Parametric Acoustic Array Research Articles

A method for selecting a proper modulation technique for the parametric acoustic array

The implementation of the parametric acoustic array has invariably being based on Berktay’s far-field solution, despite its limitations. In addition to the problems due to the approximations of the model, recent studies suggest that the frequency response of the transducers add substantial errors in the demodulation process, which are not considered by Berktay’s equation. This paper introduces a method to overcome the frequency response problem of the ultrasonic transducers, in order to reduce the harmonic distortion produced by the parametric acoustic array, by using a suitable modulation technique. The frequency response of a parametric loudspeaker has been acoustically measured. The characteristics of this response have been applied as a filter to the signal to be demodulated, using Berktay’s far-field solution. A modulation technique has been proposed, taking into consideration the frequency response of the parametric loudspeaker. The results show an improvement in the estimation of the Total Harmonic Distortion (THD) when the parametric loudspeaker frequency response is considered in the modelling. Moreover, the experimentally measured Total Harmonic Distortion of the proposed modulation technique is consistent with the predictions of the model.

Experimental Study of a Broadband Parametric Acoustic Array for Sub‐Bottom Profiling in Shallow Water

Broadband parametric acoustic arrays appear to offer advantages for shallow water sub‐bottom profiling. In this paper, the performance of a broadband parametric acoustic array system was experimentally evaluated. In tank experiments using the nonlinear parabolic wave (KZK) equation, the directivity, source level, parametric acoustic array length, and penetration depth were evaluated. Based on Berktay’s far‐field solution, the system’s emission signal was designed. According to sea trials of the broadband parametric acoustic array system as designed, a clear sub‐bottom profile was obtained. Moreover, buried pipelines in the seabed were effectively detected, verifying the system’s effectiveness.

Volterra model of the parametric array loudspeaker operating at ultrasonic frequencies.

The parametric array loudspeaker (PAL) is an application of the parametric acoustic array in air, which can be applied to transmit a narrow audio beam from an ultrasonic emitter. However, nonlinear distortion is very perceptible in the audio beam. Modulation methods to reduce the nonlinear distortion are available for on-axis far-field applications. For other applications, preprocessing techniques are wanting. In order to develop a preprocessing technique with general applicability to a wide range of operating conditions, the Volterra filter is investigated as a nonlinear model of the PAL in this paper. Limitations of the standard audio-to-audio Volterra filter are elaborated. An improved ultrasound-to-ultrasound Volterra filter is proposed and empirically demonstrated to be a more generic Volterra model of the PAL.

Nonlinear acoustics and the Acoustical Society of America

The modern era of nonlinear underwater acoustics had its roots in a remarkable session at the 57th ASA meeting at Providence RI in June, 1960, chaired by Westervelt, of Brown University, who also presented his remarkable new theory of the parametric acoustic array. This conceptual “device” enables highly directive low frequency sound to be created from the nonlinear interaction of two high frequency radiations of high directivity, which was confirmed in a model- tank study paper in this session by Brown Univ. Professor Robert Beyer and student Bellin. JASA has been the major worldwide forum for publications in nonlinear underwater acoustics, ever since, with much of the U.S. work done at the Navy Underwater Sound Laboratory and its successor, the Naval Underwater Systems Center, as well as the Applied Research Laboratories of the University of Texas at Austin. Topics have included parametric arrays, the nonlinear generation of harmonic radiations and transients, acoustic saturation, nonlinear transmitting and receiving sonar and imaging, and many practical applications to underwater sound and acoustical oceanography. Current and/or developing applications include underwater communications, backscattering measurements, penetration of narrow beams into sediments, sub-bottom profiling, and fish school quantification and classification. These and other topics are reviewed and recent implementations are described. [Work supported by ARL:UT Austin.]

Feasibility of low-frequency directive sound source with high range resolution using pulse compression technique

The lateral resolution of a parametric acoustic array with narrow directivity is expected to be superior to that of a conventional linear sound source. The range resolution of the parametric array, however, is as low as the resolution of the linear source because of its long wavelength. To improve the range resolution in maintaining a high azimuth resolution for parametric sounds, a pulse compression technique using linear frequency-modulated signals or chirp signals is explored in the present study. The generation of chirp-modulated parametric sounds was experimentally confirmed in water, and then the auto-correlation of the parametric sound demonstrates short pulse widths up to 1/10 of the primary pulse duration. The results reveal that the pulse compression is feasible for a low-frequency parametric sound source with narrow directivity in the same manner as the compression of a linear sound wave.

Automated classification of oncoming ground vehicles using acoustic echolocation and supervised machine learning

In order to perform useful tasks, robots must have the ability to notice, recognize, and respond to objects in their environment. This requires the acquisition and synthesis of information from a variety of sensors. Here we focus on acoustic echolocation measurements of approaching vehicles, where an acoustic parametric array propagates an audible signal to the oncoming target and the reflected backscattered signal is recorded using the Microsoft Kinect microphone array. Although useful information about the target is hidden inside the noisy time domain measurements, the Dynamic Wavelet Fingerprint process (DWFP) is used to create a time-frequency representation of the data. Intelligent feature selection allows the creation of a small-dimensional feature vector that best differentiates between vehicle types for use in statistical pattern classification routines. Using experimentally measured data from real vehicles at 50 m, this process is able to correctly classify vehicles into one of five known classes with 94% accuracy. Fully three-dimensional simulations allow us to study the nonlinear beam propagation and interaction with real-world targets to improve classification results.

Realization of an omnidirectional source of sound using parametric loudspeakers

Parametric loudspeakers are often used in beam forming applications where a high directivity is required. Withal, in this paper it is proposed to use such devices to build an omnidirectional source of sound. An initial prototype, the omnidirectional parametric loudspeaker (OPL), consisting of a sphere with hundreds of ultrasonic transducers placed on it has been constructed. The OPL emits audible sound thanks to the parametric acoustic array phenomenon, and the close proximity and the large number of transducers results in the generation of a highly omnidirectional sound field. Comparisons with conventional dodecahedron loudspeakers have been made in terms of directivity, frequency response, and in applications such as the generation of diffuse acoustic fields in reverberant chambers. The OPL prototype has performed better than the conventional loudspeaker especially for frequencies higher than 500 Hz, its main drawback being the difficulty to generate intense pressure levels at low frequencies.

The Distortion Analysis and Restraint of Dual-Frequency Modulation Signals

According to the parametric acoustic array theory and Berktay far-field solution, analyze the self-demodulation distortion theory of the DSB method and the square root method with the dual-frequency signal input and present the distortion calculating formulas. Bring forward an adapting modulation algorithm, which can restrain over modulation effectively. Compared with theoretical analysis, the simulation results of MATLAB are satisfactory.

A consideration on sound masking system for achieving speech privacy using parametric acoustic array speaker

Speech privacy in open spaces is becoming increasingly important in various situations. Although measures such as the use of sound partitions are already used in many cases, measures that mask speech by emitting sounds have also been considered. A method of masking meaningful speech with meaningless noise would be valuable. Because of this, previous studies have investigated the ability of meaningless steady noise to mask speech and consequently achieve speech privacy. However, the research to date has focused on evaluating speech privacy when the masking noise is emitted from the normal loud speaker system all over the room. The masking noise emitted to the area where high level of speech privacy is not required, may cause an increased psychological impression of annoyance, leading to a decline in performance. In this study, we used a highly directional sound from modulated ultrasound as a masking noise for achieving speech privacy in the narrow area. Psychological experiments were conducted in which the masking sound was transmitted to participants from frontal or above directions with a parametric acoustic array speaker. Using the experimental data, the relationships between the degree of speech privacy and frequency characteristics and directivity of parametric acoustic array speaker were investigated. The results suggested that it is possible to maintain speech privacy in the narrow area by presenting highly directional masking sound.

Use of pattern classification algorithms to interpret acoustic echolocation data from a walking-speed robotic sensor platform

An unresolved issue for autonomous walking-speed robots in unstructured outdoor environments is maintaining situational awareness. One strategy is combining information from different sensors so the robot can function in a variety of conditions and environments. The very low-cost Microsoft Kinect accessory incorporates active infrared and RGB video sensors to provide real-time depth information, as well as a 4-channel microphone array. We validate the Kinect sensors and investigate the combination of infrared (passive and active) and non-linear acoustic echolocation sensors on our mobile robotic sensor platform. By using an acoustic parametric array to generate the audible echolocation signal, a tightly-controlled beam of low-frequency sound can interrogate targets at long distances, while infrared imaging works well in the nearfield and in difficult weather conditions. Sophisticated signal processing techniques are required to combine and interpret the collected data; we present an example using pattern ...

Gas Void Fraction Estimation in Gas-Bubble-Contained Sands with Difference Frequency Waves

A nonlinearity parameter can be used for the estimation of the gas void fraction in gassy sediment. If two primary acoustic waves of different frequencies are incident on gassy sediment, nonlinear acoustic waves can be generated at the difference frequency. In the present study, the parametric acoustic array theory for the difference frequency wave was employed to estimate the nonlinearity parameters of gas-bubble-contained sands under laboratory conditions. The nonlinearity parameters of the sands were estimated between 103 and 104. Then, the estimated gas void fractions in the sands were between 10-6 and 10-4. They were similar to those estimated through the sound speed variation method. However, this linear acoustic method could not be applied to gas-bubble-contained sand with a low gas void fraction, because the sound speed variation was not observed in the sand. This study suggests that the nonlinear acoustic method seems very feasible to estimate the gas void fraction in gassy sediment with a low gas content.

A review of parametric acoustic array in air

In this review paper, we examine some of the recent advances in the parametric acoustic array (PAA) since it was first applied in air in 1983 by Yoneyama. These advances include numerical modelling for nonlinear acoustics, theoretical analysis and experimentation, signal processing techniques, implementation issues, applications of the parametric acoustic array, and some safety concerns in using the PAA in air. We also give a glimpse on some of the new work on the PAA and its new applications. This review paper gives a tutorial overview on some of the foundation work in the PAA, and serves as a prelude to the recent works that are reported by different research groups in this special issue.

Identification of a parametric loudspeaker system using an adaptive Volterra filter

Due to the parametric acoustic array effect in air, the input audible signal of a parametric loudspeaker system can be reproduced with high directivity at the target region. However, the reproduced audible signal suffers from harmonic distortion, which is the by-product of nonlinear interaction between the primary waves. In order to investigate this inherent nonlinear phenomenon, a nonlinear system identification model is developed based on an adaptive Volterra filter. Unlike the conventional loudspeaker, the nonlinear characteristic of a parametric loudspeaker system is dependent on several primary parameters in nonlinear acoustics, which include the initial pressure of the primary waves, the observing distance and angle, as well as ambient temperature and relative humidity. By using a truncated Volterra series up to the 2nd-order kernel, numerical simulations are conducted to develop a system model with one group of parameters and examine the quadratic nonlinear intensity for different parameters’ settings. Experimental measurements, which take into account of emitter’s response, are carried out to verify the modeling result and evaluate the model performance. Based on the Volterra system model, the sound pressure level and the harmonic distortion can be accurately predicted.

Parametric array technique for microbubble excitation

This study investigates the use of an acoustic parametric array as a means for microbubble excitation. The excitation wave is generated during propagation in a nonlinear medium of two high-frequency carrier waves, whereby the frequency of the excitation wave is the difference frequency of the carrier waves. Carrier waves of around 10 and 25 MHz are used to generate low-frequency waves between 0.5 and 3.5 MHz at amplitudes in the range of 25 to 80 kPa in water. We demonstrate with high-speed camera observations that it is possible to induce microbubble oscillations with the low frequency signal arising from the nonlinear propagation process. As an application, we determined the resonance frequency of Definity contrast agent microbubbles with radius ranging from 1.5 to 5 μm by sweeping the difference frequency in the range from 0.5 to 3.5 MHz.

Introduction to nonlinear acoustics

A brief review of the basic principles of fluid mechanics needed for development of linear and nonlinear ultrasonic concepts will be given. The fundamental equations of nonlinear ultrasonics will be derived and their physical properties explained. It will be shown how an originally monochromatic finite-amplitude ultrasonic wave, due to nonlinear effects, will distort during its propagation in time and space to form higher harmonics to its fundamental frequency. The concepts of shock formation will be presented. The material nonlinearity, described by the nonlinearity parameter B/A of the material, and the convective nonlinearity, described by the ultrasonic Mach Number, will be explained. Two procedures for determination of B/A will briefly be described and some B/A-values characterizing biological materials will be presented. Shock formation, described by use of the Goldberg Number,and Ultrasonic Saturation will be discussed.. An introduction to focused ultrasonic fields will be given and it will be shown how the ultrasonic intensity will vary axially and laterally in and near the focal region and how the field parameters of interest to biomedical applications may be described by use of the KZK-Model. Finally, an introduction will be given to the parametric acoustic array formed by mixing and interaction of two monochromatic, finite-amplitude ultrasonic waves in a liquid and the potentials of this mixing process in biomedical ultrasound will briefly be mentioned.

Contributions of Robert T. Beyer to acoustical science: A review.

From his appointment as Instructor at Brown University in 1945 until well after his retirement, Beyer made significant contributions to the discovery and dissemination of knowledge about physical acoustics. Beginning with water and electrolytic solutions, Beyer and his students used ultrasonics to study relaxation processes in liquified gases, organic liquids, and single crystals, and in liquid metals. The 1951 review article with Markham and Lindsay [Markham, Beyer, and Lindsay, Rev. Mod. Phys. 23, 353411 (1951)] became a starting point for all ultrasonicists. Nonlinear acoustic phenomena also received special attention by Beyer and his students, who measured absorption and distortion of finite-amplitude waves; interaction of sound with sound, noise, and turbulence, including the first empirical verification of the parametric acoustic array in both water and air; and nonlinearity parameter of fluids as diverse as water, organic liquids, and liquid metals. New apparatus and methods were developed to aid the measurements, as of radiation pressure for the determination of absorption, a radio-frequency pulse system to measure distortion, and a complex wattmeter to measure intensity in a reactive acoustic field.

Parametric acoustic arrays: A Bergen view.

At the University of Bergen (UoB), Norway, research activity in physical acoustics started in the mid‐1960s with investigations on the parametric acoustic array (PAA). The newly appointed professor in applied mathematics, Sigve Tjo/tta, had some years earlier been at Brown University and was inspired by the concept at a fundamental level, but also wanted experimental confirmation. No previous acoustical activity existed at UoB. The PAA project was started as a master project at Department of Physics, where the main activity was in nuclear, high‐energy, and ionospheric physics. Bellin and Beyer’s experiment served as a model. The results provided new information on the axial and directional properties of the difference frequency wave field. Inspired by this, theoretical modeling continued along with further measurements. Other nonlinear effects like acoustic streaming (boundary layer, density gradient) were also investigated. In 1975, a project together with SIMRAD and Norwegian Technical University resulted in a bottom penetrating PAA, later commercialized as “TOPAS.” Numerical modeling based on the KZK equation resulted in the “Bergen Code,” still in use for computing nonlinear acoustic propagation problems. In later years activity at UoB has expanded to encompass linear physical acoustics of various sorts, occasionally using PAA as a tool.

High‐powered parametric acoustic array in air.

MIT Lincoln Laboratory has developed a prototype high‐powered parametric acoustic array (HPPAA) for standoff acoustic excitation in several applications. Parametric arrays offer a highly directional, narrow beam mechanism to deliver sound in air to desired targets typically within a 100 m range. However, a difficult challenge arises in generating sufficient sound power at difference‐frequencies below 1000 Hz at a range which can be critical for many target types. An important objective of the HPPAA design maximizes the difference‐frequency pressure amplitude at range by maximizing the end‐fire array length established by the PAA. The design optimizes the trade‐offs between the three characteristic lengths that control the resultant end‐fire array length including the pump or carrier wave attenuation, PAA aperture, and acoustic saturation of air. In field demonstrations, the HPPAA generated a carrier wave pressure power approximately 155 dB re‐20‐microPa one meter from the PAA face while generating a 300 Hz difference‐frequency SPL of 90 dB re‐20‐microPa 8 meters from the PAA. These sound pressure levels at a few hundred hertz may enable safe standoff excitation and detection of buried landmines and may be useful in standoff nondestructive testing (NDT) damage detection and imaging of structures.

Parametric acoustic array development at the U.S. Navy’s New London, Connecticut laboratory.

A brief history of the development of parametric acoustic sources at the U.S. Navy Underwater Sound Laboratory (USNUSL), and its successor, the Naval Underwater Systems Center is presented. Inspired by Robert Mellen, the Parametric Sonar Group was formed to explore the practical implications of Westervelt’s idea for underwater acoustics. Spanning more than two decades, this research pursued various potential applications exploiting the unique characteristics of parametric sources, including echo‐ranging in reverberant environments, communications involving voice, data, music, and video signals, target‐strength measurements, bottom bathymetry, high‐resolution sub‐bottom profiling, and measurements of scattering and reflection from surfaces. Parametric source design procedures were developed, and guidelines were established for diagnosing and avoiding undesirable effects, such as difference‐frequency instability and spurious nonlinearities, such as direct radiation from the projector, cavitation, and receiver nonlinearity. Theoretical models extended the Westervelt theory to realistic primary fields. For planar projector arrays, parametric source design curves, valid for levels in the primary far field, enabled designers to predict performance without resorting to computer calculations. To obtain source‐levels and beam‐widths within the primary near field, where the secondary signal is generated, a computer program, called CONVOL5 in its present configuration, can be used to obtain more detailed performance predictions.

Audio parametric arrays in air.

Following Westervelt’s discovery and theoretical description of the parametric acoustic array reported at the 1960 meeting of the ASA at Brown Univ., and the measurements in both water and air at ultrasonic frequencies, confirming Westervelt’s predictions qualitatively, reported by Bellin and Beyer at that same meeting, it was not until 1973 that Bennett and Blackstock reported convincing quantitative comparisons of theory and experiment for a parametric array in air. While their experiments produced difference frequencies in the audio range, not until the late 1990s did application of the parametric array to directional audio transmission in air achieve any commercial success. This presentation will briefly review the history of developments leading up to modern‐day audio parametric arrays. Challenges associated with transducer performance, signal processing, and acoustical measurements specific to audio parametric arrays in air will be described. Features of several audio parametric arrays currently on the market will be discussed. Finally, results will be presented from an ongoing collaboration between the author’s group at Univ. of Texas at Austin and Professor Pierre Khuri‐Yakub’s group at Stanford Univ. to create audio parametric arrays in air using capacitive micromachined ultrasonic transducers to transmit the ultrasonic primary beams.