Mark S. Wochner

50 kHz capacitive micromachined ultrasonic transducers for generation of highly directional sound with parametric arrays

In this study, we examine the use of capacitive micromachined ultrasonic transducers (CMUTs) with vacuum- sealed cavities for transmitting directional sound with parametric arrays. We used finite element modeling to design CMUTs with 40-mum- and 60-mum-thick membranes to have resonance frequencies of 46 kHz and 54 kHz, respectively. The wafer bonding approach used to fabricate the CMUTs provides good control over device properties and the capability to fabricate CMUTs with large diameter membranes and deep cavities. Each CMUT is 8 cm in diameter and consists of 284 circular membranes. Each membrane is 4 mm in diameter. Characterization of the fabricated CMUTs shows they have center frequencies of 46 kHz and 55 kHz and 3 dB bandwidths of 1.9 kHz and 5.3 kHz for the 40-mum- and 60-mum-thick membrane devices, respectively. With dc bias voltages of 380 V and 350 V and an ac excitation of 200 V peak-to-peak, the CMUTs generate average sound pressure levels, normalized to the devices surface, of 135 dB and 129 dB (re 20 muPa), respectively. When used to generate 5 kHz sound with a parametric array, we measured sound at 3 m with a 6 dB beamwidth of 8.7deg and a sound pressure level of 58 dB. To understand how detector nonlinearity (e.g., the nonlinearity of the microphone used to make the sound level measurements) affects the measured sound pressure level, we made measurements with and without an acoustic low-pass filter placed in front of the microphone; the measured sound levels agree with numerical simulations of the pressure field. The results presented in this paper demonstrate that large-area CMUTs, which produce high-intensity ultrasound, can be fabricated for transmitting directional sound with parametric arrays.

Numerical simulation of finite amplitude wave propagation in air using a realistic atmospheric absorption modela)

A nonlinear system of fluid dynamic equations is modeled that accounts for the effects of classical absorption, nitrogen and oxygen molecular relaxation, and relative humidity. Total variables rather than acoustic variables are used, which allow for the inclusion of frequency-independent terms. This system of equations is then solved in two dimensions using a fourth-order Runge-Kutta scheme in time and a dispersion-relation-preserving scheme in space. It is shown that the model accurately simulates wave steepening for propagation up to one shock formation distance. For a source amplitude of 157dB re 20μPa, the Fourier component amplitudes of the analytical and computed waveforms differ by 0.21% at most for the first harmonic in a lossless medium and 0.16% at most for the first harmonic in a medium that includes thermoviscous losses. It is shown that the absorption due to classical effects and molecular relaxation demonstrated by the model is within 1% of the analytical model and the computed dispersion du...

Cubic nonlinearity in shear wave beams with different polarizations

A coupled pair of nonlinear parabolic equations is derived for the two components of the particle motion perpendicular to the axis of a shear wave beam in an isotropic elastic medium. The equations account for both quadratic and cubic nonlinearity. The present paper investigates, analytically and numerically, effects of cubic nonlinearity in shear wave beams for several polarizations: linear, elliptical, circular, and azimuthal. Comparisons are made with effects of quadratic nonlinearity in compressional wave beams.

Sound propagation in water containing large tethered spherical encapsulated gas bubbles with resonance frequencies in the 50 Hz to 100 Hz range

The efficacy of large tethered encapsulated gas bubbles for the mitigation of low frequency underwater noise was investigated with an acoustic resonator technique. Tethered latex balloons were used as the bubbles, which had radii of approximately 5 cm. Phase speeds were inferred from the resonances of a water and balloon-filled waveguide approximately 1.8 m in length. The Commander and Prosperetti effective-medium model [J. Acoust. Soc. Am. 85, 732-746 (1989)] quantitatively described the observed dispersion from well below to just below the individual bubble resonance frequency, and it qualitatively predicted the frequency range of high attenuation for void fractions between 2% and 5% for collections of stationary balloons within the waveguide. A finite-element model was used to investigate the sensitivity of the waveguide resonance frequencies, and hence the inferred phase speeds, to changes in individual bubble size and position. The results indicate that large tethered encapsulated bubbles could be used mitigate low frequency underwater noise and that the Commander and Prosperetti model would be useful in the design of such a system.

6F-4 50-kHz Capacitive Micromachined Ultrasonic Transducers for Generating Highly Directional Sound with Parametric Arrays

Capacitive micromachined ultrasonic transducers (CMUTs) were designed, fabricated, and characterized to generate highly directional sound in air with parametric arrays. Finite element modelling was used to design the vacuum-sealed CMUTs to operate at frequencies of approximately 50 kHz and to generate pressures of approximately 139 dB (0 dB = 20 muPa RMS) at the face of the transducer. CMUTs with 40-mum and 60-mum thick membranes were fabricated and characterized. The characterized devices had center frequencies of 46 kHz and 55 kHz and bandwidths of 2.0 kHz and 5.4 kHz, respectively. Driven with a 200-V peak-to-peak excitation voltage, the devices generated up to 115 dB and 107 dB of pressure at 3 m, respectively. The devices were used to produce a narrow (8.7deg) beam of 5 kHz sound, which at 3 m was 58 dB.

Attenuation of standing waves in a large water tank using arrays of large tethered encapsulated bubbles

The use of bubble resonance effects to attenuate low-frequency underwater sound was investigated experimentally in a large water tank. A compact electromechanical sound source was used to excite standing wave fields at frequencies ranging between 50 and 200 Hz in the tank. The source was then surrounded by a stationary array of tethered encapsulated air bubbles, and reduction in standing wave amplitude by as much as 26 dB was observed. The bubbles consisted of either thin-shelled latex balloons with approximately 5 cm radii or thicker-shelled vinyl boat fenders with 6.9 cm radii. The effects of changing the material and thickness of the bubble shells were found to be in qualitative agreement with predictions from Churchs model for sound propagation in a liquid containing encapsulated bubbles [J. Acoust. Soc. Am. 97, 1510-1521 (1995)]. Although demonstrated here for low frequency noise abatement within a tank, which is useful for quieting acoustic test facilities and large tanks used for marine life husbandry, the eventual aim of this work is to use stationary arrays of large tethered encapsulated bubbles to abate low frequency underwater noise from anthropogenic sources in the marine environment.

Short-range shock formation and coalescence in numerical simulation of broadband noise propagation.

The number of jet and rocket noise studies has increased in recent years as researchers have sought to better understand aeroacoustic source and radiation characteristics. Although jet and rocket noise is finite-amplitude in nature, little is known about the existence of shock formation and coalescence close to the source. A numerical experiment is performed to propagate finite-amplitude noise and determine the extent of the nonlinearity over short distances with spherical spreading. The noise is filtered to have a haystack shape in the frequency domain, as is typical of such sources. The effect of the nonlinearity is compared in both the temporal and frequency domains as a function of distance. Additionally, the number of zero-crossings and overall sound pressure level is compared at several distances. The results indicate that the center frequency plays a particularly important role in the amount of coalescence and spectral redistribution that occurs. The general applicability of these results to actual near-field finite-amplitude jet and rocket noise experiments is also presented.

Mitigation of low‐frequency underwater sound using large encapsulated bubbles and freely‐rising bubble clouds.

Low-frequency anthropogenic noise may affect marine life, motivating the need to minimize its potential impact. Bubbles cause significant dispersion and attenuation of underwater sound at frequencies near the individual bubble resonance and can potentially be used to abate this noise. Such effects have been reported for large encapsulated bubbles with resonance frequencies below 100 Hz, and significant attenuation due to bubble resonance phenomena and acoustic impedance mismatch was observed in a tank experiment [J. Acoust. Soc. Am. 127:2015 (2010); J. Acoust. Soc. Am. 128:2279 (2010)]. Both of these mechanisms were found to significantly reduce down-range radiated acoustic pressure, as much as 40 dB, at low frequencies (60 to 1000 Hz) in a series of lake experiments where a sound source was surrounded by an array of tethered resonant toroidal air bubbles, a cloud of freely-rising sub-resonant bubbles, and various combinations of the two. Hydrophones were placed at various depths and ranges to determine the effect of the bubbles on the radiated field. The effects of void fraction and bubble size variation on the spectrum of the radiated sound were also investigated. [Work supported by Shell.]

Attenuation of low-frequency underwater sound using bubble resonance phenomena and acoustic impedance mismatching

Air bubbles can be a significant source of attenuation in underwater sound propagation, but such effects have not been experimentally verified for low frequencies in part due to the difficulty in creating large stable bubbles. This work is in part an extension of a previously reported study on the acoustic effects of large resonant encapsulated air bubbles in a 1‐D waveguide [J. Acoust. Soc. Am. 127, 2015 (2010)]. Now, both bubble resonance effects and impedance contrast effects are investigated in a large (2600 m3) tank. Both of these mechanisms are shown to attenuate sound at low frequencies (50–200 Hz) through experiments in which a sound source is surrounded by a column of freely rising sub‐resonant bubbles, a matrix of tethered resonant air balloons, and a combination of the two. Experiments with a matrix of thicker‐shelled encapsulated bubbles demonstrate decreased attenuation due to weaker resonant interaction. The use of a polydipserse versus monodisperse encapsulated bubble size distribution demo...

The design and characterization of capacitive micromachined ultrasonic transducers (CMUTs) for generating high-intensity ultrasound for transmission of directional audio

A directional source of audio sound created using a parametric array, sometimes called an audio spotlight, generates a sound beam that is much narrower than the sound beam generated by a conventional source. These directional sources require the transmission of a modulated high-intensity ultrasonic carrier wave. Capacitive transducers are well-suited for parametric array audio applications because they can efficiently generate high-intensity ultrasound with a relatively wide bandwidth. CMUTs with vacuum-sealed cavities are particularly advantageous because they lack squeeze-film damping, which increases bandwidth but reduces displacement, and because their sealed cavities and permanently attached membranes make them relatively robust. In this paper, we present the basic design constraints of CMUTs intended to generate low-frequency high-intensity airborne ultrasound. In addition, we describe a new method for fabricating these CMUTs that results in uniform cavity depths and a thick insulating oxide layer. Measurement of a fabricated devices input impedance and small-signal displacement demonstrates the success of the new fabrication method and shows good agreement with theory.