Electroacoustic Responses Research Articles

Generalized homogenization model of piezoelectric materials for ultrasonic transducer applications

Although piezocomposite (PC) materials have increasingly attracted researchers, there is still a need to properly and easily derive their properties. We develop a generalized homogenization model (GHM) that accounts for Smith and Cha approaches to evaluate the equivalent characteristics of piezocomposites. This method could be applied to all connectivities patterns, but restricted herein to 2-2 and 1-3 piezocomposites for comparison with Smith (1-3) and Cha (2-2) analytical results. In the proposed GHM is a parameter θ, is changed for various connectivities. The 1-3 and 2-2 PZT-7A/Araldite D (PCs) data are used and equivalent characteristics of these Pcs are determined as function of volume fraction of PZT-7A piezoelectric. Results show that the electromechanical coefficients are well fitted by Voigt and Reuss models. Results obtained for some parameters show that the proposed GHM is consistent with the analytical existing models used for the 1-3 and 2-2 connectivities and is in line with measured values from Chan and Unsworth (1989). Based on the GHM 2-2 configuration results of piezocomposite materials, the electroacoustic responses of transducers having some of these properties are simulated using the KLM model. A performance trade-off was chosen, resulting in an improved thickness coupling coefficient and a lowered acoustical impedance, and a similar approach as that on a pure PZT-7A.

Pre-test analysis to reproduce random pressure fields with multi-channel acoustic control

Pre-test analysis is proposed as a methodology to design control strategies for environmental acoustic tests. In the space industry these tests are used to assess the survivability of hardware, which is exposed to harsh dynamic loads in service. In this work an array of loudspeakers and microphones is used in a laboratory to reproduce the acoustic environment generated during the launch of a spacecraft. Active control is required to design the signals driving the loudspeakers. Pre-test analysis uses an internal model of the electro-acoustic plant responses to automatically design the control parameters required by multi-channel random control algorithms. The outcome of pre-test analysis is the automatic selection of the control microphones, and a set of realizable test references in the form of a spectral density matrix. The methodology proposed in this work to design the control strategies is validated experimentally on system level tests. Compared with other approaches, proper design of the control strategy for the environmental acoustic test significantly enhances the correlation between the measured and desired pressure responses.

Piezoelectric P(VDF-TrFE) film inkjet printed on silicon for high-frequency ultrasound applications

We investigated the innovative processing of poly(vinylidene fluoride-trifluoroethylene) P(VDFx-TrFE1-x) (x = 83 mol. %) by inkjet printing to deliver uniform and thickness-controlled layers on silicon substrates. Here, we provide detailed processing steps and optimize film deposition conditions. The thickness coupling factor for a P(VDF-TrFE) film around 11 μm thick was 22%, demonstrating good electromechanical performance after poling. These multilayer structures were specifically for high-frequency, single-element ultrasonic transducer applications. The measurements of electro-acoustic responses were in water. The maximal frequency was centered at 33.2 MHz and had a fine axial resolution at 22 μm, corresponding to a fractional bandwidth at −6 dB of 100%. In the context of technological evolutions aimed at miniaturized devices and integrated electronics, these results allow for the consideration of complex structures such as multi-element transducers for high-frequency imaging applications.

Spherical-Omnidirectional Piezoelectric Composite Transducer for High- Frequency Underwater Acoustics.

Spherical-omnidirectional acoustic source has become a powerful tool to provide a near-ideal omnidirectional beam pattern for acoustic tests and communications. Current spherical-omnidirectional acoustic sources do not combine an omnidirectional beam pattern with a high transmitting voltage response (TVR) in the frequency range of above 200 kHz. This work presents the design, fabrication, and measurements of a high-frequency spherical-omnidirectional transducer that can provide a near-ideal omnidirectional beam pattern and a high TVR. The active element of the transducer consists of six identical square coupons with spherical curvature 1-3 piezoelectric composites operating in thickness mode. Electroacoustic responses of the fabricated transducer in water were measured. The measured resonance frequency of the transducer was 280 kHz. The maximum TVR was 161.3 dB re 1 μ Pa/V@1 m. The horizontal and vertical beamwidths of the transducer were 360° and 346°, respectively. Measurements show that the spherical piezoelectric composite transducer has a favorable spherical-omnidirectional behavior and a high TVR at high frequency. These results demonstrate that the spherical piezoelectric composite transducer is potentially a strong candidate for a high-frequency underwater acoustic source that requires an omnidirectional response.

Linear viscoelasticity of weakly cross-linked hydrogels

Chemically cross-linked polyacrylamide (PA) hydrogels have been studied extensively, but gels with a very low cross-linker ratio have received relatively little attention. In this paper, the rheology of bis-cross-linked PA hydrogels is examined with acrylamide concentrations ca approximately 10% w/v and cross-linker to acrylamide ratios cbis/ca in the range 0.2–0.9 mmol mol−1. Linear viscoelastic responses were measured during and following gelation. The effective cross-link formation rate increases with the cross-linker ratio, with a plateau modulus that is weakly quadratic in the cross-linker ratio. The gels exhibit distinctly different features from their counterparts in the literature with higher cross-linker ratios. Plateau regions in the dynamic moduli spectra are less pronounced, and the loss tangents are several orders of magnitude higher than for gels with a higher cross-linker ratio but a comparable plateau modulus. Time-temperature superposition of viscoelastic spectra furnishes a disentanglement activation energy that decreases with increasing temperature, from ≈25 to 6kBT. Time-concentration superposition for samples prepared above the percolation threshold furnishes a critical relaxation exponent Δ≈0.38, which is much smaller than previously reported for the universal sol-gel transition. These distinct features can be attributed to the predominance of entanglements. We show that a Wiechert model with a power-law distribution of relaxation times faithfully reproduces dynamic moduli spectra from the creep compliance. Together, the results provide a foundation with which to interpret acoustic and electroacoustic rheological responses (at MHz frequencies) and the linear viscoelasticity of DNA-cross-linked gels (to be reported elsewhere).

Porous lead zirconate titanate ceramics with optimized acoustic properties for high-frequency ultrasonic transducer applications

Acoustic properties of porous PZT ceramics intended to be used as backing in high-frequency transducer applications are investigated using a novel method where an electroded piezoelectric thick film is deposited on the backing under test. Two backings with pore sizes 1.5 μm and 10 μm were obtained by sintering a mixture of ceramic powder and an organic template, their porosity was evaluated by scanning electron microscopy at 15%, leading to a density around 6.5 g/cm3. The electroacoustic impulse responses of these devices were measured considering the backing as a propagation medium, the initial thickness of which was chosen small enough to allow back-wall echoes to be detected and large enough to be able to separate the signals in time domain. Then the thickness of the backing was reduced (from around 2 mm to less than 1 mm) and the measurements were repeated. Acoustic properties were then deduced: attenuation coefficients reaching 4 dB/mm/MHz and group velocities around 3400 m/s were obtained, leading to an acoustic impedance around 22 MRa. Such combination of high attenuation and moderate acoustical impedance make these materials an interesting solution for high-resolution ultrasonic imaging transducers.

Super-Cell Piezoelectric Composite With 1-3 Connectivity.

The standard fabrication method for 1-3 piezocomposites for ultrasound transducers is the "dice and fill" method (DFM) in which lateral periodicity is introduced. This contributes to the appearance of spurious modes that can drastically affect the performance of the device if they appear near its thickness mode frequency, thus limiting the effective frequency range. A new 1-3 piezocomposite fabricated with a super-cell structure [1-3 super cell (13SC)] was designed in order to overcome these limitations. It consists of the merging of several periodic cells with 47% PZT volume fraction and epoxy resin as the matrix. Two lateral periodicities in one direction are defined as well as two different kerfs. The chosen cell shape is composed of five nonaligned square section rods ( 1 ×1 mm 2 ). For comparison of performance, two regular 1-3 piezocomposites (the same materials and equivalent periodicities) were fabricated by DFM. Electroacoustic responses in water were measured for the three composites being considered as transducers. Successive regular thinnings (from 2.8 to 1.1 mm) were carried out for each sample to increase the operating frequency (from around 0.4 to 1.3 MHz) and study the evolution of the characteristics (bandwidth and sensitivity). The experimental results confirmed the behavior of those obtained with numerical simulations, showing that the 13SC composite can be used in this entire frequency range, unlike regular composites.

A cMUT probe for ultrasound-guided focused ultrasound targeted therapy.

Ultrasound-mediated targeted therapy represents a promising strategy in the arsenal of modern therapy. Capacitive micromachined ultrasonic transducer (cMUT) technology could overcome some difficulties encountered by traditional piezoelectric transducers. In this study, we report on the design, fabrication, and characterization of an ultrasound-guided focused ultrasound (USgFUS) cMUT probe dedicated to preclinical evaluation of targeted therapy (hyperthermia, thermosensitive liposomes activation, and sonoporation) at low frequency (1 MHz) with simultaneous ultrasonic imaging and guidance (15 to 20 MHz). The probe embeds two types of cMUT arrays to perform the modalities of targeted therapy and imaging respectively. The wafer-bonding process flow employed for the manufacturing of the cMUTs is reported. One of its main features is the possibility of implementing two different gap heights on the same wafer. All the design and characterization steps of the devices are described and discussed, starting from the array design up to the first in vitro measurements: optical (microscopy) and electrical (impedance) measurements, arrays' electroacoustic responses, focused pressure field mapping (maximum peak-to-peak pressure = 2.5 MPa), and the first B-scan image of a wire-target phantom.

Dual-frequency transducer for nonlinear contrast agent imaging

Detection of high-order nonlinear components issued from microbubbles has emerged as a sensitive method for contrast agent imaging. Nevertheless, the detection of these high-frequency components, including the third, fourth, and fifth harmonics, remains challenging because of the lack of transducer sensitivity and bandwidth. In this context, we propose a new design of imaging transducer based on a simple fabrication process for high-frequency nonlinear imaging. The transducer is composed of two elements: the outer low-frequency (LF) element was centered at 4 MHz and used in transmit mode, whereas the inner high-frequency (HF) element centered at 14 MHz was used in receive mode. The center element was pad-printed using a lead zirconate titanate (PZT) paste. The outer element was molded using a commercial PZT, and curved porous unpoled PZT was used as backing. Each piezoelectric element was characterized to determine the electromechanical performance with thickness coupling factor around 45%. After the assembly of the two transducer elements, hydrophone measurements (electroacoustic responses and radiation patterns) were carried out and demonstrated a large bandwidth (70% at -3 dB) of the HF transducer. Finally, the transducer was evaluated for contrast agent imaging using contrast agent microbubbles. The results showed that harmonic components (up to the sixth harmonic) of the microbubbles were successfully detected. Moreover, images from a flow phantom were acquired and demonstrated the potential of the transducer for high-frequency nonlinear contrast imaging.

O uso de emissões otoacústicas como ferramenta auxiliar no diagnóstico de efeitos da exposição ao ruído

Resumo A exposição a níveis elevados de ruído é causa de perda auditiva sensorioneural, a qual poderia ser diagnosticada mais precocemente. O objetivo do estudo, realizado em 2009, foi investigar respostas eletroacústicas para as Emissões Otoacústicas Evocadas por Estímulo Transiente (EOAT) e as Emissões Otoacústicas Evocadas - Produto de Distorção (EOAPD) em trabalhadores expostos a níveis de ruído iguais ou superiores a 85dB(A). Meatoscopia, imitânciometria, EOAT e EOAPD foram rea- lizados em 270 trabalhadores metalúrgicos do sexo masculino, entre 18 a 30 anos, divididos em GI (160 trabalhadores com 1 a 5 anos de exposição) e GII (110 trabalhadores com exposição maior que 5 anos). Entre as EOAT do GI, 82% passaram e 5,62% falharam bilateralmente, já no GII, 80% passaram e 7,3% falharam bilateralmente. Entre as EOAPD no GI, 96,9% passaram e 0,6% falhou bilateralmente. No GII, 97,3% passaram e 0,9% falhou bilateralmente. Diferenças foram significantes para EOAPD entre os grupos, em 4 e 5 kHz. Os resultados obtidos mostraram que as respostas para EOAT estiveram ausentes em maior número quando comparadas aos registros das EOAPD, mesmo quando os limiares auditivos obtidos na Audiometria Tonal Liminar estavam normais. Assim, a EOAT é uma importante ferramenta para a detecção precoce de alterações na fisiologia coclear decorrente da exposição ocupacional ao ruído, permitindo que medidas preventivas sejam adotadas mais precocemente pelas empresas.

Electromechanical Properties of Piezoelectric Integrated Structures on Porous Substrates

The fabrication of piezoelectric transducers for high resolution medical imaging applications requires a backing material to damp the piezoelectric resonance, resulting in a shorter time response, i.e. improved resolution, but lower sensitivity. Thus, the choice of such a substrate must be made according to its acoustical properties, namely the ratio of acoustic impedance of the backing material and the piezoelectric layer. Moreover, this backing material must have a relatively high attenuation, and provide good surface properties such as a low roughness and diffusion potential. Finally, the substrate material must have a good mechanical behavior at high temperature (around 900°C for the sintering of the piezoelectric thick film). In this context a review of available materials is first given, and the corresponding list is found to be quite limited. Several PZT/PGO piezoelectric thick films deposited by screen-printing have been fabricated on dense alumina [1] and have delivered good electromechanical performance, but the substrate attenuation was too low to be used as a backing for high frequency transducers. In this paper, new porous Al 2 O 3 substrates have been fabricated and used for the fabrication of a batch of piezoelectric films. The effects of a barrier layer and bottom electrode (conductive material type and thickness) are studied and related to the electromechanical performance of the piezoelectric thick film and then to the electro-acoustic properties of the ultrasonic transducer integrating these structures. Finally, these results are compared with previous studies [2] using porous PZT substrate. These comparisons are performed in terms of fabrication facility, electromechanical constants of the thick films (in thickness mode) and electro-acoustic responses of ultrasonic transducers with center frequencies over 20 MHz.

Ultrasonic transducer based on highly textured PMN-PT piezoelectric ceramic

A successful fabrication process of textured ceramics by homo-template grain growth (HTGG) is described. Two samples fabricated with this technique were characterised. Functional properties obtained are competitive with for example a thickness coupling factor over 50%. Different characteristics were compared with those obtained by a unit cell model previously developed for piezocomposites and extended to the case of two piezoelectric phases. Behaviours of several connectivities (0–3, 3–0 and 3–3) are calculated and results show that the spatial arrangement does not have a great influence on effective parameters. A transducer based on a textured ceramic sample has been fabricated and characterised. The measured performance shows the efficiency of these new piezoelectric materials. Finally, electroacoustic responses of identical single element transducer configurations have been simulated. They show that a sensitivity increase of 3dB can be obtained at equivalent relative bandwidth of 40% with textured ceramic sample in comparison with PMN-PT ceramic used as a matrix in the textured ceramic.

SAW characteristics of AlN films sputtered on silicon substrates

This article is focused on the analysis of the electroacoustic response of surface acoustic wave (SAW) filters made of aluminium nitride (AlN) thin films on various types of Si wafers. AlN films with (0 0 · 2) orientation were deposited by RF reactive sputtering of an Al target in Ar and N 2 admixtures on Si(1 0 0) and (1 1 1) wafers with resistivities ranging between 10 and 2000 Ω cm. The electroacoustic response of SAW filters with an acoustic wavelength of 40 μm was analysed by measuring the S ij parameters with a network analyser. We have determined that the out-of-band loss is directly related to the Si substrate resistivity, varying from 26 dB for 10 Ω cm to 55 dB for 2000 Ω cm. The SAW velocity depends on the orientation of the Si wafer, being approximately 4700 m/s for Si(1 1 1) and 5100 m/s for Si(1 0 0). The electroacoustic responses of the SAW filters were fitted by computations based on a simple circuital model that takes into account parasitic effects such as airborne electromagnetic coupling and conduction through the substrate. This procedure provides accurate values of the electromechanical coupling factor k 2 even for devices with poor characteristics. Good quality SAW filters of AlN on high resistivity Si(1 0 0) wafers with k 2 larger than 0.12% are demonstrated.

Electroacoustic verification of FM benefits in advanced hearing aid circuitry

Miniature FM receivers attached to ear-level hearing aids can provide significant improvements in speech recognition in noisy environments. These receivers are designed to couple to the hearing aid via a boot connection with limited or no adjustment of hearing aid settings. Ideally, the circuitry allows the FM signal to be approximately 10 dB more intense than the typical signal from the hearing aid microphone, allowing for an FM advantage. Furthermore, for intense input levels, the output limiting should not differ for the hearing aid compared to the hearing aid and FM receiver combined, i.e., FM transparency. Following procedures recommended by the American Speech, Language, and Hearing Association, the electroacoustic responses of digital and conventional aids were measured with and without coupling to FM systems at typical conversational input levels and maximum input levels. The rms difference between the electroacoustic responses of the hearing aid alone and coupled to the FM was used to quantify the FM advantage at typical input levels and the FM transparency at high input levels. The finding of great variability in both FM advantage and transparency supports the need for additional fitting controls or design modifications to obtain the maximum FM benefit.

Discharge rate of the auditory nerve during noise revealed by electrocochlear stimulation

The purpose of this study was to evaluate the average discharge rate of all fibres in the whole auditory nerve ( R wn) when a broad-band noise with steady-state effects is applied to the ear. We assessed the R wn parameter by detecting the state of refractoriness of the nerve during noise stimulation using an electric stimulus (ES) as a probe. The technique, applied in awake pre-implanted guinea pigs (Charlet de Sauvage et al., 1994), made it possible to obtain electro-acoustic responses (EARs), from which an estimate of the R wn parameter could be deduced. Negative current pulses of 100 μs duration, each followed by an identical pulse of positive polarity for charge balance, were applied between round window and indifferent vertex electrodes at intervals of 160 ms. The 120 ms wide-band noise masker started 92 ms before every other negative ES. The signal on the stimulating electrodes was averaged over a 5.12 ms window in synchrony with the negative pulse. EARs were obtained by alternately subtracting recordings during noise from those during silence. The R wn parameter was determined by comparing experimental and computed EAR patterns. For this purpose, a model of unit response incorporating changes in amplitude and conduction velocity during the relative refractory period was designed. The recovery function of the firing probability in response to ES was evaluated. Fibres were classified in different categories according to their background discharge rates. The probability of response of single fibres to ES in each category was calculated on the basis of their interval histograms during silence and noise. Individual spikes were combined accordingly to obtain the computed EAR waveform. R wn was determined by adjusting the EAR amplitude of the model in relation to that of the experimental EAR. R wn generally increases in a linear fashion with respect to noise intensity expressed in dB, thus following the increase in loudness perception estimated by Weber’s law. At the highest noise levels, R wn tends to saturate. The estimated saturation rate was found to be about 380 spikes/s.

Computational methods and experimental measurements of scattered ultrasonics transient fields

The aim of this study is to find the transient acoustic field scattered by a target with a known dimension and geometry. The target is a rigid reflector immersed in an isotropic, homogeneous, and lossless fluid. For that purpose, the presented model takes into account not only the impulse response of the insonified medium, but also that of all the elements which constitute the electroacoustic line. The main elements of the latter are a transmitter-receiver system and an ultrasonic probe modeled by its electroacoustic and acoustoelectric impulse responses. In the hypothesis of a linear, invariant, and causal measurement line, the detected pressure is expressed in the form of a time convolution between the impulse response of all the elements which compose the electroacoustic line and the impulse response of the medium. This last depends on the geometry of transducer, target, their relative position, and the propagation medium. A signal processing method of the experimental data is developed in order to evaluate the global impulse response system. The results of this process will then be used to obtain, by simulation, the effective pressure signal. Some experimental results are presented to show the good agreement with the theoretical model proposed.

Differential sensitivity to rotation measured on potentials evoked by electrical stimulation of the guinea-pig ear

Responses to electrical stimulation of the ear applied between round-window and vertex electrodes were recorded in awake guinea-pigs from the same electrodes or from separate vertex/mastoid subdermal needle electrodes. They were averaged during opposite phases of sinusoidal rotation or before and after constant velocity rotation. In both cases the responses were subtracted from each other and yielded differential per- or post-rotatory “electrovestibular” responses. For comparison, responses were also recorded in the same animals and conditions of electrical stimulation during silence and during presentation of a broad-band noise. The difference yielded “electroacoustic” responses. In round-window records, electrovestibular and electroacoustic responses presented typical compound nerve action potential patterns. Electrovestibular responses could be recorded for head angular velocities as low as 3° sec −1 at 0.1 Hz. Response amplitude showed a logarithmic relation to head velocity. Changes in amplitude, as a function of time after rotation, were comparable to those reported for vestibular nerve fibre responses. In vertex/mastoid records, electroacoustic responses presented a sequence of peaks similar to the click-evoked auditory brain-stem responses, and electrovestibular responses presented two peaks, presumably representing contributions of central vestibular structures. Such “electrovestibulography” permits the study of an individual ear and makes available to the investigator a large range of vestibular stimulation conditions.

Differential sensitivity to rotation measured on potentials evoked by electrical stimulation of the guinea-pig ear**

Responses to electrical stimulation of the ear applied between round-window and vertex electrodes were recorded in awake guinea-pigs from the same electrodes or from separate vertex/mastoid subdermal needle electrodes. They were averaged during opposite phases of sinusoidal rotation or before and after constant velocity rotation. In both cases the responses were subtracted from each other and yielded differential per- or post-rotatory “electrovestibular” responses. For comparison, responses were also recorded in the same animals and conditions of electrical stimulation during silence and during presentation of a broad-band noise. The difference yielded “electroacoustic” responses. In round-window records, electrovestibular and electroacoustic responses presented typical compound nerve action potential patterns. Electrovestibular responses could be recorded for head angular velocities as low as 3° sec−1 at 0.1 Hz. Response amplitude showed a logarithmic relation to head velocity. Changes in amplitude, as a function of time after rotation, were comparable to those reported for vestibular nerve fibre responses. In vertex/mastoid records, electroacoustic responses presented a sequence of peaks similar to the click-evoked auditory brain-stem responses, and electrovestibular responses presented two peaks, presumably representing contributions of central vestibular structures. Such “electrovestibulography” permits the study of an individual ear and makes available to the investigator a large range of vestibular stimulation conditions.