Measurement Of Acoustic Power Research Articles

Dynamic acoustic optimization of pulse tube refrigerators for rapid cooldown

Pulse tube refrigerators are a critical enabling technology for many disciplines that require low temperatures. These refrigerators dominate the total power consumption of most modern cryostats, including those that reach millikelvin temperatures using additional cooling stages. In state-of-the-art commercial pulse tube refrigerators, the acoustic coupling between the driving compressor and the refrigerator is fixed and optimized for operation at base temperature. We show that this optimization is incorrect during the cooldown process, which results in wasted power consumption by the compressor and slow cooldown speed. After developing analytic expressions that demonstrate the need for acoustic tuning as a function of temperature, we dynamically optimize the acoustics of a commercial pulse tube refrigerator and show that the cooldown speed can be increased to 1.7 to 3.5 times the original value. Acoustic power measurements show that loss mechanism(s)—and not the capacity of the compressor—limit the maximum cooling available at high temperatures, suggesting that even faster cooldown speeds can be achieved in the future. This work has implications for the accessibility of cryogenic temperatures and the cadence of research in many disciplines such as quantum computing.

Fully Printed Flexible Ultrasound Transducer for Medical Applications

AbstractThe fabrication of a fully printed, lead‐free, polymer piezoelectric transducer is presented and the characterization of its structural, dielectric, and ferroelectric properties at different processing stages is demonstrated. The performance of poly(vinylidene fluoride‐trifluoroethylene) transducers with resonance frequency analyses, acoustic power measurements, and pulse‐echo experiments is evaluated. Notably, for the first time for a fully printed transducer, an optimal performance in the medical ultrasound range (<15 MHz) is demonstrated with acoustic power >1 W cm−2, which is promising for applications in epidermal and wearable electronics. Overall, the findings provide a strong foundation for future research in the area of flexible ultrasound transducers.

Image-Guided Measurement of Radiation Force Induced by Focused Ultrasound Beams.

The radiation force balance (RFB) is a widely used method for measuring acoustic power output of ultrasonic transducers. The reflecting cone target is attractive due to its simplicity and long-term stability, at a reasonable cost. However, accurate measurements using this method depend on the alignment between the ultrasound beam and cone axes, especially for highly focused beams utilized in therapeutic applications. With the advent of dual-mode ultrasound arrays (DMUAs) for imaging and therapy, image-guided measurements of acoustic output using the RFB method can be used to improve measurement accuracy. In this article, we describe an image-guided RFB measurement of focused DMUA beams using a widely used commercial instrument. DMUA imaging is used to optimize the alignment between the acoustic beam and reflecting cone axes. In addition to image-guided alignment, DMUA echo data is used to track the displacement of the cone, which provides an auxiliary measurement of acoustic power. Experimental results using a DMUA prototype with [Formula: see text] shows that 1-2 mm of misalignment can result in 5%-14% error in the measured acoustic power. In addition to the use of B-mode image guidance for improving measurement accuracy, we present preliminary results demonstrating the benefit of displacement tracking using real-time DMUA imaging during the application of (sub)therapeutic focused beams. Displacement tracking provides a direct measurement of the radiation force with high sensitivity and follows the expected dependence on changes in amplitude and duty cycle (DC) of the focused ultrasound (FUS) beam. This could lead to simpler, more reliable methods for measuring acoustic power based on the radiation force principle. Combined with appropriate computational modeling, the direct measurement of acoustic radiation force could lead to reliable dosimetry in situ in emerging applications such as transcranial FUS (tFUS) therapies.

Langevin's Ultrasonic Metrology.

This article proposes that Paul Langevin should be considered the originator of ultrasonic metrology. He established the theoretical foundation for the use of radiation pressure for the measurement of acoustic power, by considering the energy density at a target in a beam. This approach was used for calibrating the ultrasonic transducers he helped to develop for submarine detection and underwater communications during the First World War (WWI). Within a decade, he developed two calibrated devices to measure acoustic power and average intensity, a torsion balance, and the first electronic power meter. He patented a piezoelectric non-resonant quartz hydrophone and a quartz probe to explore transducer surface vibration. Design criteria for the instruments included a rugged design that could allow measurements to be carried out not only in the laboratory but also at sea. Langevin was also influential in establishing an ultrasonics transducer laboratory in Toulon in 1923 where these instruments were used. New quartz pulse-echo transducer designs were developed and evaluated there, and performance certification was provided for manufactured transducers for both naval and civil systems. Early workers in ultrasonics recognized Langevin's original and pioneering contributions, which were the enabling technology for modern ultrasonic metrology.

Measurement of the Instantaneous Acoustic Power of Diagnostic Ultrasound Equipment by Piezoelectric-Array Acoustic Power Meter

Measurement of the Instantaneous Acoustic Power of Diagnostic Ultrasound Equipment by Piezoelectric-Array Acoustic Power Meter

Acoustic Power Measurement and Thermal Bioeffect Evaluation of Therapeutic Langevin Transducers.

We recently proposed an analytical design method of Langevin transducers for therapeutic ultrasound treatment by conducting parametric study to estimate the effect of compression force on resonance characteristics. In this study, experimental investigations were further performed under various electrical conditions to observe the acoustic power of the fully equipped transducer and to assess its heat-related bioeffect. Thermal index (TI) tests were carried out to examine temperature rise and thermal damage induced by the acoustic energy in fatty porcine tissue. Acoustic power emission, TI values, temperature characteristics, and depth/size of thermal ablation were measured as a function of transducer’s driving voltage. By exciting the transducer with 300 Vpp sinusoidal continuous waveform, for instance, the average power was 23.1 W and its corresponding TI was 4.1, less than the 6 specified by the Food and Drug Administration (FDA) guideline. The maximum temperature and the depth of the affected site were 74.5 °C and 19 mm, respectively. It is shown that thermal ablation is likely to be more affected by steep heat surge for a short duration rather than by slow temperature rise over time. Hence, the results demonstrate the capability of our ultrasonic transducer intended for therapeutic procedures by safely interrogating soft tissue and yet delivering enough energy to thermally stimulate the tissue in depth.

Research on sound power measurement of multi-element ultrasonic focusing transducer based on cross-spectrum method.

In this paper, aiming at the problem that a multi-array focused ultrasonic transducer working in a high power range has a nonlinear influence on the measurement accuracy, a study of acoustic power measurement based on the near field cross-spectrum method was carried out. The focused acoustic field of a multi-array transducer was derived theoretically by combining the cross-spectrum method and the Westervelt nonlinear acoustic propagation equation. Finite element simulation was used to establish the model of the focused sound field of the multi-array transducer under different excitation conditions, and the influence law of each harmonic on the total sound power under specific excitation conditions was obtained. A cross-spectrum measurement system was built to scan the two near focusing planes under different excitation conditions. The total energy, each harmonic energy, and their proportion in the focusing region under the corresponding excitation were obtained through calculation and processing. The theoretical and simulation results were verified, and the harmonic energies were compensated in the calculation of the total ultrasonic power. The measurement results were compared with those obtained by radiation force balances. It was found that the maximum measurement deviation of compensated ultrasonic power was 7.89%, which met the requirements of acoustic measurement. The accuracy of the method and conclusion was verified under the power range of 10-60W.

Large-aperture 256-element fully populated random array for therapeutic ultrasound applications: Experimental study of its functionality and characterization of the radiated field

Recently, a new method has been proposed for designing random high-intensity focused ultrasound (HIFU) arrays with the maximum possible filling factor and polygonal elements of equal area, based on capacity constrained tessellation [Rosnitskiy et al., IEEE Trans. UFFC (2018)]. Following this approach, a 256-element piezocomposite array with a radius of curvature of 150 mm, a diameter of 200 mm, and an operating frequency of 1.2 MHz was designed at the Laboratory for Industrial and Medical Ultrasound (LIMU) of Moscow State University. The array was subsequently manufactured by Imasonic (France). It can be driven using a Verasonics ultrasound engine. Electrical matching was performed for each element using matching network created by Image Guided Therapy (France). This report presents the system design procedure and experimental results for array characterization using acoustic holography and acoustic power measurements using the radiation force balance. In addition, a theoretical study was carried out using the “HIFU beam” simulator developed at LIMU (freely available at www.limu.msu.ru) to assess the array's ability to achieve highly nonlinear regimes with shocked pressure waveforms. The possibility of using the array for HIFU applications such as acoustic levitation and manipulation, histotripsy and transcostal HIFU therapy is discussed. [Work supported by RSF 19-12-00148.]

Extended tube acoustic metamaterial: Its modeling and application to a kitchen hood

In this work, acoustic metamaterials based on the concept of extended tube are investigated. The metastructure consists of an array of units, each featuring a thin tube protruding from a perforation into a backing cavity to form a Helmholtz resonator. First, an impedance model for a single-layer extended tube structure is developed using the equivalent circuit method. For the tube in which the viscothermal effect can not be ignored, an equivalent fluid model is used to characterize its acoustic property and end corrections are introduced to take into account the flow distortion effect at the two ends. Then, two cost-effective samples are fabricated and tested using the impedance tube where good accuracy between the prediction and measurement is found. The structural simplicity and low-frequency absorption property allow the extended tube acoustic metamaterials to be utilized in scenarios where installation space for acoustic absorption materials is constrained. As an example, we demonstrate its use on a commercial kitchen hood. Standard acoustic power measurement shows its superior noise reduction performance over conventional porous materials of the same thickness.

Pyroelectric ultrasound sensor model: directional response

Ultrasound is typically measured using phase-sensitive piezoelectric sensors. Interest in phase-insensitive sensors has grown recently, with proposed applications including ultrasound attenuation tomography of the breast and acoustic power measurement. One advantage of phase-insensitive detectors, in contrast to conventional phase-sensitive detectors, is that they do not suffer from a narrow directional response at high frequencies due to phase cancellation. A numerical model of a phase-insensitive pyroelectric ultrasound sensor is presented. The model consists of three coupled components run in sequence: acoustic, thermal, and electrical. The acoustic simulation models the propagation and absorption of the incident ultrasound wave. The absorbed acoustic power density is used as a heat source in the thermal simulation of the time-evolution of the temperature in the sensor. Both the acoustic and thermal simulations are performed using the k-Wave MATLAB toolbox with an assumption that shear waves are not supported in the medium. The final component of the model is a pyroelectric circuit model which outputs the sensor response based on the temperature change in the sensor. The modelled pyroelectric sensor response and directional dependence are compared to empirical data.

Towards silent micro-air vehicles: optimization of a low Reynolds number rotor in hover

The demand in micro-air vehicles is increasing as well as their potential missions. Either for discretion in military operations or noise pollution in civilian use, noise reduction of micro-air vehicles is a goal to achieve. Aeroacoustic research has long been focusing on full scale rotorcrafts. At micro-air vehicle scales however, the hierarchization of the numerous sources of noise is not straightforward, as a consequence of the relatively low Reynolds number that ranges typically from 5000 to 100,000 and low Mach number of approximately 0.1. This knowledge, however, is crucial for aeroacoustic optimization and blade noise reduction in drones. This contribution briefly describes a low-cost, numerical methodology to achieve noise reduction by optimization of micro-air vehicle rotor blade geometry. Acoustic power measurements show a reduction of 8 dB(A). The innovative rotor blade geometry allowing this noise reduction is then analysed in detail, both experimentally and numerically with large eddy simulation using lattice Boltzmann method. Turbulence interaction noise is shown to be a major source of noise in this configuration of low Reynolds number rotor in hover, as a result of small scale turbulence and high frequency unsteady aeroadynamics impinging the blades at the leading edge.

A new radiation force balance method for measuring diverging piston source power in the frequency range 20–100 kHz: Theory and experimental verification

The purpose of this work was to present the results of a theoretical and experimental study in which a new acoustic output power measurement method was developed for a strongly diverged ultrasonic beam (ka < 3, where k is the circular wave number and a is the radius of transducer) using acoustic radiation force. To the best of our knowledge, there is no acceptable and effective method to measure acoustic power P for diverging piston transducers from 20 kHz to 100 kHz in the range ka < 3, it is a unsolved problem acutely. In the study, we used radiation force balance (RFB) method with a novel concave semi-spherical absorbing target in far field to measure the acoustic power up to 54 W. Based on the phase inverse mirror-image model we developed and Bridge's product theorem, the axial radiation force F on the target was first calculated and measured. The maximum difference of the ratio rP/cF (=P/cF) between theoretic and experimental values was smaller than 5%, where c is speed of sound in water. The reproducibility test of acoustic power measurements using two independent methods showed that the measurement uncertainty evaluated less than 10% by the new RFB method was much smaller than that (30%) by the traditional acoustic pressure method in underwater acoustics. It is indicated that the new RFB method is a primary and effective method for acoustic power measurement at least up to 20 W for frequency range 20 kHz to 100 kHz. The similar method was extended to power measurement for the rectangular transducer.Based on this method a new primary method of hydrophone calibration was also developed.

Nonlinear acoustic-power measurement based on fundamental focal axial vibration velocity for high-intensity focused ultrasound

The acoustic power (AP) of high-intensity focused ultrasound (HIFU) shows great potential for ensuring the efficacy and safety of tumor treatment. By considering the energy of the harmonics, an easily applicable nonlinear AP measurement method for HIFU based on the fundamental focal axial vibration velocity (F-FAVV) is proposed. The focal pressures of the harmonics with respect to the surface vibration velocity are simulated, and a piecewise function of the required harmonic order is developed based on the 40-dB attenuation criterion of the harmonic-to-fundamental ratio. With the relationships between the power gain and the FAVVs of the harmonics, the dependence of AP on the F-FAVV is achieved by summing the harmonic powers. The APs of HIFU under various surface vibration velocities are verified by experimental measurements of the F-FAVV using a laser vibrometer and the corresponding integration results over the transducer surface. Good agreement between the numerical and experimental results demonstrates the feasibility of accurate AP measurement for HIFU using the F-FAVV and suggests the potential for applications in biomedical engineering.

Study on the Mechanism of Ultrasonic Power Measurement Sensor based on Pyroelectric Effect

PVDF pyroelectric sensor has been widely applied in many fields, such as intruder alarm. Nowadays, this sensor shows a potential for ultrasonic power measurement. However, the transformation mechanism between the acoustic and pyroelectric signals has not been particularly studied until now. In this paper, a physical model was introduced for theoretical study of the mechanism of energy transformations. In addition, a simulation program based on finite-element analysis method was built up for analyzing the ultrasound propagation characteristics and the temperature rise on the PVDF, as well it predicted the waveform and amplitude of the generated pyroelectric signal. Besides that, a PVDF pyroelectric sensor was fabricated and used for acoustic power measurement experiment. Finally, the experiment and simulation results were compared, confirming that the physical model is suitable for pyroelectric sensor characteristics analysis. It can also provide useful suggestions for the design and fabrication of PVDF pyroelectric sensors with high sensitivity.

Acoustic power measurement of linear compressors

The linear compressor works as an important driver for high frequency regenerative cryocoolers. The acoustic power output of the compressor is a critical parameter in the design and the optimization of a linear compressor. To measure this parameter, several approaches based on different theories have been developed. In this paper, the RC load approach and the back chamber approach have been applied to a linear compressor to measure the acoustic power output. The results measured by the approaches indicate a good consistency with the theoretical calculation and reveal the connections between different approaches.The difference between the acoustic power at the piston surface and the exit of a linear compressor has been analyzed based on the experimental results from the RC load approach and the back chamber approach. The volume flow rate difference which accounts for the acoustic power difference is studied theoretically. Furthermore, based on the RC load approach, the optimum impedance together with the impedance cloudy map for the linear compressor to reach its highest efficiency has been obtained by analyzing the experimental and the theoretical results.

Acoustic Power Measurement for a Cylindrical Concave Transducer by using the Self-Reciprocity Method

A novel solution of measuring the acoustic power for a convergent cylindrical wave is proposed based on the self-reciprocity method, which turns the acoustic-power measurement to an electrical-parameters measurement. The formula for estimating the acoustic power was derived for a cylindrical concave transducer. The diffraction and the reflection corrections were also investigated for the precise measurement of the acoustic power. The performance of this method was compared with that of the radiation force balance (RFB) method. The experimental results showed good agreement. Moreover, the self-reciprocity method is more robust to the environment than the traditional RFB method and in practice can be extended to low-level acoustic power measurements.

Experimental Study of the Focused Ultrasonic System for Differential Diagnostics of the Human Hearing

In this paper the analysis of relevance and novelty of application of piezoelectric transducer for noninvasive diagnostics of a cochlea of the human by means of the focused ultrasound is carried out. To date, clinical use of ultrasound is numerous and diverse, and ultrasound diagnosis is one of the fastest growing methods in medicine. Revealed a previously unknown effect of spatial notch filtering quarter wavelength layers diffuse heat radiation. It is shown that the effect, together with a focused acoustic thermometer, allows for greater accuracy and allows measurement of the body's deep body temperature in real time.The design of the converter consists of a piezoceramic, an acoustic lens with the help of which the transformation of a flat wave front into an acoustic system with a flat piezoelement. The flat, concave lens is made of solid material, the rate of ultrasonic diffusion in which exceeds its value in water (biological tissue) for the studied acoustic system, organic glass was chosen. It is provided the theory, calculation and a design of measuring means: a high-frequency wattmeter for measurement of the electric power consumed by a piezoelectric transducer; the radiometer for measurement of acoustic power of the focused ultrasonic bunches. With their help detailed pilot study of the acoustic system is conducted, namely, parameters of her acoustic field and efficiency are determined. The theory, calculation and design are given: free float radiometer for measuring acoustic power of both focused and unfocused ultrashort beams; high-frequency wattmeter without active elements for measuring the power consumption of a piezus emitter.The experimental study of the layout of the acoustic focusing system is to measure the emitted acoustic and consumed electrical power of the piezoelectric element with the lens. The task of measuring acoustic power is solved using a radiometer. The measurement of the consumed electric power is made by means of a high-frequency wattmeter, which contains a multiplication scheme in the form of a circular scheme on four diodes and a current transformer. The received frequency dependences of radiated acoustic power, electric power consumed and the efficiency factor are obtained.The conclusion about expediency of use of this type of the electro-acoustic transducer for differential diagnostics of hearing of the human is drawn. The calculated intensity is sufficient for both diagnostic and therapeutic use of the developed acoustic system. This type of piezoelectric transducer will make it possible to distinguish between diseases of the middle ear of a person from internal diseases, and therefore to carry out differential diagnostics of the auditory system. This is especially important in clinical practice for choosing a further recovery strategy: if the loss occurs in the middle ear - the hearing aid is used, if in the inner one - is performed surgically by implantation of the cochlear implant.Ref. 10, fig. 6, tabl. 2.

Focused ultrasound transducer spatial peak intensity estimation: a comparison of methods

Characterisation of the spatial peak intensity at the focus of high intensity focused ultrasound transducers is difficult because of the risk of damage to hydrophone sensors at the high focal pressures generated. Hill et al (1994 Ultrasound Med. Biol. 20 259–69) provided a simple equation for estimating spatial-peak intensity for solid spherical bowl transducers using measured acoustic power and focal beamwidth. This paper demonstrates theoretically and experimentally that this expression is only strictly valid for spherical bowl transducers without a central (imaging) aperture. A hole in the centre of the transducer results in over-estimation of the peak intensity. Improved strategies for determining focal peak intensity from a measurement of total acoustic power are proposed. Four methods are compared: (i) a solid spherical bowl approximation (after Hill et al 1994 Ultrasound Med. Biol. 20 259–69), (ii) a numerical method derived from theory, (iii) a method using measured sidelobe to focal peak pressure ratio, and (iv) a method for measuring the focal power fraction (FPF) experimentally. Spatial-peak intensities were estimated for 8 transducers at three drive powers levels: low (approximately 1 W), moderate (~10 W) and high (20–70 W). The calculated intensities were compared with those derived from focal peak pressure measurements made using a calibrated hydrophone. The FPF measurement method was found to provide focal peak intensity estimates that agreed most closely (within 15%) with the hydrophone measurements, followed by the pressure ratio method (within 20%). The numerical method was found to consistently over-estimate focal peak intensity (+40% on average), however, for transducers with a central hole it was more accurate than using the solid bowl assumption (+70% over-estimation). In conclusion, the ability to make use of an automated beam plotting system, and a hydrophone with good spatial resolution, greatly facilitates characterisation of the FPF, and consequently gives improved confidence in estimating spatial peak intensity from measurement of acoustic power.

Accurate acoustic power measurement for low-intensity focused ultrasound using focal axial vibration velocity

Low-intensity focused ultrasound is a form of therapy that can have reversible acoustothermal effects on biological tissue, depending on the exposure parameters. The acoustic power (AP) should be chosen with caution for the sake of safety. To recover the energy of counteracted radial vibrations at the focal point, an accurate AP measurement method using the focal axial vibration velocity (FAVV) is proposed in explicit formulae and is demonstrated experimentally using a laser vibrometer. The experimental APs for two transducers agree well with theoretical calculations and numerical simulations, showing that AP is proportional to the square of the FAVV, with a fixed power gain determined by the physical parameters of the transducers. The favorable results suggest that the FAVV can be used as a valuable parameter for non-contact AP measurement, providing a new strategy for accurate power control for low-intensity focused ultrasound in biomedical engineering.

Prediction of tonal ducted fan noise

This study introduces an analytical model aiming at predicting the tonal acoustic sources generated and radiated by the rotor–stator interaction in a fan stage. This model is able to cope with complex three-dimensional stator geometries and it fully accounts for cascade effects, characteristic of modern fan stages. It is based on a proper description of the rotor wake coupled with an analytic cascade response function and with an acoustic analogy. The proposed model is validated for the first time against acoustic sources and sound power measurements, on the Advanced Noise Control Fan from NASA. On this configuration representative of an actual fan stage, the model is shown to predict tonal sources and powers accurately, in function of the rotational velocity and of the stator-vane count. Another realistic configuration, namely the low-pressure CME2 research compressor, is then considered in order to demonstrate the suitability of the model to be used as a design tool in an industrial context. A parametric study concerning both the stator vane sweep and lean angles is performed on this rotor–stator stage. The model produces predictions consistent with studies from literature, quantifying the effectiveness of swept and leaned vanes as a tonal noise reduction mechanism. This parametric study allows defining an optimal stator design for minimal noise emission.