High Acoustic Attenuation Research Articles

High Acoustic Impedance and Attenuation Backing for High-Frequency Focused P(VDF-TrFE)-Based Transducers.

Backing materials with tailored acoustic properties are beneficial for miniaturized ultrasonic transducer design. Whereas piezoelectric P(VDF-TrFE) films are common elements in high-frequency (>20 MHz) transducer design, their low coupling coefficient limits their sensitivity. Defining a suitable sensitivity-bandwidth trade-off for miniaturized high-frequency applications requires backings with impedances of >25 MRayl and strongly attenuating to account for miniaturized requirements. The motivation of this work is related to several medical applications such as small animal, skin or eye imaging. Simulations showed that increasing the acoustic impedance of the backing from 4.5 to 25 MRayl increases transducer sensitivity by 5 dB but decreases the bandwidth, which nevertheless remains high enough for the targeted applications. In this paper, porous sintered bronze material with spherically shaped grains, size-adapted for 25-30 MHz frequency, was impregnated with tin or epoxy resin to create multiphasic metallic backings. Microstructural characterizations of these new multiphasic composites showed that impregnation was incomplete and that a third air phase was present. The selected composites, sintered bronze-tin-air and sintered bronze-epoxy-air, at 5-35 MHz characterization, produced attenuation coefficients of 1.2 and >4 dB/mm/MHz and impedances of 32.4 and 26.4 MRayl, respectively. High-impedance composites were adopted as backing (thickness = 2 mm) to fabricate focused single-element P(VDF-TrFE)-based transducers (focal distance = 14 mm). The center frequency was 27 MHz, while the bandwidth at -6 dB was 65% for the sintered-bronze-tin-air-based transducer. We evaluated imaging performance using a pulse-echo system on a tungsten wire (diameter = 25 μm) phantom. Images confirmed the viability of integrating these backings in miniaturized transducers for imaging applications.

Comparison of Pulsed Electroacoustic Measurements and AF-NUMIT3 Modeling of Polymers Irradiated with Monoenergetic Electrons

Successful spacecraft design and charging mitigation techniques require precise and accurate knowledge of charge deposition profiles. This paper compares models of charge deposition and transport using a venerable deep dielectric charging code, AF-NUMIT3, with direct measurements of charge profiles via pulsed electroacoustic (PEA) measurements. Eight different simulations were performed for comparison to PEA experiments of samples irradiated by 50 or 80 keV monoenergetic electrons in vacuum and at room temperature. Two materials, polyether-ether ketone (PEEK) and polytetrafluoroethylene (PTFE), were chosen for their very low conductivities so that minimal charge migration would occur between irradiation and PEA measurements. PEEK was found to have low acoustic attenuation, while PTFE has high acoustic attenuation through the sample thicknesses of 125 and for each material. The measurements were directly compared to AF-NUMIT3 simulations to validate aspects of the code and to investigate the importance of various simulation options, as well as to characterize the PEA instrumentation, measurement methods, and signal processing used. The measurement and simulation values for magnitude of charge deposition, penetration depth, and charge deposition spatial profiles are largely in agreement, though spatial and temporal distributions in incident electron flux and effects of radiation-induced conductivity (RIC) and delayed RIC during the deposition process complicate the process. This work provides an experimental validation of the AF-NUMIT3 deep dielectric charging code and insight into the accuracy and precision of the PEA method.

Research on the sensor for detection of carburized case depth based on nonlinear ultrasound

Carburized case depth may be characterized by using nonlinear ultrasound. However, the excitation and receiving sensors are difficult to control in the same conditions in each experiment and the operation process is complicated, therefore, a specialized sensor needs to be developed. In this work, the influence of the backing materials with different constituents and thickness on the characteristics of sensors is investigated. The structure of the specialized sensor is designed, applied, and verified in the detection of carburized case depth based on nonlinear ultrasound. The results show that the backing layer consists of three-dimensional graphene (3DG) 1wt%/tungsten powder 80wt%/epoxy resin has high acoustic impedance and attenuation characteristics, the backing layer with thicknesses of 8 mm has high sensitivity and residual vibration suppression, the specialized senor simplifies the detection procedure, and improves the efficiency and stability of detection.

Effect of probe pulse duration in picosecond ultrasonics

Picosecond ultrasonics is a powerful tool for nanoscale metrology, giving access to dimensions and mechanical, thermal, and optical properties of nanomaterials. By monitoring the temporal evolution of the interaction of light with coherent acoustic phonons, also known as Brillouin oscillations, phonon lifetime and optical absorption can be measured. However, the extraction of these quantities can be inaccurate due to the common assumption of the infinite coherence length of probe pulses. Here, we demonstrate the effect of probe pulse duration on picosecond ultrasonic measurements numerically and experimentally. We establish a model that shows how the probe coherence length affects the measured signal loss and how we can overcome this limitation and measure an upper limit of the acoustic attenuation factor. The model is verified experimentally on a GaAs bulk substrate by varying the probe pulse duration, showing a strong effect for sub-100 fs pulses. Finally, we applied to CH3NH3PbBr3, where we reveal a high acoustic attenuation factor, which is in line with recent claims of strong anharmonicity in halide perovskites.

Predicting ablation zones with multislice volumetric 2-D magnetic resonance thermal imaging

Background High-intensity focused ultrasound (HIFU) serves as a noninvasive stereotactic system for the ablation of brain metastases; however, treatments are limited to simple geometries and energy delivery is limited by the high acoustic attenuation of the calvarium. Minimally-invasive magnetic resonance-guided robotically-assisted (MRgRA) needle-based therapeutic ultrasound (NBTU) using multislice volumetric 2-D magnetic resonance thermal imaging (MRTI) overcomes these limitations and has potential to produce less collateral tissue damage than current methods. Objective To correlate multislice volumetric 2-D MRTI volumes with histologically confirmed regions of tissue damage in MRgRA NBTU. Methods Seven swine underwent a total of 8 frontal MRgRA NBTU lesions. MRTI ablation volumes were compared to histologic tissue damage on brain sections stained with 2,3,5-triphenyltetrazolium chloride (TTC). Bland-Altman analyses and correlation trends were used to compare MRTI and TTC ablation volumes. Results Data from the initial and third swine’s ablations were excluded due to sub-optimal tissue staining. For the remaining ablations (n = 6), the limits of agreement between the MRTI and histologic volumes ranged from −0.149 cm3 to 0.252 cm3 with a mean difference of 0.052 ± 0.042 cm3 (11.1%). There was a high correlation between the MRTI and histology volumes (r 2 = 0.831) with a strong linear relationship (r = 0.868). Conclusion We used a volumetric MRTI technique to accurately track thermal changes during MRgRA NBTU in preparation for human trials. Improved volumetric coverage with MRTI enhanced our delivery of therapy and has far-reaching implications for focused ultrasound in the broader clinical setting.

Improvement in Accuracy of a Multi-Joint Robotic Ultrasonic Inspection System for the Integrity of Composite Structures

Composite materials have attracted significant attention with regard to the manufacturing of structures that require weight reduction, such as automobiles and aircraft, because they are more resistant to corrosion and fatigue than conventional metal materials. However, such materials exhibit a reliability degradation problem, i.e., their mechanical and physical properties deteriorate due to the occurrence of delamination and voids. Ultrasonic inspection methods have been widely applied for nondestructive detection of such defects in structures; however, the application of these approaches has been impeded due to high anisotropy and acoustic attenuation. In addition, the existing ultrasonic inspection methods require considerable time and cost for the inspection of large materials or structures. These problems were addressed in this study by developing an automatic ultrasonic inspection system; this was achieved by adopting a squirter-type water injection device, which uses a multi-joint robot and the through-transmission ultrasonic method. In addition, a software program to correct axis misalignment was developed and verified to solve the deterioration in defect detectability and accuracy that was caused by axis misalignment, which may occur during the use of the developed system. This development was accomplished after measuring the coordinates of the deformed mechanical part using a three-dimensional laser measuring instrument.

How Can Active Exhaust Systems Contribute to the Reduction of CO <sub>2</sub> Emission and Comply with Future Pass-By Noise Limits?

<div class="section abstract"><div class="htmlview paragraph">The pass-by noise limits of passenger vehicles according to ISO 362 / R51.03 [<span class="xref">1</span>, <span class="xref">2</span>] will be further reduced by 2 dB in 2024 in Europe. Since the pass-by noise is substantially influenced by exhaust noise, the effort for the exhaust system needs to be increased substantially. This results in systems with larger mufflers or higher backpressure. However, the more stringent CO<sub>2</sub>-emission targets require ever more efficient powertrains, which calls for rather lower backpressure to optimize the engine design.</div><div class="htmlview paragraph">This paper describes how compact active exhaust lines can support a design for low backpressure and high acoustic attenuation at the same time. For two passenger vehicles with gasoline engines active exhaust lines are investigated in detail and the results are compared to the series production exhaust lines. Thus, in one exemplary case, the pass-by noise of a limousine could be reduced from 70 dB(A) to 68 dB(A) without any change in the vehicle design except the improved exhaust system. In a second example, the backpressure of a passenger vehicle was reduced by > 100 mbar and at the same time a lower tailpipe noise could be achieved with essentially the package space.</div></div>

Enhancement of Neurite Outgrowth by Warming Biomaterial Ultrasound Treatment.

Ultrasound is a method for enhancing neurite outgrowth because of its thermal effect. In order to reach the working temperature to enhance neurite outgrowth, long-time treatment by ultrasound is necessary, while acknowledging that the treatment poses a high risk of damaging nerve cells. To overcome this problem, we developed a method that shortens the ultrasonic treatment time with a warming biomaterial. In this study, we used Fe3O4 nanoparticle-embedded polycaprolactone (PCL) as a sonosensitized biomaterial, which has an excellent heating rate due to its high acoustic attenuation. With this material, the ultrasonic treatment time for enhancing neurite outgrowth could be effectively shortened. Ultrasonic treatment could also increase neuronal function combined with the warming biomaterial, with more promoter neuronal function than only ultrasound. Moreover, the risk of overexposure can be avoided by the use of the warming biomaterial by reducing the ultrasonic treatment time, providing better effectiveness.

Phased array ultrasonic testing of micro-flaws in additive manufactured titanium block

While titanium components manufactured by additive manufacturing have been widely used in direct molding of complex components, their performance is strongly affected by existing internal flaws generated in the unique manufacturing process. Thus, how to efficiently and accurately characterize geometrical characteristics of internal flaws is critical for enhancing applications of additive manufactured titanium components. In the present work, an effective non-destructive method by using phased array ultrasonic testing is proposed to characterize sub-millimeter artificial deep bottom holes in additive manufactured TC18 titanium block. Specifically, a phased array ultrasonic testing platform integrated with total focusing method-based post-processing algorithm is established. Flat bottom holes with a diameter of 0.8 mm and a depth of 5.0 mm in 55 mm-sized cube titanium block are detected using both linear and annular array transducers. Experimental results show that pre-existing holes can be characterized by both linear and annular transducers, in despite of accompanied high acoustic attenuation. Furthermore, the annular phased array ultrasonic testing has higher detection accuracy and resolution than the linear phased array one, for its stronger capability of sound field focusing. More importantly, the annular phased array ultrasonic testing shows similar high testing accuracy in different relative orientations between forming orientation of the titanium component and sound wave propagation direction. These findings provide an effective strategy for the non-destructive ultrasonic testing of titanium components by additive manufacturing.

Acoustic doppler velocimeter backscatter for suspended sediment measurements: Effects of sediment size and attenuation

Use of backscatter output from a 10-MHz acoustic Doppler velocimeter (ADV) for quantifying suspended sediment concentration was investigated in a laboratory setting. The ADV was immersed in a 125-liter solid polycarbonate circulation tank in which known concentrations of non-cohesive quartz sediments (63–500 µm size range) were mixed in freshwater to obtain homogeneous solutions. Six different size fractions under a wide range of concentration conditions up to 10 kgm−3 were used in the tests. Empirical relations between the acoustic backscatter strength and concentration for each sediment size fraction revealed that there are size-dependent upper and lower bounds on measurable concentrations. Above or below an optimum number of particles in suspension, the performance degrades due to insufficient scatterers which results in a lower bound, and high acoustic attenuation which results in an upper bound on concentration. Both upper and lower bounds increased with increasing sediment concentration, providing, in general, a wider applicability range for larger particles. The range of measurable conditions was broad enough to make the approach useful for sand sizes and concentrations commonly encountered in nature. Effect of sound attenuation was also examined in detail by correcting the acoustic backscatter for transmission losses due to spreading, absorption of energy by water and attenuation due to suspended sediments. The results showed that the effect of transmission losses varies significantly depending on sediment size and concentration, which especially becomes important at high concentrations. This is the first time this effect is quantified to show the direct impact of signal attenuation on suspended sediment measurements using an ADV.

Synthetic Polymer Aerogels in Particulate Form.

Aerogels have been defined as solid colloidal or polymeric networks of nanoparticles that are expanded throughout their entire volume by a gas. They have high surface areas, low thermal conductivities, low dielectric constants, and high acoustic attenuation, all of which are very attractive properties for applications that range from thermal and acoustic insulation to dielectrics to drug delivery. However, one of the most important impediments to that potential has been that most efforts have been concentrated on monolithic aerogels, which are prone to defects and their production requires long and costly processing. An alternative approach is to consider manufacturing aerogels in particulate form. Recognizing that need, the European Commission funded “NanoHybrids”, a 3.5 years project under the Horizon 2020 framework with 12 industrial and academic partners aiming at aerogel particles from bio- and synthetic polymers. Biopolymer aerogels in particulate form have been reviewed recently. This mini-review focuses on the emerging field of particulate aerogels from synthetic polymers. That category includes mostly polyurea aerogels, but also some isolated cases of polyimide and phenolic resin aerogels. Particulate aerogels covered include powders, micro granules and spherical millimeter-size beads. For the benefit of the reader, in addition to the literature, some new results from our laboratory concerning polyurea particle aerogels are also included.

Numerical simulation and laboratory measurements on an open tunable acoustic barrier

A new open, thin and low frequency acoustic barrier is presented. These barriers, based on arrays of isolated pickets produce high acoustic attenuation in a selective range of frequencies related to their geometry and distribution. These open barriers are acoustically competitive with traditional ones, which are based on continuous and rigid materials. To show its versatility to in attenuating different selected ranges of frequencies, a compact numerical model is presented.Different cases are analysed and compared with experimental results. The accuracy of the experimental results compared to the simulated ones allow us to use the compact model to design these barriers in order to reduce both industrial and traffic noise on demand and to introduce them into the noise control market.

Vibro-acoustic model of an active aircraft cabin window

This paper presents modeling and design of an active structural acoustic control (ASAC) system for controlling the low frequency sound field transmitted through an aircraft cabin window. The system uses stacked piezoelectric elements arranged in a manner to generate out-of-plane actuation point forces acting on the window panel boundaries. A theoretical vibro-acoustic model for an active quadruple-panel system is developed to characterize the dynamic behavior of the system and achieve a good understanding of the active control performance and the physical phenomena of the sound transmission loss (STL) characteristics. The quadruple-panel system represents the passenger window design used in some classes of modern aircraft with an exterior double pane of Plexiglas, an interior dust cover pane and a glazed dimmable pane, all separated by thin air cavities. The STL characteristics of identical pane window configurations with different piezoelectric actuator sets are analyzed. A parametric study describes the influence of important active parameters, such as the input voltage, number and location of the actuator elements, on the STL is investigated. In addition, a mathematical model for obtaining the optimal input voltage is developed to improve the acoustic attenuation capability of the control system. In general, the achieved results indicate that the proposed ASAC design offers a considerable improvement in the passive sound loss performance of cabin window design without significant effects, such as weight increase, on the original design. Also, the results show that the acoustic control of the active model with piezoelectric actuators bonded to the dust cover pane generates high structural vibrations in the radiating panel (dust cover) and an increase in sound power radiation. High active acoustic attenuation can be achieved by designing the ASAC system to apply active control forces on the inner Plexiglas panel or dimmable panel by installing the actuators on the boundaries of one of the two panels. In some cases, increasing the actuator numbers in the structure advances the active control performance by controlling more structural modes; however, this decreases the STL of the passive control system because of the increase in structure-borne sound transmission paths of the stiffer piezoelectric actuators.

Hierarchical Assembly of Tungsten Spheres and Epoxy Composites in Three-Dimensional Graphene Foam and Its Enhanced Acoustic Performance as a Backing Material.

Backing materials play important role in enhancing the acoustic performance of an ultrasonic transducer. Most backing materials prepared by conventional methods failed to show both high acoustic impedance and attenuation, which however determine the bandwidth and axial resolution of acoustic transducer, respectively. In the present work, taking advantage of the structural feature of 3D graphene foam as a confined space for dense packing of tungsten spheres with the assistance of centrifugal force, the desired structural requirement for high impedance is obtained. Meanwhile, superior thermal conductivity of graphene contributes to the acoustic attenuation via the conversion of acoustic waves to thermal energy. The tight contact between tungstate spheres, epoxy matrix, or graphene makes the acoustic wave depleted easily for the absence of air barrier. The as-prepared 3DG/W80wt%/epoxy film in 1 mm, prepared using ∼41 μm W spheres in diameter, not only displays acoustic impedance of 13.05 ± 0.11 MRayl but also illustrates acoustic attenuation of 110.15 ± 1.23 dB/cm MHz. Additionally, the composite film exhibits a high acoustic absorption coefficient, which is 94.4% at 1 MHz and 100% at 3 MHz, respectively. Present composite film outperforms most of the reported backing materials consisting of metal fillers/polymer blending in terms of the acoustic impedance and attenuation.

Temperature and pressure effects on elastic properties of lead magnesium niobate-lead titanate relaxorferroelectric material

Lead magnesium niobate—lead titanate (PMN-PT) exhibits exceptional electromechanical properties, considered as a highly efficient transduction material for vibration energy harvesting and acoustic sensing applications. It is reported in the literature that the PMN-PT undergoes structural phase transitions with changes in temperature and the chemical composition. We seek to gain insight into the phase diagram of PMN-PT using temperature and pressure dependence of the elastic properties. Single crystal PMN-PT with chemical composition close to the morphotropic phase boundary (MPB) was used in a resonant ultrasound spectroscopy (RUS) study performed in the temperature range from room temperature to 773 K and the pressure range from near vacuum to 3.4 MPa. At atmospheric pressure, significantly high acoustic attenuation of the PMN-PT material is observed at the temperatures below 400 K. Strong stiffening is observed in the temperature range of 400 K–673 K, followed by a gradual softening at higher temperatures. With the varying pressure, we observed an increased pressure sensitivity of elastic properties of the PMN-PT material that can be localized to the temperature regime where the strong stiffening is observed. As time allows, the behavior of PMN-PT upon cooling below room temperature will also be discussed.

Noise reduction in ultrasonic computerized tomography by preprocessing for projection data

In this study, an ultrasonic computerized tomography (CT) using time-of-flights (TOFs) has been used for the nondestructive inspection of steel billets with high acoustic attenuation. One of the remaining problems of this method is noise in CT images, which makes it difficult to distinguish defects from noise. Conventionally, noise is suppressed by a low-pass filter (LPF) in the process of filtered back projection (FBP). However, it has been found that there is residual noise even after filtering. To cope with this problem, in this study, the noise observed in ultrasonic testing was examined. As a result, it was found that the TOF data used for CT processing contains impulse noise, which remains in the CT image even after filtering, owing to the existence of transducer directivity. To remove impulse noise selectively, we propose a noise reduction technique for ultrasonic CT for steel billet inspection, that is, preprocessing (outlier detection and removal) of TOF data. The performance of the proposed technique was evaluated experimentally. The obtained results suggest that the proposed technique can remove impulse noise selectively and markedly improve the quality of the CT image. Hence, the proposed technique can improve the performance of ultrasonic CT for steel billet inspection.

Use of AE Testing Data for Condition Monitoring in Wind Turbine Gearbox

Acoustic emission (AE) method of nondestructive check is based on exertion wave radiation and their registration during fast local material structure reorganization. It is used as a means of analysis of materials, constructions, productions control and diagnosis during operating time. In the article, it is applied to structural health monitoring of Wind Turbine Gearbox (WTG). Acoustic emission testing has been used for years to test metallic structures. More recently it has become the primary method of testing WTG; all are present in the failure of WTG. AE has been very successful at detecting all of these failure mechanisms and sometimes identifying them from amplitude analysis of the AE signals. However in large structures, the high acoustic attenuation in WTG precludes amplitude analysis unless the origin of the individual signals can be identified and corrections for the distances traveled applied to the signal amplitudes. The usual method of testing WTG structures has been to apply an array of sensors spaced so that a moderate amplitude AE signal occurring midway between them will just barely trigger each sensor.

High temperature resonant ultrasound spectroscopy study on Lead Magnesium Niobate—Lead Titanate (PMN-PT) relaxor ferro-electric material

Lead magnesium niobate-lead titanate [(1-x)PbMg1/3Nb2/3O3-xPbTiO3] is a perovskite relaxor ferroelectric material exhibiting superior electromechanical coupling compared to the conventional piezoelectric materials. In this work, non-poled single crystal PMN-PT material with the composition near morphotropic phase boundary (MPB) was investigated in the temperature range of 400 K—800 K where the material is reported to be in the cubic phase. High temperature resonant ultrasound spectroscopy (HT-RUS) technique was used to probe temperature dependency of elastic constants derived from the measured resonant modes. Non-monotonic resonant frequency trends in the temperature regime indicate stiffening of the material, followed by gradual softening typically observed in heated materials. Elastic constants confirmed this stiffening in the temperature range of 400 K—673 K, where the stiffness constants C11 and C44 increased approximately by 40% and 33% respectively. Acoustic attenuation, derived from the quality factor (Q), exhibits a minimum around the temperature where the stiffness is maximum and, significantly higher attenuation observed at temperatures below 400 K. The temperature range 395 K—405 K was identified as a transition temperature range, where the material showed an abrupt change in the resonant spectrum and, the material emerges from the MPB characterized by this very high acoustic attenuation. This transition temperature compares favorably with dielectric constant measurements reported in the literature.

Ultrasonic test of carbon composite/copper joints in the ITER divertor

The vertical targets of the ITER divertor consist of high flux units (HFU) actively cooled: CuCrZr tubes armoured by tungsten and carbon/carbon fibre composite (CFC). The armour is obtained with holed parallelepiped blocks, called monoblocks, previously prepared and welded onto the tubes by means diffusion bonding. The monoblock preparation consists in the casting of a layer of copper oxygen free (Cu OFHC) inside the monoblock hole.Each HFU is covered with more than 100 monoblocks that have to be joined simultaneously to the tube. Therefore, it is very important to individuate any defects present in the casting of Cu OFHC or at the interface with the CFC before the monoblocks are installed on the units.This paper discusses the application of non-destructive testing by ultrasound (US) method for the control of the joining interfaces between CFC monoblocks and Cu OFHC, before the brazing on the CrCrZr tube.In ENEA laboratory an ultrasonic technique (UT) suitable for the control of these joints with size and geometry according to the ITER specifications has been developed and widely tested. Real defects in this type of joints are, however, still hardly detected by UT. The CFC surface has to be machined to improve the mechanical strength of the joint. This results in a surface not perpendicular to the ultrasonic wave. Moreover, CFC is characterized by high acoustic attenuation of the ultrasonic wave and then it is not easy to get information regarding the Cu/CFC bonding. Nevertheless, the UT sharpness and simplicity pushes to perform some further study. With this purpose, a sample with artificial defects induced on the CFC/Cu interface during the Cu casting, has been manufactured and investigated, both by pulse-echo technique and by different techniques. The results obtained by the UT on this sample are reported and compared with X-ray method.

Non-destructive methods for the defect detection in the ITER high heat flux components

This paper discusses the application of non-destructive testing (NDT) by ultrasonic technique for the control of the joining interfaces of the ITER divertor vertical target plasma facing units. The defect detection capability has to be proved for both metal to metal and metal to carbon/carbon fibre composite (CFC) joints because these two types of joints have to be realized for the manufacturing of the high heat flux units. In this paper the UT results coming from the investigation performed during the manufacturing, but also after the thermal fatigue testing (up to 20 MW/m 2) of six mock-ups manufactured using the Hot Radial Pressure technology (HRP) in ENEA labs are presented and compared with the evidences from the final destructive examination. Regarding the Cu/CFC joint, the effectiveness of the ultrasonic test has been deeply studied due to the high acoustic attenuation of CFC to ultrasonic waves. To investigate the possibility to use the ultrasonic technique for this type of joint, an ‘ad hoc’ flat Cu/CFC joint sample, that reproduces the actual annular joint interfaces, was manufactured. This flat sample has the advantage of being easily tested by probes with different geometry and frequency. UT results are compared with X-ray and eddy current testing of the same sample.