Design of Piezoelectric Acoustic Transducers for Underwater Applications.

Interest in underwater transducers has persisted since the mid-1900s. Underwater transducers are designed in various shapes using various materials depending on the purpose of use, such as to achieve high power, improve broadband, and enhance beam steering. Therefore, in this study, an analysis is conducted according to the structural shape of the transducer, exterior material, and active material. By classifying transducers by structure, the transducer design trends and possible design issues can be identified. Researchers have constantly attempted new methods to improve the performance of transducers. In addition, a methodology to overcome this problem is presented. Finally, this review covers old and new research, and will serve as a reference for designers of underwater transducer.

Reflected Wave Reduction Based on Time-Delay Separation for the Plane Array of Multilayer Acoustic Absorbers.

This paper presents a control technique for reducing the reflection of acoustic signals for the plane array of multilayer acoustic absorbers underwater. In order to achieve this, a plane array of multilayer acoustic absorbers is proposed to attenuate low-frequency noise, with each unit consisting of a piezoelectric transducer, two layers of polyvinylidene fluorides and three layers of the acoustic window. Time-delay separation is used to find the incident and reflected acoustic signals to achieve reflected sound reduction. Experimental comparison of the attenuation rate of the reflected acoustic signal when performing passive and active controls is considered to verify the effectiveness of the time-delay separation technique applied plane array absorbers. Experiments on the plane array of smart skin absorbers confirmed that the reduction of reflected acoustic signals makes it suitable for a wide range of underwater applications.

Design and Analysis of an Active Reflection Controller That Can Reduce Acoustic Signal Refer to the Angle of Incidence.

Techniques for reducing the reflection of acoustic signals have recently been actively studied. Most methods for reducing acoustic signals were studied using the normal-incidence wave reduction technique. Although the technique of canceling an object from the normal incidence wave is essential, research on reducing acoustic signals according to the angle of incidence is required for practical applications. In this study, we designed, fabricated, and experimented with an active reflection controller that can reduce acoustic signals according to the angle of incidence. The controller consists of a transmitter on one layer, a receiver sensor on two layers, and an acoustic window on three layers. To reduce the reflected signal, a combination of the time delay and phase was applied to the controller to minimize the acoustic signal by up to -23 dB at an angle of 10°. A controller array simulation was performed based on the results of a controlled experiment. In conclusion, our proposed controller can reduce acoustic signals according to the angle of incidence, which makes it suitable for many applications.

Comparison between Resonance and Non-Resonance Type Piezoelectric Acoustic Absorbers.

In this study, piezoelectric acoustic absorbers employing two receivers and one transmitter with a feedback controller were evaluated. Based on the target and resonance frequencies of the system, resonance and non-resonance models were designed and fabricated. With a lateral size less than half the wavelength, the model had stacked structures of lossy acoustic windows, polyvinylidene difluoride, and lead zirconate titanate-5A. The structures of both models were identical, except that the resonance model had steel backing material to adjust the center frequency. Both models were analyzed in the frequency and time domains, and the effectiveness of the absorbers was compared at the target and off-target frequencies. Both models were fabricated and acoustically and electrically characterized. Their reflection reduction ratios were evaluated in the quasi-continuous-wave and time-transient modes.

Multipoint-Detection Strain Sensor with a Single Electrode Using Optical Ultrasound Generated by Carbon Nanotubes.

With the development of wearable devices, strain sensors have attracted large interest for the detection of human motion, movement, and breathing. Various strain sensors consisting of stretchable conductive materials have been investigated based on resistance and capacitance differences according to the strain. However, this method requires multiple electrodes for multipoint detection. We propose a strain sensor capable of multipoint detection with a single electrode, based on the ultrasound pulse-echo method. It consists of several transmitters of carbon nanotubes (CNTs) and a single polyvinylidene fluoride receiver. The strain sensor was fabricated using CNTs embedded in stretchable polydimethylsiloxane. The received data are characterized by the different times of transmission from the CNTs of each point depending on the strain, i.e., the sensor can detect the positions of the CNTs. This study demonstrates the application of the multipoint strain sensor with a single electrode for measurements up to a strain of 30% (interval of 1%). We considered the optical and acoustic energy losses in the sensor design. In addition, to evaluate the utility of the sensor, finger bending with three-point CNTs and flexible phantom bending with six-point CNTs for the identification of an S-curve having mixed expansion and compression components were carried out.

Single-Shot Waterless Low-Profile Photoacoustic System: Near-Field Volumetric Imaging In Vivo for Blood Vessels Based on Capacitive Micromachined Ultrasonic Transducer (CMUT).

Intensive research on photoacoustics (PA) for imaging of the living human body, including the skin, vessels, and tumors, has recently been conducted. We propose a PA measurement system based on a capacitive micromachined ultrasonic transducer (CMUT) with waterless coupling, short measurement time (<1 s), backward light irradiation, and a low-profile ultrasonic receiver unit (<1 cm). We fabricate a 64-element CMUT ring array with 6.2 mm diameter and 10.4 MHz center frequency in air, and 100% yield and uniform element response. To validate the PA tissue characterization, we employ pencil lead and red ink as solid and liquid models, respectively, and a living body to target moles and vessels. The system implements a near-field imaging system consisting of a 6 mm polydimethylsiloxane (PDMS) matching layer between the object and CMUT, which has a 3.7 MHz center frequency in PDMS. Experiments were performed in a waterless contact on the PDMS and the laser was irradiated with a 1 cm diameter. The experimental results show the feasibility of this near-field PA imaging system for position and depth detection of skin, mole, vessel cells, etc. Therefore, a system applicable to a low-profile compact biomedical device is presented.

Acoustic radiation force for analyzing the mechanical stress in ultrasound neuromodulation.

Objective. Although recent studies have shown that mechanical stress plays an important role in ultrasound neuromodulation, the magnitude and distribution of the mechanical stress generated in tissues by focused ultrasound transducers have not been adequately examined. Various acoustic radiation force (ARF) equations used in previous studies have been evaluated based on the tissue displacement results and are suitable for estimating the displacement. However, it is unclear whether mechanical stress can be accurately determined. This study evaluates the mechanical stress predicted by various AFR equations and suggests the optimal equation for estimating the mechanical stress in the brain tissue.Approach. In this paper, brain tissue responses are compared through numerical finite element simulations by applying the three most used ARF equations-Reynolds stress force ((RSF)), momentum flux density tensor force, and attenuation force. Three ARF fields obtained from the same pressure field were applied to the linear elastic model to calculate the displacement, mechanical stress, and mean pressure generated inside the tissue. Both the simple pressure field using a single transducer and the complex standing wave pressure field using two transducers were simulated.Main results. For the case using a single transducer, all three ARFs showed similar displacement. However, when comparing the mechanical stress results, only the results using the RSF showed a strong stress tensor at the focal point. For the case of using two transducers, the displacement and stress tensor field of the pattern related to the standing wave were calculated only from the results using the RSF.Significance. The model using RSF equation allows accurate analysis on stress tensor inside the tissue for ultrasound neuromodulation.