Electroacoustic Efficiency Research Articles

Measurement of ultrasonic power and electro-acoustic efficiency of high power transducers

In this paper, an improved method for the measurement of acoustic power and electro-acoustic efficiency of high power ultrasonic transducers is presented. The measuring principle is described, the experimental results are given. In comparison with traditional methods, the method presented in this paper has the advantages of simplicity, economy and practicality. The most important is that it can measure the output acoustic power and the electro-acoustic efficiency of the transducer under the condition of high power and practical applications, such as ultrasonic cleaning and soldering.

A 256-element ultrasonic phased array system for the treatment of large volumes of deep seated tissue

A 256-element phased array has been designed, constructed, and tested for ablative treatment of large focal volumes of deep seated tissue. The array was constructed from a 1.1-MHz, 1-3 composite piezoelectric spherical shell with a 10-cm radius of curvature and a 12-cm diameter. The array was tested to determine its electroacoustic efficiency and inter-element coupling under high acoustic power conditions. A series of in vivo porcine experiments demonstrated the ability to produce deep seated tissue lesions in thigh muscle using the large scale phased array. The array was used to heat and coagulate tissue volumes >5 cm(3) in a single ultrasound exposure using multiple foci and temporally scanned power deposition patterns. The spatial and temporal experimental results for large, heated focal volumes correlated very well with the simulated temperature response model for homogeneous tissue. A 25-cm(3) tissue volume was coagulated in a 90-min period using overlapping large ultrasound exposures.

Influence of nonparallelism of the surfaces of a piezoelectric layer on the frequency band of an electroacoustic piezoelectric transducer

The influence of nonparallelism of the surfaces of a piezoelectric transducer on its impedance and frequency band is analyzed theoretically. The model of a wedge-shaped piezoelectric layer taken for the analysis consists of a set of n plane-parallel elements arranged in succession along the length of the transducer with a gradual increase in the thickness of the piezoelectric layer. The analysis performed demonstrates the possibility of significantly expanding the band of working frequencies of a piezoelectric transducer when its electroacoustic conversion efficiency is reduced. In some cases, however, a decrease in conversion efficiency can be employed to optimize an acoustic device, for example, in creating a filter-type piezoelectric transducer, where electromagnetic energy is converted into acoustic energy successively from one cell of the filter to another. The proposed method for expanding the frequency band can be useful, for example, in creating high-frequency acousto-optic Bragg cells.

High-efficiency 2-kW thermoacoustic driver

A moving-magnet electrodynamic driver will be described which is intended to maintain an acoustic resonance within a thermoacoustic air conditioner containing an inert gas mixture of helium and argon at 30 bar. It is energized by a linear motor with a Bl-product of 41 N/A and a coil resistance of 0.24 Ω. An auxiliary spring system augments the magnetic stiffness (154 kN/m) to provide a driver mechanical resonance frequency near 60 Hz. An Inconel bellows forms a flexure seal which provides an effective piston area that maximizes electroacoustic conversion efficiency by making the Joule heating losses in the coil equal to the mechanical dissipation in the magnetic suspension. The driver’s electroacoustic conversion efficiency is predicted to be in excess of 83%. [Work supported by the Office of Naval Research.]

Load characteristics of bolted Langevin torsional transducer II

A torsional vibration system for ultrasonic welding plastics has been assembled and the characteristics demonstrated. The results have been reported in our preceding report [1]. 1.53 ms −1 of the vibration velocity and 90% of the electroacoustical efficiency were obtained using a board radiator connected to the edge of the torsional transducer. However, the region of the acoustic load resistance was too small to get excellent performance as an ultrasonic welding system. In this presentation, we designed and fabricated an angular pillar radiator which was longitudinally connected to the half wavelength straight horn and a bolted Langevin torsional transducer for a torsional vibration system. 95% of the efficiency, 1.61 ms −1 of the vibration velocity and 42 W of the acoustic output power at 18 Ns m −1 of the acoustic load resistance was observed. More than 90% efficiency was also observed in a wide range of acoustic load resistance (10 ~ 93 Ns m −1).

Design and in vitro results of a high intensity ultrasound interstitial applicator

Interstitial technologies were proposed for the treatment of deep-seated and unresectable tumours. This study was designed to demonstrate the possibility of producing coagulation necrosis for a short exposure duration (20 s) and a large volume with an interstitial ultrasonic applicator equipped with a plane transducer. The applicator was evaluated in terms of electroacoustic efficiency (58% at 10.7 MHz), temperature pattern and in vitro pig liver tissues destruction. The temperature elevation reached 52, 39, 22 and 15 °C at, respectively, 2.5, 5, 7.5 and 10 mm from the applicator surface along the propagation axis. The evident cigar-shaped lesions spread on a depth of (8 ± 2) mm from the surface applicator, (10 ± 1) mm high and (3 ± 1) mm wide. The temperature measurements showed the limitations encountered with a 10 MHz transducer. The high increase in temperature of tissues close to the transducer can induce vaporisation and a gaseous barrier opaque to ultrasound. Owing to the shape and the operating frequency of the transducer, we hope to obtain the same satisfactory performances in vivo with weakly perfusion-dependent lesions. Axial and longitudinal applicator motions will enable us to treat more important volumes.

Design and preliminary results of an ultrasound applicator for interstitial thermal coagulation

The extracorporeal high-intensity focused ultrasound (HIFU) techniques are not still transposable for tumors of the digestive tract because of their locations. This study was designed to evaluate the feasibility of interstitial applicators (3.8 mm O.D.) to comply with this therapeutic lack and to demonstrate the possibility of producing coagulation necrosis by a specially designed probe and with a short exposure time (20 s). The active surface of the applicators consists of plane water-cooled PZT transducers working at 10 or 5 MHz. They were evaluated in terms of acoustic power emitted as a function of the frequency, and applied electrical input (electroacoustic efficiency of 75% at their working frequency) and in vitro and in vivo pig liver tissue destruction. The in vitro and in vivo necroses depth from the applicator surface ranged from 8 to 20 mm. This showed the advantage of a nondivergent source: the pressure decay is only due to the tissue absorption in the Fresnel zone. The lesions dimensions are slightly dependent on perfusion: 8 ± 2 mm deep in vitro for a 10.7-MHz transducer working at 14 W/cm2 against 10 mm in vivo. Operating at 5 MHz makes it possible to increase the therapeutic heating depth. For example, at a similar close-to-transducer temperature, the 5-MHz applicator induced, at a depth of 10 mm, a temperature elevation of 27°C against 15°C for that working at 10 MHz.

Underwater impulsive acoustical generation from steam bubbles

One of the most common techniques to generate impulsive sound underwater is through the use of sparkers. The sound is generated through high-voltage arcing across two electrodes. Research at NCCOSC, RDT&E division has shown that modest sound levels can be produced from electrodes operated at nonarcing voltages. In this mode the main products in the electrode spacing are steam bubbles. The expansion and subsequent collapse of a single steam bubble is what generates the underwater sound. The advantages to the use of nonarcing electrodes are longer electrode life, greater reproduciblity, and the use of light-weight components. The disadvantages include lower electroacoustical conversion efficiency and lower electrical power input. The electroacoustical conversion efficiencies measured reached 0.7%. Extrapolation of efficiency data show that 1% or better efficiency may be achievable. A high-speed imaging was developed to capture the history of events in the electrode spacing. Images of various bubble events are shown and interpreted. [Work supported by ONR.]

The considerations and guides of the wattmeter method for measuring output acoustical power of Langevin-type transducers — I: theory

The vibrational amplitude ratios of a number of positions in some typical vibration systems under different loads, especially of amplitudes at positions close to nodes and anti-nodes (related to no-load conditions) are given. Results according to the wattmeter method for measuring output acoustical power and electro-acoustic efficiency of high-power ultrasonic vibration systems (Langevin-type transducers and horns) have been simulated, and theoretical errors of this method calculated. This points out that there are some limits for the wattmeter method in high load conditions, but the method has practical values for measuring output acoustical power, especially for sonochemistry. Finally, based on the above analysis, some guidelines and suggestions for convenient on-line measurement are proposed. The vibration velocity of the radiation surface is discussed as an important quantity in analyzing acoustic input in high-intensity ultrasound applications.

Directionally solidified Tb xDy 1− x(M yFe 1− y) giant magnetostrictive materials and their applications in transducer

The grain-aligned Tb x Dy 1− x (M y Fe 1− y ) 1.9 ( x=0.50, 0.30, y=0.05, 0.10, 0.15, M=transition metal) giant magnetostrictive materials with the size of Φ(6∼15)×(150∼200) mm have been produced using a newly developed directional solidification apparatus. The magnetic properties of λ s = 1820 ppm and d 33=1.1 ppm/Oe (under the compressive pressure of 12.5 MPa) have been obtained in Tb 0.5Dy 0.5(Fe 0.95)Mn 0.05) 1.9 alloy under the alloy by drawing it down to the cooling zone at the speed of 12 mm min −1 and heat treating the alloy at 950°C for 6∼8 h. XRD studies reveal a [110] crystallographic alignment in the directionally solidified Tb 0.5Dy 0.5(Fe 0.95MN 0.05) 1.9 alloy. A transducer designed to work in underwater sonar system has been developed by using this material. It has an resonance frequency of 2.4 kHz, emission current sensitivity of 170 dB, mechanical quality factor of 3 and an electro-acoustic efficiency of 43%.

Capped ceramic underwater sound projector

The cymbal-type transducer developed at the Materials Research Laboratory at Penn State Univ., which is similar to the ring/shell flextensional transducer, is currently being investigated as a shallow-water (<250 m) sound projector for the 10–75 kHz frequency range. The standard size cymbal transducer is 12.7 mm in diameter and approximately 2 mm in total thickness. It consists of a poled lead zirconate-titanate (PZT) disk capped symmetrically on each face by identical thin metal caps. Each cap is shaped with a cavity on its inner surface so that an air space exists between the PZT disk and the top of the cap. The size of the air space, the thickness of the cap, and the type of cap material all influence the operating performance and allowable operating depth of the device. Parameters necessary to characterize an underwater projector such as resonance frequency, mechanical Q, electroacoustic efficiency, transmit voltage response (TVR), and source level will be presented. Experimental data will be compared with values calculated from the Ansys■ finite-element program. Results from single elements as well as cymbal arrays (both potted and unpotted) will be shown. [This work was funded by the Office of Naval Research Contract No. N00014-96-1-1173.]

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.

Magnetohydrodynamic fluid pump

Magnetohydrodynamic transduction has been successfully demonstrated in the conversion of acoustic to electric energy in a thermoacoustic prime mover [Swift, J. Acoust. Soc. Am. 83, 350 (1988); Swiftet al., J. Acoust. Soc. Am. 78, 767 (1985)]. The reciprocal effect [Swift and Garrett, J. Acoust. Soc. Am. 81, 1619 (1987)]—the magnetohydrodynamic generation of sound—is also potentially attractive for production of high-amplitude standing waves in gases since the moving fluid does not require a flexure or piston seal. Results will be presented for an experimental device which compresses gases above a fluid-filled U-tube driven magnetohydrodynamically at resonance. Electroacoustic efficiencies for mercury and potassium chloride conductivity standards will be related to transducer parameters (e.g., magnetic induction, electrode dimensions, tube diameter, fluid viscosity, etc.). [Work supported by the Office of Naval Research.]

High-efficiency shock-wave generator for extracorporeal lithotripsy.

In extracorporeal lithotripsy, the electro-acoustic efficiency of electrohydraulic generators is limited by the inductance of the electrical discharge circuit. A new shock-wave generator is described that uses a coaxial discharge line enabling electro-acoustic efficiency to be greatly increased. The line is built using a para-electric ceramic with a relative dielectric constant of 1700, manufactured for use in high-voltage impulse mode. A coaxial spark gap, with minimal inductance, has been developed to obtain the triggered breakdown of the discharge line. Shock waves are created with a coaxial electrode plugged directly into the spark gap and immersed in an electrolyte of degassed saline. Electrode gap and electrolyte resistivity are adjusted to match the resistivity of the electrolyte volume between the underwater electrodes to the characteristic impedance of the line. The discharge line generates in the medium a rectangular current pulse with an amplitude of about 6000 A and a rise time of 50 ns. Compared with conventional generators, measurements of the expansive peak pressure pulse show an increase of 105% at 10 kV, 86.5% at 12 kV and 34.5% at 14 kV charging voltage. Electro-acoustic efficiency is found to be 11% instead of 5.5% for a conventional discharge circuit.

Shipboard electronics thermoacoustic cooler

A thermoacoustic refrigerator that was optimized for preservation of biological samples in space, was modified for use as a cooler for the CV-2095 shipboard radar electronics rack. The thermoacoustic cooler was tested in the laboratory and demonstrated at sea aboard USS DEYO (DD-989). In the laboratory, using a calibrated heat load, the data acquisition system was able to account for the total energy balance to within 4%. At the highest operating power aboard ship, 227 W of acoustic power was used to provide 419 W of useful cooling power, corresponding to a coefficient of performance of 1.85. Taking into account the 54% electroacoustic efficiency of the loudspeakers, the shipboard electronics thermoacoustic cooler (SETAC) provided 1 W of cooling for each watt of electrical power input. [Work supported the Office of Naval Research and the Navy Science Assistance Program.]

Shipboard electronics thermoacoustic cooler

A thermoacoustic refrigerator that was optimized for preservation of biological samples in space, was modified for use as a cooler for the CV‐2095 shipboard radar electronics rack. The thermoacoustic cooler was tested in the laboratory and demonstrated at sea aboard USS DEYO (DD‐989). In the laboratory, using a calibrated heat load, the data acquisition system was able to account for the total energy balance to within 4%. At the highest operating power aboard ship, 227 W of acoustic power was used to provide 419 W of useful cooling power, corresponding to a coefficient of performance of 1.85. Taking into account the 54% electroacoustic efficiency of the loudspeakers, the shipboard electronics thermoacoustic cooler (SETAC) provided 1 W of cooling for each watt of electrical power input. [Work supported the Office of Naval Research and the Navy Science Assistance Program.]

Hydrodynamic siren as a broadband underwater sound projector

The siren mechanism is adapted to underwater uses for sound production. Such a hydrodynamic siren works as a powerful, broadband mixed sound projector as well as a less powerful, more noiselike sound projector. The broadband mixed sound consists of the harmonics due to the rotor rotation rate and the hydrodynamic cavitation noise. The cavitation noise level at rotor rotation is almost the same as that when the rotor is not rotated. The maximum acoustic pressure of a model siren is 189 dB re: 1 μPa at 1 m when integrating between 0 and 10 kHz, and 179 dB re: 1 μPa at 1 m as to one peak line spectrum. The siren gain attained by the rotation rate harmonics is approximated at about 20 dB. The maximum overall electro-acoustic conversion efficiency is estimated as 1.2% for the rotor–stator clearance of 0.15 mm and the applied water pressure of 8.2 kg/cm2, that is, 0.80 MPa. Since the siren has a dual hole configuration, a third set of harmonics is virtually derived from the greatest common divisor obtained from the numbers of holes in the two sets of holes (i.e., 8 holes and 20 holes give an effective virtual 4 holes). The radiation directionality and depth dependence of the model siren are also examined. The experimental results on the hydrodynamic cavitation noise at nonrotation of the rotor almost match those revealed in previous literature on the noise from cavitating water jets and hydrofoils. However, the results show a strong dependence of the spectrum level on the receiving position. More detailed knowledge on the cavitating jet length, the bubble concentration along it, the vibration of the surviving bubbles, etc., are needed to understand the hydrodynamic cavitation noise more intensively. Although the siren cannot avoid the harmonic generation due to the rotation rate, a siren as a noise maker is proposed by considering a random configuration of many holes and random occurrence of hole opening in one rotation period.

Study of vibrational characteristics for piezoelectric sandwich ultrasonic transducers

Electro-acoustic efficiency is an important parameter for high power ultrasonic transducers. When a transducer is acoustically matched into its load, the electro-acoustic efficiency reaches the potential efficiency, which depends only on the transducer's unloaded parameters, not on the acoustic load. The relation between the potential efficiency and the contacting loss caused by bonding at interfaces and the relation between the potential efficiency and the dimensions of the end sections of the sandwich transducer are examined. The effects of the loss at the contacting surfaces on the transducer's resonant frequency and the potential efficiency are studied. The results show that there is an optimum design of the sandwich transducer which maximizes the potential efficiency and minimizes the effect of the contacting loss on the transducer's resonant frequency.

Water depth and drive voltage dependence of the acoustic parameters of a barrel-stave flextensional projector

A low-frequency barrel-stave flextensional projector for sonar and general underwater acoustics applications has been built at the Defence Research Establishment Atlantic in Dartmouth, Nova Scotia. The projector was calibrated at the Seneca Lake sonar test facility at Dresden, NY, as a joint research venture with the Naval Undersea Warfare Center, New London, Connecticut. The transmitting voltage response (TVR), mechanical quality factor (Qm), electrical admittance, and electroacoustic efficiency (ηea) were measured for driving voltages up to 5.0 kVrms at four depths: 30, 61, 91, and 123 m. At 5.0 kVrms, the resonance frequency (f0) increased from 780 to 840 Hz, Qm decreased from 4.4 to 3.3, and ηea decreased from 87% to 71%, as the depth was increased from 30 to 123 m. The change in each acoustic parameter, averaged over the four depths of interest, was −10 Hz/kVrms for f0, +0.4 dB/kVrms for the TVR at resonance, and +0.1/kVrms for Qm, as the driving voltage was increased from 0.1 to 5.0 kVrms. The dependence of the parameters on depth and driving voltage are discussed in terms of the interactions between the rubber boot and the staves. [Work sponsored in part by the U.S. Office of Naval Research.]

Investigation of a new wide-band transducer with piezoelectric layered ring

A type of transmitting transducer with a piezoelectric layered ring is described. It is wide band, applicable to low-frequency operation, and has horizontal nondirectivity. Theoretical analysis and structural design are both discussed. Experimental results show: (1) the flat and smooth transmitting frequency response of the experimental transducer ranges from 9 to 20 kHz with the fluctuation not exceeding +/-1 dB; (2) within this wide band the electroacoustic efficiency reaches 30%-60%; and (3) when a small signal is used as excitation, the theoretical frequency response curve coincides very well with the experimental curve.