping properties and thermal conductivity of graphene nanoplatelets/acrylonitrile-butadiene rubber nanocomposites. The merits of graphene nanoplatelet-modified underwater acoustic absorbing materials were higher damping, better underwater sound absorption, and better mechanical properties with unaffected density in comparison to other inorganic and rigid fillers or porous fillers.

The Interaction of Resonance And Bragg Scattering Effects for the Locally Resonant Phononic Crystal with Alternating Elastic and Fluid Matrices

Abstract Three-dimensional (3D) locally resonant phononic crystals (LRPCs) are studied with the aim of optimising the sub-wavelength band gaps of such composites. By analysing their effective acoustic properties, it has been found that the effective acoustic speed of the composite will drop to zero when local resonance arise, and will increase monotonically when Bragg scattering effects occur. Moreover, if the matrix is a low-shear-speed medium, local resonators can significantly reduce the effective acoustic speed of the composite and, therefore, lower the frequency where Bragg scattering effects occur. Hence, a specific LRPC with alternating elastic and fluid matrices is proposed, whose resonance and Bragg gaps are already close in frequency. The fluid matrix behaves as a wave filter, which prevents the shear waves from propagating in the composite. By using the layer-multiple-scattering theory, the coupling behaviour of local resonance and Bragg scattering band gaps has been investigated. Both gaps are enhanced when they move closer to each other. Finally, a gap-coupled case is obtained that displays a broad sub-wavelength band gap. Such proposal excels at the application of underwater acoustic materials since the arrangement of structure can be handily adjusted for tuning the frequency of coupled gap.

A Parametric Analysis on the Sound Absorbing Performance of the Locally Resonant Sonic Material Using a Mass-Damper-Spring Model

The locally resonant sonic material (LRSM) is a kind of structural composite. Such composite typically consists of an elastic matrix periodically embedded with metallic spheres, which are coated with soft rubber. Owing to its capability of controlling the low frequency sound, the LRSM has a promising prospect in the application of underwater acoustic materials. This paper proposes a mass-damper-spring model to explain the sound absorbing mechanism of the LRSM, and derives analytical formulae to evaluate the absorbing performance. After reasonable simplification, the analytical formulae can intuitively illustrate the relationship between the absorbing performance and the parameters of the LRSM. The correctness of the physical model was verified by comparing the analytical evaluation with the numerical result calculated by the layer-multiple-scattering method. The result shows that the sound absorption of the LRSM is induced by the energy dissipation of the damped local resonator subjected to excitations. The influence of the parameters on the absorbing performance of the LRSM is analysed systematically. It is shown that a resonator with a heavier core and a stiffer coat can produce a better sound absorbing performance.

Effect of nanostructures of modified clay–carbon black on physico‐mechanical, electrical, and acoustic properties of elastomer‐based composites

Elastomeric composites based on nitrile rubber (NBR), carbon black (CB), and organically modified nanoclay (NC) were prepared using a laboratory two‐roll mixing mill. Influences of the hybrid filler system (CB+NC) on various properties of NBR compound were analyzed. It was found that the addition of hybrid filler (CB+NC) over only carbon black enhances various properties. It was also found that the addition of nanoclay to the rubber matrix effectively improved key properties. Acoustics and electrical properties were modified with reduced water absorption because of layered clay platelets. The lower volume resistivity of NBR composites reflected better electrical conductivity attributed to the presence of nanoclay leading to effective filler connectivity. X‐ray diffraction and transmission electron microscopy measurements revealed that nanoclays were mostly intercalated and were uniformly dispersed. Use of calcium stearate facilitated dispersion of nanoclay in the rubber matrix which was observed through the formation of nanostructures including “nano” and “halo” units. Time temperature superposition in dynamic mechanical analysis test of the composites indicated lower mechanical loss in the frequency range of interest. The advantages accruing due to overall property enhancement, including lower water absorption, and better electrical and excellent acoustic properties of NBR composites make it suitable as underwater acoustic transparent materials for transducer encapsulation application. POLYM. COMPOS., 37:1786–1796, 2016. © 2014 Society of Plastics Engineers

펄스 튜브를 이용한 수중 음향 성능 측정 시스템 개발

Underwater acoustic materials are installed in order to reduce reflection, transmission and radiation of an underwater structure. The acoustic performance of the materials should be evaluated in accurately-controlled environment in terms of temperature and static pressure. In this paper, two pulse tubes, which are equipped with temperature and pressure controllers, are designed and developed to evaluate echo reduction(ER) and transmission loss(TL) of underwater acoustic materials. The procedures of the evaluation are suggested and the validation is carried out by comparing theoretical values to experimental results for a simple stainless steel specimen and free surface. In result, it is validated that developed pulse tubes are able to measure ER and TL with 2 dB tolerance.

Influence of Dynamic Mechanical Parameters on the Sound Insulation Properties of Underwater Acoustic Materials

In order to study the effect of dynamic mechanical parameters of the underwater acoustic materials on sound insulation properties, the underwater acoustic materials composed of both viscoelastic rubber with different blend ratio are made. The results showed that dynamic mechanical parameters of underwater acoustic materials have certain influence on the sound insulation properties. In order to obtain better sound insulation properties, the loss factor and the storage modulus are should be optimized. The sound insulation performance of the underwater acoustic materials made in the experiment in the loss fator rang 0.2 to 0.3 and the storage modulus lower than 20MPa is better.

The synthetic kinetics and underwater acoustic absorption properties of novel epoxyurethanes and their blends with epoxy resin

Reaction kinetics between isocyanate-terminated prepolyurethane (PPU) and glycidol using dibutyltin dilaurate (DBTDL) as a catalyst was investigated by monitoring the change in the intensity of the absorbance peak of NCO stretching band at 2,270 cm−1 on Fourier transform infrared spectrum at different temperatures. The results indicated that the reactions of TDI- and IPDI-type PPU with glycidol followed second-order kinetics, and their activation energies could be efficiently reduced by DBTDL. For TDI-type PPU, the reaction activation energies were 80.37 kJ mol−1 without catalyst, 49.86 kJ mol−1 with 0.1 % of DBTDLs, and 37.85 kJ mol−1 with 0.2 % of DBTDLs, respectively. For IPDI-type PPU, the reaction activation energies were 69.16 kJ mol−1 without catalyst, 63.05 kJ mol−1 with 0.1 % of DBTDLs, and 55.57 kJ mol−1 with 0.2 % of DBTDLs, respectively. This corresponding TDI- and IPDI-type epoxyurethane (EPU) were blended with epoxy resins (EPs) and cured by the Michael adduct of ethlylenediamine with butyl acrylate (molar ratio = 1:1) curing agent, to prepare EPU/EP blend elastomers for underwater acoustic absorption materials. The TDI-type EPUs had good acoustic absorption properties and the average acoustic absorption coefficient of TDI-type EPU was 0.75, the maximum acoustic absorption coefficient was 0.94; the EPUs blended with E-51 EP had better acoustic absorption properties than those from E-44; and the EPU from PPG-2000 had better underwater acoustic absorption properties than that from PPG-1000.

Evaluation of sound velocity inside underwater acoustic materials using test panel acoustic measurements

Acoustic materials can be used as external coating to reduce radiated noise or target strength of immersed structures, or to improve performance of sonar systems. In order to optimise the design and the use of such coatings, it is necessary to have a good estimate of their intrinsic properties, in particular the complex sound velocity inside the materials. The type of material studied here consists in a viscoelastic slab containing a given proportion of micro-voids. Determination of the complex longitudinal velocity is not an easy task, and direct methods or experimental set-ups do not give sufficient results. The method presented here consists in determining the complex longitudinal velocity by solving an inverse problem, from acoustic measurements of the reflection and transmission coefficients of a test panel in a water tank. Best results, including attenuation, are obtained by over-determining the problem, using all data available. Results are consistent to the known physical behaviour of such materials, regarding influence of frequence and hydrostatic pressure.

On analysis of passive underwater acoustic damping materials

A theoretical approach has been developed to evaluate the noise reduction characteristics of underwater acoustic damping materials. In this study, materials proposed in the literature are considered and analyzed. The analysis model is based on the elastic theory and the transfer function method. The numerical result of a filled rubber materials system is verified using an experimental measurement from the literature for high frequency range (∼7 MHz). However, material suitable for a lower frequency range (∼ 20 kHz) is desired in this study. So acoustic reflective properties of materials consisting of different components are analyzed and discussed.

Passive underwater acoustic damping materials with Rho-C rubber–carbon fiber andmolecular sieves

The effect of damping of underwater acoustic signals by composites with carbon fiber andmolecular sieves of various pore sizes in Conathane EN-1556 based matrix is presented inthis paper. The measurements were made in a water-filled pulse tube set-up for afrequency range of 4–12 kHz. The results show that molecular sieves of pore size3 Å are a better selection than others. Also, molecular sieves in the form of pellets gavegood underwater acoustic damping and processability. With 11% mass loading of3 Å pellets in Conathane EN-1556 RhoC rubber containing 0.25% volume of carbon fiber,an echo reduction of 17–24 dB was obtained for a frequency range of 7–12 kHz.

Low-frequency impedance measuring device for underwater acoustic materials

A laboratory testing device for measuring the direct and transfer impedances of acoustic materials below 1 kHz has been developed. The 3-ft-long test chamber accommodates 4-in.-diam material samples. Excitation of the sample is by means of an impedance head consisting of a piezoelectric tonpilz driver and force gage. Responses of the excitation and of the sample backing plate are sensed by accelerometers. The material can be hydrostatically pressurized up to 500 psia. A unique feature of the device is a water layer that couples the impedance head to the material sample. This layer permits materials to be tested after deformation under hydrostatic loading. Design features and performance characteristics of the testing device are described. Results of calibration tests and measurements on an air-rubber material are presented. [Work supported by the Office of Naval Research.]

Reflection coefficient measurement techniques for underwater acoustic panels

A new measurement method for evaluating underwater acoustic materials is proposed. It is based on a two-hydrophones technique and allows the determination of the complex reflection coefficient for size-limited acoustical panels. The results are shown to be independent of the incident plane wave amplitude and the distance between the hydrophones and the panel. Therefore, this technique involves direct and quick measurements so that it is possible to reduce the effects of scattered waves from the edges of the sample by averaging. Simulations have been made to check the stability of the method versus noise. The experiment covered the frequency range 1–12 kHz. Techniques (intensity, impedance, and pressure measurements) tested at G.E.R.D.S.M. (Groupe d'Etudes et de Recherches en Détection Sous-Marine) using surfacic sensors are also presented. Their accuracies are discussed through a mathematical model based on Helmholtz integral equation. Results obtained by the various methods on finite panels are compared.