High Acoustic Output Research Articles

Characteristics of thermoacoustic conversion and coupling effect at different temperature gradients

The thermoacoustic engine that can convert heat into sound offers a promising methodology of energy usage. However, the low energy conversion rate limited the exploitation of this technology. To obtain high acoustic power and energy outputs, effects of temperature gradients on thermoacoustic characteristics were numerically investigated in this study. The results indicated that temperature gradients had significant impacts on the output sound pressure and heat flux. Meanwhile, complicated coupling effects among the temperature, fluid velocity and sound pressure were observed during the thermoacoustic oscillations. Deep insight into the phase changes implied that the temperature difference would initiate the oscillations of the fluid movement and induce the sound pressure afterwards. The study also demonstrated that large temperature differences and abrupt temperature changes were favorable for the acoustic power output.

Lag-One Coherence as a Metric for Ultrasonic Image Quality.

Reliable assessment of image quality is an important but challenging task in complex imaging environments such as those encountered in vivo. To address this challenge, we propose a novel imaging metric, known as the lag-one coherence (LOC), which leverages the spatial coherence between nearest-neighbor array elements to provide a local measure of thermal and acoustic noise. In this paper, we derive the theory that relates LOC and the conventional image quality metrics of contrast and contrast-to-noise ratio (CNR) to channel noise. Simulation and phantom studies are performed to validate this theory and compare the variability of LOC to that of conventional metrics. We further evaluate the performance of LOC using matched measurements of contrast, CNR, and temporal correlation from in vivo liver images formed with varying mechanical index (MI) to assess the feasibility of adaptive acoustic output selection using LOC feedback. Simulation and phantom results reveal a lower variability in LOC relative to contrast and CNR over a wide range of clinically relevant noise levels. This improved stability is supported by in vivo measurements of LOC which show an increased monotonicity with changes in MI compared to matched measurements of contrast and CNR (88.6% and 85.7% of acquisitions, respectively). The sensitivity of LOC to stationary acoustic noise is evidenced by positive correlations between LOC and contrast ( ) and LOC and CNR ( ) at high acoustic output levels in the absence of thermal noise. Results indicate that LOC provides repeatable characterization of patient-specific trends in image quality, demonstrating feasibility in the selection of acoustic output using LOC and its application for in vivo image quality assessment.

Conformal cymbal array: A broadband, wide beamwidth underwater acoustic projector

The development of “cymbal”-based acoustic source technology in recent years has been an essential advancement in improving NRL's capability for shallow-water acoustics. The cymbal's unique attributes of high acoustic output from a lightweight, thin profile configurable array is a central technology in target identification programs at NRL. This talk will present a review of cymbal array development over the past ten years. It will start with conventional piezoelectric ceramic (PZT) cymbal elements and end with current research on advanced cymbal designs utilizing new formulations of piezoelectric single crystal materials. In particular, the contributions of Kim Benjamin to the initial development work of the conformal array design will be highlighted. [Work sponsored by the Naval Research Laboratory.]

First look: Acoustic calibration of carbon nanotube transducers

Material researchers at the University of Texas at Dallas (UT-D) have recently been reporting on the development of underwater acoustic carbon nanotube (CNT) yarn sheets capable of high acoustic output at low frequencies with broad bandwidth. The principle transduction mechanism for their approach is through thermal acoustic means as opposed to conventional underwater transducers that utilize electromechanical vibrations. This presentation will begin with an overview of the CNTs including the design of a first generation packaging technique that was incorporated for the fabrication of three prototypes. The prototypes were acoustically calibrated in April 2013 at the US Navy acoustic calibration facility in Okahumpka, Florida. The presentation will include measured unbiased and biased transmitting voltage responses (TVRs) and directivity patterns over a two and a half decade band. Final discussions will include on-going research directions for further development. [Work supported by NAVSEA Division Newport.]

High amplitude nonlinear acoustic wave driven flow fields in cylindrical and conical resonators

A high fidelity computational fluid dynamic model is used to simulate the flow, pressure, and density fields generated in a cylindrical and a conical resonator by a vibrating end wall/piston producing high-amplitude standing waves. The waves in the conical resonator are found to be shock-less and can generate peak acoustic overpressures that exceed the initial undisturbed pressure by two to three times. A cylindrical (consonant) acoustic resonator has limitations to the output response observed at one end when the opposite end is acoustically excited. In the conical geometry (dissonant acoustic resonator) the linear acoustic input is converted to high energy un-shocked nonlinear acoustic output. The model is validated using past numerical results of standing waves in cylindrical resonators. The nonlinear nature of the harmonic response in the conical resonator system is further investigated for two different working fluids (carbon dioxide and argon) operating at various values of piston amplitude. The high amplitude nonlinear oscillations observed in the conical resonator can potentially enhance the performance of pulse tube thermoacoustic refrigerators and these conical resonators can be used as efficient mixers.

Optimizing piezoelectric panel speakers using the simulated annealing algorithm

This article focuses on the optimization of piezoelectric panel speakers. Two piezoelectric ceramic plates serve to excite the diaphragm in the speaker. In light of an optimization procedure, the optimal position on the diaphragm to mount the piezoelectric ceramic plates is determined. A finite-element model is established using the energy method, where the electrical system, mechanical system, and acoustic loading of the transducer are considered as a coupled system. The simulated annealing algorithm is exploited to attain a design that enables low fundamental resonance frequency and high acoustic output. Experiments were conducted to verify the numerical model. The experimental results were in good agreement with the numerical prediction, in which the performance of the optimized configuration was found to be significantly improved over the non-optimal design.

Helmholtz-like resonators for thermoacoustic prime movers

In a thermoacoustic prime mover, high acoustic output power can be achieved with a large-diameter stack and with a cavity with a large volume attached at the open end of the resonator containing the stack. The combination of resonator and cavity makes the device Helmholtz-like, with special characteristics of the resonant frequencies and quality factor, Q. Analysis of its acoustic behavior based on a model of a closed bottle presents features that are useful for the development of such prime movers for energy conversion from heat to sound. In particular, the arrangement produces in the cavity a high sound level, which is determined by the Q of the system. Comparison with a half-wave resonator type of prime mover, closed at both ends, shows the advantages of the Helmholtz-like device.

Dynamic micro speaker with dual suspension

The invention relates to a micro-speaker with a second suspension made of highly resilient material installed inside of the speaker. The speaker is able to provide high level acoustic output with low distortion rate in a wide frequency range with a very small and slim structure and it may prevent a disconnection of a lead connecting the voice coil and the electrode. The speaker includes a yoke, a permanent magnet, a plate, a vibration plate integrated with a first suspension, a voice coil, a frame and a protector. The speaker further comprises a second suspension made of highly resilient material installed between the plate and the vibration plate, and is characterized in that the voice coil is attached to the lower surface of the second suspension and the vibration plate is attached to the upper surface of the second suspension and the outer periphery of the second suspension is fixed to the frame.

Whispering in your ear: a recent history of subminiature transducers

For 50 years, the history of hearing aid transducer development could be read in a simple photograph and timeline, where the size of the transducer was inversely correlated to the year it was developed. This size curve corresponds to hearing aid development over the same time period, progressing from body‐worn aids in the 1940s to large behind‐the‐ear (BTE), smaller BTE's, in‐the‐ear (ITE), and successively deeper in‐ the‐canal (ITC) devices through the 1990s. Although size still matters, in the last decade there has been less effort in reducing the size of hearing aids and more on increasing features, usability, and comfort. Development of hearing aid transducers has become similarly multidimensional. This paper charts the progress from the linear smaller‐is‐better design model to lower vibration, higher acoustic output, lower current draw, reduced radio frequency interference, wider bandwidth, and other multifaceted challenges of modern hearing aid transducer design.

Phased arrays and devices for high‐intensity focused ultrasound

Focused ultrasound (FUS) is rapidly attracting attention as an alternative to surgery for the treatment of tumors and other disorders. Such therapeutic procedures were first proposed over 60 years ago, but have only recently become clinically feasible. These new successes can be attributed primarily to two advances: improvement in noninvasive monitoring techniques and the development of high‐power ultrasound arrays. As the field progresses, there is a demand for increased transducer bandwidth, allowing tissue‐specific frequency optimization, and even imaging, while maintaining high acoustic power outputs. These arrays have been made possible via dice‐and‐fill manufactured 1‐3 piezocomposite arrays. Transducers with over 500 elements have been tested in the clinic, and designs have been proposed for at least 5000 elements. Meanwhile, the rapid development of high‐power, broadband IC technology has made it possible to build electronic components for amplifiers and switching at a practical cost (under $50/ch...

Hot topics in biomedical ultrasound: ultrasound therapy and its integration with ultrasonic imaging

Since the development of biomedical ultrasound imaging from sonar after WWII, there has been a clear divide between ultrasonic imaging and ultrasound therapy. While imaging techniques are designed to cause as little change as possible in the tissues through which ultrasound propagates, ultrasound therapy typically relies upon heating or acoustic cavitation to produce a desirable therapeutic effect. Concerns over the increasingly high acoustic outputs of diagnostic ultrasound scanners prompted the adoption of the Mechanical Index (MI) and Thermal Index (TI) in the early 1990s. Therapeutic applications of ultrasound, meanwhile, have evolved from deep tissue heating in sports medicine to include targeted drug delivery, tumor and plaque ablation, cauterization via high intensity focused ultrasound (HIFU), and accelerated dissolution of blood clots. The integration of ultrasonic imaging and therapy in one device is just beginning, but the promise of improved patient outcomes is balanced by regulatory and practical impediments.

Does contrast ultrasonography damage liver cell?

To confirm whether the microbubble activated by sonication causes liver damage, the authors performed two experiments. Experiment 1 followed Shigeta's experiment. As an ultrasound contrast agent (UCA), Levovist was used and sonicated by Acuson Sequoia™ 512 by Siemens. Wister rats of 300 g were divided into four groups: (A) administration of ultrasonic contrast agent (UCA) and ultrasonic exposure, (B) administration of UCA, (C) ultrasonic exposure, and (D) control. Experiment 1 did not show the evidence of hepatic damage such as Shigeta et al. reported. Hence, we conducted Experiment 2 with a much higher acoustic output. But no clear damage was observed in both optical and electron microscopic specimens.

Optimal Implementation of Miniature Piezoelectric Panel Speakers Using the Taguchi Method and Genetic Algorithm

A thin and miniature piezoelectric panel speaker has been developed in this paper. The system is modeled using the energy method in conjunction with the finite element method. The electrical system, mechanical system and acoustic loading are considered in combined fashion. The genetic algorithm (GA) and the Taguchi method are employed to achieve optimal designs with low fundamental frequency and high acoustic output. The designs resulting from the optimization are then implemented and evaluated by experiments. These experiments were carried out on the basis of quantitative and subjective performance measures. The experimental results indicate that the piezoelectric panel speakers are capable of producing comparable acoustic output with significantly less electrical power than the conventional voice-coil panel speakers.

Therapy/imaging array-based system and technology for intense ultrasound surgery

Minimally invasive, miniature (2.2- × 50-mm aperture, 3.3-mm diameter) dual-mode linear arrays have been developed into low-cost disposable probes with high acoustic power output (120 W/cm2 at the source), high transmit efficiency (>65% typical), and good imaging performance (50% fractional bandwidth, >100-mm-deep field of view). These therapy/imaging probes have been integrated into a flexible intense ultrasound surgery platform which also includes conventional diagnostic imaging probes. A system architecture has been developed which includes a 64-channel therapy driver with software selection of array aperture and phasing (λ/16), frequency (0.5–8 MHz), drive amplitude (5 W/channel, nominal), rotational steering (±180 deg), and temporal sequencing/switching of imaging/therapy/monitoring modes. System software includes graphical and text-based script mode control of therapeutic treatment. Real-time monitoring of electric power per channel, temperature sensors, and thermal effects provide a range of feedback and safety. Numerous system and probe technological issues such as electrical interconnect and matching, acoustic coupling, thermal control, and maintaining probe efficiency have been addressed. The array-based imaging/therapy system has produced encouraging results in preclinical studies of bulk tissue ablation and imaging.

Computer assisted optimization of an electromagnetic transducer design for implantable hearing aids

A simple, contactless electromagnetic transducer design for implantable hearing aids is investigated. It consists of a coil and a permanent magnet, both of which are intended for implantation in the middle ear. The transducer is modeled and optimized using computer simulations, followed by experimental verification. It is shown that the proposed transducer design can, because of its size and geometry, allow implantation through the external auditory canal, and provide a sufficiently high acoustic output corresponding to approximately 120 dB sound pressure level. It can be optimized to be tolerant of radial displacements between coil and magnet of up to 1 mm .

Investigation of Curved Polymeric Piezoelectric Active Diaphragms

Piezoelectric active diaphragms hold promise as an alternative to using passive diaphragms driven by voice coils for sound generation and noise cancellation applications. This paper presents an in-depth investigation of the acoustic response for curved polymeric piezoelectric active diaphragms. Simple analytical models were derived and experimentally validated to predict the structural dynamic and acoustic responses for generic polyvinylidene fluoride (PVdF) constant curvature active diaphragms with variable geometric parameters (width, radius, subtended angle, thickness). These models are useful for design purposes and for capturing the overall behavioral trends. To analyze the acoustic response mechanisms further, three-dimensional numerical models were also developed and experimentally validated. Parametric studies based upon these models reveal the potential of high acoustic outputs (over 100 dB in the far field) from optimized geometric configurations with subtended angles differing from the conventionally utilized flat, semicircular or circular configurations. These studies, corroborated by experiments on a variety of active diaphragm prototypes, conclude that the acoustic output is governed by the structural ring resonance, which can be designed such that the most efficient acoustic radiation is within the particular frequency range of operation.

Acoustic modeling of miniature loudspeakers

There is currently considerable interest in developing models that more accurately predict the response of systems with small loudspeakers, such as in cellular telephones. There has also been a trend towards developing hands-free communication with cell phones, thus requiring a relatively high acoustic output from a small source. Work will be reported on the development of a model for a cell phone to help address such research issues. It has been shown that boundary layer effects can cause significant frequency shifts in the resonance peaks associated with systems having elements with dimensions on the order of the boundary layer thickness. This phenomena has been observed in the presence of ports with diameter of 1 mm and less. These effects are being investigated in a cell-phone system, and the results obtained to date will be presented.

Application of High Temperature Superconducting Wire to Magnetostrictive Transducers for Underwater Sonar

ABSTRACT A low‐frequency underwater acoustic transducer integrating high‐temperature superconducting (HTS) coils and terbium‐dysprosium (TbDy) magnetostrictive material was designed, fabricated and tested. This represents a novel application for high‐temperature superconductivity and is a first example of an integrated system involving HTS coils cooled by a mechanical cryocooler. It also brings HTS technology together with a novel magnetic materials technology. The HTS coils were fabricated using react‐and‐wind BiSrCaCuO‐2223 HTS wires. They produce a peak field of 0.1 Tesla at 50 K. A single‐stage, Stirling‐cycle cryocooler was used to cool the coils and the TbDy driver to cryogenic temperatures (50‐65K). The coils provide an AC magnetic field superimposed on a DC bias field, which produces oscillatory strain within the magnetostrictive rod; this motion is transmitted through a cryostat to two head masses which project sound into the surrounding environment. High power acoustic output can be obtained by operating the transducer at its resonance frequencies of 520 Hz in air and 430 Hz underwater. This development demonstrates that, unlike low temperature superconductors, HTS wires can be considered for AC applications due to the low losses in these superconductors and the higher heat capacities of materials at temperatures above liquid helium.

A low-frequency directional flextensional transducer and line array

A unique low-frequency (900 Hz) class IV flextensional transducer that produces an enhanced far-field pressure on one side and canceled far-field pressure on the other side has been developed. The transducer radiating surface consists of a thick-walled elliptical aluminum shell and a U.S. Navy type III piezoelectric stack along its major axis with two active sections and one inactive section. The directionality is achieved by simultaneously exciting the shell into an omnidirectional and dipole operation by driving stack into both extensional and bending modes. Both measurements and modeling on this device show a front to back pressure ratio of more than 30 dB, producing cardioid-type radiation patterns over an octave band, for a single transducer element. The transducers measured mechanical Q is 8, coupling coefficient is 0.25, and electroacoustic efficiency is 80% and produced a source level of 215 dB re: 1 μPa at 1 m when driven at a field limit of 394 kV/m (10 kV/in.) at resonance. The uniqueness of this transducer is its directional beam patterns (directivity index=3.4 dB) and high acoustic output power from a small (less than a third of a wavelength) single element. Six of these transducers were placed in a closely packed line array two-wavelengths long. The array successfully produced narrow directional sound beams (directivity index=8.7 dB) with a front to back ratio greater than 30 dB and a source level of 225 dB re: 1 μPa at 1 m.

The Application of High Energy Density Transducer Materials to Smart Systems

ABSTRACTRecent NUWC research efforts in the field of high power sonar transducers designed to produce high acoustic outputs over significant bandwidths while being of minimal size and weight have been aided by advances in the continuing development of several new high energy density transducer drive materials. Both Terfenol-D, a rare earth magnetostrictive material, and lead magnesium niobate, a relaxor ferroelectric, have demonstrated a tenfold increase in field-limited energy density over a typical very hard lead zirconate titanate (i.e., Clevite PZT-8) piezoelectric ceramic. The Center's focus is to double the demonstrated performance of each material and to address such issues as hysteresis reduction in the magnetostrictive material and coupling coefficient improvements in the electrostrictive materials. Poly (vinylidene fluoride-trifluoroethylene) can also be considered a high energy density material because of its excellent energy density and its broad bandwidth possibilities. The application of these material technologies, either separately or as hybrid composites, to smart material design will be detailed.