Sound Projector Research Articles

Realising acoustic projectors using systems of metamaterial lenses

In this work, we demonstrate how the use of acoustic metamaterials allows using concepts from optics even when ray optics is not applicable. In particular, we use two compact metasurfaces, designed to be acoustic lenses, to realise a Keplerian acoustic telescope. We use the telescope in front of a loudspeaker to realise a sound projector, capable of delivering sound in specific positions within a diameter of 4m from the source. We first evaluate the accuracy and the limitations of our device by running classical microphone measurements, highlighting the shape of the delivered field and the contrast of the “image” with the surroundings. We then run two indoor localization experiments involving human participants, using either tonal signals or classical music. In the first experiment, we set the device to deliver sound in a room and asked participants to find it, while they were conducting a soundwalk across the space. In the second experiment, the participants were sitting in a fixed position and we moved the sound around them, once again asking them to locate where they thought it was coming from. Our results show the similarities of concepts like “aberrations” and “focusing” with optics, when metamaterials are used even when a single loudspeaker, opening possibilities for localised acoustics special effects.

A review of the effect of various type of artificial sounds to odontocetes

Reactions of bottlenose dolphins, Pacific white-sided dolphins, false killer whales, Risso's dolphins, Dall's porpoises to artificial underwater sounds were observed for developing countermeasures to reduce by-catch on gill nets. Related papers published during 1980-1995 in Japan were re-visited. Various sound projectors having dominant frequencies from 2 kHz to 200 kHz with source levels reaching up to 200dB re 1μPa. Reactions of dolphins and porpoises to the sounds were observed in a pool, a net enclosure, or an open sea. Clear escaping reactions from a sound source were observed by sounds with exposed level above 170dB at the subject animal. Reactions to sounds below 160dB were not stable. The dolphins got accustomed to the sound after multiple projections. On the other hand, frequency and amplitude modulated sounds seemed to be effective even if the sound pressure level was about 120dB. Efficacy of acoustic pingers to control wild dolphins and porpoises was limited. The exposure level to change behavior was estimated above 170dB that depends on the source level of a device and distance to the animal. Acclimation for the sounds by dolphins and porpoises were not negligible. Avoiding successive transmission and amplitude/frequency modulations induced behavioral reactions.

Improved thermoacoustic sound projectors by vibration mode modification

The problem of coupled panel-cavity vibrations is addressed by studying the resonance modes in high power, low frequency, encapsulated thermoacoustic sound projectors using scanning laser vibrometry and sound pressure measurements. We fabricated and analyzed the performance of large area thermoacoustic sound projectors based on freestanding carbon nanotube sheets, encapsulated between a heat sink and various rigid vibrating plates. For thin, simply-supported vibrating plates we observed a strong deviation of resonance frequency and sound intensity from theoretical prediction for in vacuo plates. The domination of symmetric, odd modes for thin plates, with substantially improved sound radiation, is attributed to the coupled panel-cavity modes produced by harmonically varying pressure within the cavity. The observed modified mode shapes result from a minimum energy principle, wherein the potential energy stored in the entrained gas is minimized by reducing the overall change of the cavity gas volume. The effect of plate and cavity thickness on thermoacoustic projector performance in air is described for various devices. The optimized ultralight, low volume sound projectors can generate a remarkably high sound pressure level in air of over 120 dB re 20 μPa @ 1 m in a frequency range of 500 - 3000 Hz.

Experimental validation of axially polarized multilayer piezoelectric composite cylindrical transducers with adjustable multifrequency

The axially polarized multilayer piezoelectric composite cylindrical transducers with adjustable multifrequency capability have been proposed by adjusting the external electric resistance and the ratio of piezoelectric layer numbers between the actuator part and the sensor part, which have promising potential in designing the novel cymbal transducer for underwater sound projector and ultrasonic radiator applications. In the previous studies, the multilayer models were established to guide the design of the transducers with arbitrary layer number, and analyzed the dynamic characteristics theoretically. In this work, an experimental study is performed to validate the theoretical models and predictions. Piezoelectric rings with multiple concentric annular electrodes are designed to characterize the multilayer piezoelectric composite cylindrical transducers. The top surface of the piezoelectric rings is divided into two separate parts. One part is covered by multiple concentric annular electrodes, corresponding to the piezoelectric layers, and the other part is uncovered, corresponding to the elastic layers. Four prototypes are fabricated and each consists of four concentric annular electrodes. The impedance spectra are measured by the impedance analyzer to obtain the resonance and anti-resonance frequencies. Effects of two adjusting methods on the dynamic characteristics are evaluated experimentally. The experimental results basically coincide with the theoretical ones. This comprehensive experimental work assures the feasibility of using axially polarized multilayer piezoelectric composite cylindrical transducers with adjustable multifrequencies and confirms the benefit of the developed theoretical models for guiding the fabrication and optimization of this type of transducers.

Signal processing ocean ambient sound for environmental awareness

Signal processing methods have been developed that use ocean ambient sound to estimate environmental parameters such as the seabed properties. The techniques are straightforward, but in realistic environments there are a variety of complications. The methods considered use a vertically oriented hydrophone line array and practical issues arise such as array motion and deformation. These array parameters are often unknown and represent mismatch with the processing assumptions (e.g., a stationary, straight array). If ignored, performance can degrade significantly so robust signal processing techniques are needed to mitigate. Further, when using ambient sound, as opposed to a sound projector, additional averaging time is needed; this is typically on the order of one or two minutes. Stability in the measurements is essential to obtain the necessary coherent gain. Motion-correcting averaging methods, Doppler correction, adaptive beamforming and supergain are applied to ambient sound processing and will be discussed. Further, data will be analyzed to illustrate the relevant issues and potential remedies in real acoustic environments. Measurements from recent experiments will be presented to illustrate the performance including data collected on the New England Shelf Break in April/May 2021. [Work supported by the Office of Naval Research.]

Boundary Pass underwater listening station cabled hydrophone array system

An advanced cabled real-time underwater listening station was deployed in May 2020 in Boundary Pass, British Columbia, Canada, to measure underwater radiated noise of commercial vessels and to detect and track vocalizing marine mammals. This system was sponsored by Transport Canada and is operated jointly by Vancouver Fraser Port Authority and JASCO Applied Sciences (Canada) Ltd. The system consists of two synchronized tetrahedral hydrophone arrays, each with 1.65 m hydrophone spacings, deployed 300 m apart on the seabed in 190 m water depth. The hydrophone frames are connected to shore by two 2.8 km fiber-optic cables, and also support ADCP and CTD sensors, sound projectors for calibrations, and video cameras. The Boundary Pass location was chosen because of its relatively deep water and because the major inbound and outbound commercial shipping lanes leading to Vancouver become narrow and adjacent here, allowing acoustic measurements of ships passing in both directions. In this presentation we will discuss the system technical design and sensor specifications. We will also discuss its deployment and the data processing and presentation systems, with a brief overview of the substantial measurements obtained in the first full year of operation.

A spatial sound delivery system for virtual and augmented reality

Virtual and Augmented Reality (VR and AR) are increasingly becoming a consumer commodity, where sound plays a central role, usually delivered through headphones. Headphones, however, do not allow direct interactions between users outside the virtual world: other solutions may be necessary. In this work, we present a different approach: a sound projector, which can deliver spatial sound at a distance by forming a beam of audible sound. Our device consists of a portable speaker and two metamaterial lenses of different focal length, arranged in a telescope. We test our device by delivering signals between 5 and 6 kHz to moving people, tracked by a camera. We benchmark its performance with measurements and a pilot study, with human participants. Finally, we connect our projector to a VR headset and show how it can be used to deliver messages in VR. In this work, we discuss the limitations and perspectives of similar metamaterial-based delivery systems.

Auditory brainstem responses to stimulus offset in the bottlenose dolphin (Tursiops truncatus)

The auditory brainstem response (ABR) is typically associated with the onset of a sound stimulus; however, ABRs can also be produced at sound offset. Here, ABRs in response to stimulus offset were examined in normal hearing (NH) and hearing impaired (HI) bottlenose dolphins. Tests were conducted in San Diego Bay, with the dolphin positioned underwater in front of a piezoelectric sound projector. Scalp activity was amplified (94 dB) and filtered (300–3000 Hz) before synchronous, weighted averaging and digitization. Each ABR was the averaged response to 512 stimuli; two ABRs were obtained for each stimulus condition. Stimuli included spectrally pink noisebursts (bandwidth: 20–160 kHz) and tonebursts (40-kHz; 113-kHz for the NH dolphins only). Stimulus level, risetime, and duration were systematically manipulated across experimental trials. Regardless of onset/offset envelope (cosine or linear), large amplitude offset responses were observed to 40-kHz tonebusts presented at higher levels with faster rise times for the NH but not the HI animals. One interpretation of these findings is that the offset response may be arising from more basal regions of the cochlea than the onset response. [Work supported by US Navy Living Marine Resources Program.]

High power-density thermoacoustic sound projectors

Design, fabrication, and performance of large size and high power-density encapsulated thermoacoustic sound projectors driven by short pulses will be presented. The analysis of enhanced energy conversion efficiency using proper signal processing and low-dimensional thermodynamics will be given. [Work supported by the Office of Naval Research and Army Research Office.]

A review of single crystal underwater transducers

The unique material properties of lead magnesium niobate-lead titanate (PMN-PT) single crystals enable compact, broadband, high power sound projectors that cannot be realized using conventional ceramics. This paper shall highlight single crystal transducer prototypes developed at the Naval Undersea Warfare Center, Division Newport, and elsewhere demonstrating that the broadband, high power performance promised by single crystal’s material properties can be realized in practical underwater transducers. Various single crystal transducer designs for underwater applications, including longitudinal vibrators, fully active and active-passive air-backed cylinder transducers, free-flooded ring transducers and bender bar transducers, will be shown. These transducer designs cover a wide span of frequencies, and utilize both 33- and 32- operating modes. Tuned bandwidths of up to 2.5 octaves of frequency, as well as source level increases of up to 15 dB at the band edges, will be demonstrated. t will also be shown that these technologies provide stable performance under high drive and duty cycle conditions. Finally, the impact of using these single crystal sources on the drive electronics and power consumption shall also be discussed.

벤더 디스크형 저주파 수중 음향 발생기의 특성분석

We have performed characteristic analysis on developed bender-disc type underwater sound projector which is used for producing low frequency and high power sound source. The circular unit bender- disc is 60 mm in radius and 27 mm in hight and both sides of the unit disc are radiating surfaces which vibrate in flexural mode of unimorph bender disc. The thickness and radius of piezoelectric layers of the bender disc are 2.5 mm and 60 mm respectively. PZT4 and PZT5A were used for the piezoelectric layer to compare its characteristics. The array projector consists of 12 unit discs which are aligned in a normal direction of the radiating surface with gap size of 13 mm. Transmitting voltage responses (TVR) and source level (SL) of the unit disc was analysed by lumped parameter models and 2d axisymmetric finite element models varying design parameters. The acoustic performance of the projector was experimently evaluated and analyzed on TVR, SL, horizontal beam pattern, and linearity.

Thermophones for sonar applications

Thermophones are electrically driven thermoacoustic sound projectors which have historically been used as a primary source of sound. Thermophones have a sound spectra that are largely determined by their housing and support and efficiencies that are determined by the device’s ability to maintain a dynamic temperature gradient across a gaseous layer surrounding the thin heating element. A number of factors make thermophones an attractive technology for underwater use including the relative ease and low cost of production, the large thermal reservoir provided by the surrounding aquatic environment, and the ability to tune the spectra over a broad range of frequencies. We present calibrated acoustic underwater tests performed on thermophones which demonstrate the potential for a new class of sonar transducers. Small, 6.35 cm diameter, inert gas filled laminate pouch thermophones were fabricated which provide a low frequency resonance. Additionally, a liquid filled thermophone demonstrates a smooth response over a wide frequency band.

Carbonized Electrospun Nanofiber Sheets for Thermophones.

Thermoacoustic performance of thin freestanding sheets of carbonized poly(acrylonitrile) and polybenzimidazole nanofibers are studied as promising candidates for thermophones. We analyze thermodynamic properties of sheets using transport parameters of single nanofibers and their aligned and randomly electrospun thin film assemblies. The electrical and thermal conductivities, thermal diffusivity, heat capacity, and infrared blackbody radiation are investigated to extract the heat exchange coefficient and enhance the energy conversion efficiency. Spectral and power dependencies of sound pressure in air are compared with carbon nanotube sheets and theoretical prediction. Despite lower thermoacoustic performance compared to that of CNT sheets, the mechanical strength and cost-effective production technology of thermophones make them very attractive for large-size sound projectors. The advantages of carbonized electrospun polymer nanofiber sheets are in the low frequency domain (<1000 Hz), where the large thermal diffusion length diminishes the thermal inertia of thick (∼200 nm) nonbundled fibers and the high intrinsic thermal conductivity of fibers enhances the heat exchange coefficient. Applications of thermoacoustic projectors for loudspeakers, high power SONAR arrays, and sound cancellation are discussed.

Passive acoustical oceanography

Acoustical Oceanography includes the use of both active and passive acoustic systems to measure various properties of the ocean environment. Active methods use a specifically designed and controlled sound projector while passive methods exploit naturally occurring ocean sounds such as breaking waves, ship noise, or marine mammal vocalizations. Historically, active methods have been the preferred choice when precise propagation times or calibrated sound levels are important. However, passive sensing has a strong appeal due in part to the ease in which listening-only systems can be deployed. There have been a number of advances in passive sensing even for cases where propagation times and sound levels are of interest. For example, it has been shown that surface wave noise can be used to measure vertical propagation times through the seabed and water column. More recently, there is experimental evidence that indicates noise correlations may also be used to determine horizontal propagation times in the seabed. Although replacing controlled projectors with noise sources is an attractive option there are limitations to how and when noise can be used this way. These limitations as well as experimental and modeling results from surface and ship noise will be described in this presentation.

Effects of Sound on the Behavior of Wild, Unrestrained Fish Schools.

To assess and manage the impact of man-made sounds on fish, we need information on how behavior is affected. Here, wild unrestrained pelagic fish schools were observed under quiet conditions using sonar. Fish were exposed to synthetic piling sounds at different levels using custom-built sound projectors, and behavioral changes were examined. In some cases, the depth of schools changed after noise playback; full dispersal of schools was also evident. The methods we developed for examining the behavior of unrestrained fish to sound exposure have proved successful and may allow further testing of the relationship between responsiveness and sound level.

Place specificity of dolphin auditory evoked potentials assessed with high-pass masking noise

Auditory evoked potential measurements are commonly used for hearing assessment in marine mammals for which psychophysical testing is not practical. Evoked potential measurements typically employ electrical signals consisting of short-duration tone-pips or sinusoidal amplitude modulated tones, which are then presented to piezoelectric underwater sound projectors. Although tone-pip and short-duration tonal stimuli may possess relatively narrow frequency bandwidth, the resulting physiological responses may possess much greater bandwidth, especially for lower frequency stimuli at higher stimulus levels. In this study, high-pass masking noise techniques were used to examine the place specificity of auditory evoked responses from click, tone-pip, and sinusoidal amplitude modulated tones in bottlenose dolphins (Tursiops truncatus). The experimental methods for generating and spectrally equalizing masking noise and click stimuli will be presented, along with the effect of compensated clicks with uncompensated clicks and ABR latencies and amplitudes. [Funded by U.S. Navy Living Marine Resources Program.]

Losses in piezoelectrics derived from a new equivalent circuit

Miniaturization of piezoelectric devices such as ultrasonic motors, transformers, and sound projectors requires high power density maintained in the piezoelectric materials. During operation, heat generation due to material losses, however, hinders the realization of high power density. As a result, it is very important to understand the loss mechanisms in piezoelectric materials to successfully realize device miniaturization and at the same time maintain a good device performance. There are three fundamental losses in piezoelectric materials: dielectric, elastic, and piezoelectric. The first two components have been intensively investigated, whereas piezoelectric loss has not received much attention, which leaves some phenomena inexplicable. To verify its significance, in this paper, a new methodology has been presented to calculate the dielectric, elastic and piezoelectric losses based on a new equivalent circuit (EC) deduced from the revised Hamilton’s Principle. The improvement of the new methodology lies in the fact that all the three losses, including the piezoelectric loss, have been fully taken in consideration. The derivations and the calculation results indicate that the piezoelectric loss is not only non-negligible, but is also larger than other two components for hard PZT materials. The significance of the piezoelectric loss component has been therefore verified. During the above verification process, an elegant and concise measuring procedure of all the losses, including piezoelectric component, has been presented from an EC aspect for the first time. The calculation and measurement also indicate that the largest mechanical quality factor exists at a frequency between resonance and antiresonance, which may suggest a new optimal working frequency for piezoelectric devices from the loss reduction viewpoint.

Modeling and optimization of adjustable multifrequency axially polarized multilayer composite cylindrical transducer

A novel adjustable multifrequency axially polarized multilayer composite cylindrical transducer is developed in this paper. The transducer is composed of two parts: an actuator part and a sensor part. Each part is considered as a multilayer piezoelectric/elastic composite structure. The actuator part is utilized to actuate the transducer, while the senor part is used to adjust its dynamic characteristics through connecting to an external electric resistance. Based on the plane stress assumption, the radial vibration of this new kind of transducer is analyzed, and its input electric admittance is derived analytically. Comparisons with the earlier works are conducted to validate the theoretical solution. Furthermore, numerical analysis is performed to study the effects of the external electric resistance on the transducer’s dynamic characteristics, such as resonance and anti-resonance frequencies, as well as the corresponding electromechanical coupling factor. Numerical results show that the multifrequency cylindrical transducer can be designed through adjusting the external electric resistance and the ratio of piezoelectric layer numbers between the actuator part and the sensor part. In addition, the optimized transducer can be proposed at the matching electric resistance. The proposed cylindrical transducer plays an important role in designing the cymbal transducer, which can be used in underwater sound projectors and ultrasonic radiators.

Underwater acoustics education at Portland State University

The Northwest Electromagnetics and Acoustics Research Laboratory (NEAR-Lab) is in the Electrical and Computer Engineering Department at Portland State University (PSU) in Portland, Oregon. The NEAR-Lab was founded in 2005 and is co-directed by Lisa M. Zurk and Martin Siderius. A primary interest is underwater acoustics, and students at undergraduate and graduate levels (occasionally also high school students) regularly participate in research. This is synergistic with underwater acoustics education at PSU, which includes a course curriculum that provides opportunities for theoretical and experimental research and multiple course offerings at both the undergraduate and graduate level. The research generally involves modeling and analysis of acoustic propagation and scattering, acoustic signal processing, algorithm development, environmental acoustics, and bioacoustics. The lab maintains a suite of equipment for experimentation including hydrophone arrays, sound projectors, a Webb Slocum glider, an electronics lab, and an acoustic tank. Large-scale experiments that include student participation have been routinely conducted by successful collaborations such as with the APL-University of Washington, NATO Centre for Maritime Research and Experimentation, and the University of Hawaii. In this talk, the state of the PSU underwater acoustics program will be described along with the courses offered, research activities, experimental program, collaborations, and student success.

Thermal management of thermoacoustic sound projectors using a free-standing carbon nanotube aerogel sheet as a heat source

Carbon nanotube (CNT) aerogel sheets produce smooth-spectra sound over a wide frequency range (1–105 Hz) by means of thermoacoustic (TA) sound generation. Protective encapsulation of CNT sheets in inert gases between rigid vibrating plates provides resonant features for the TA sound projector and attractive performance at needed low frequencies. Energy conversion efficiencies in air of 2% and 10% underwater, which can be enhanced by further increasing the modulation temperature. Using a developed method for accurate temperature measurements for the thin aerogel CNT sheets, heat dissipation processes, failure mechanisms, and associated power densities are investigated for encapsulated multilayered CNT TA heaters and related to the thermal diffusivity distance when sheet layers are separated. Resulting thermal management methods for high applied power are discussed and deployed to construct efficient and tunable underwater sound projector for operation at relatively low frequencies, 10 Hz–10 kHz. The optimal design of these TA projectors for high-power SONAR arrays is discussed.