Kyle S. Spratt

A modal architecture for artificial reverberation

The method of analyzing an acoustic space by way of modal decomposition is well established. In this work, a computational structure employing modal decomposition is introduced for synthesizing artificial reverberation, implementing the modes using a collection of resonant filters, each driven by the source signal and summed in a parallel structure. With filter resonance frequencies and dampings tuned to the modal frequencies and decay times of the space, and filter gains set according to the source and listener positions, any number of acoustic spaces and resonant objects may be simulated. While convolutional reverberators provide accurate models but are inflexible and computationally expensive, and delay network structures provide only approximate models but are interactive and computationally efficient, the modal structure presented in this work provides explicit, interactive control over the parameters of each mode, allowing accurate modeling of acoustic spaces, movement within them and morphing among...

Radiation damping of, and scattering from, an arbitrarily shaped bubble

The work by Strasberg on small volume oscillations of bubbles with arbitrary shape [J. Acoust. Soc. Am. 25, 536-537 (1953)] is here extended to include the effects of radiation damping. An expression for the far-field scattering amplitude from an arbitrarily shaped bubble is given in terms of a quantity that is mathematically equivalent to the electrostatic capacitance of the bubble shape. This general approach is then applied to prolate and oblate spheroidal geometries, for which simple analytic expressions are available for the electrostatic capacitance, and the resulting far-field scattering amplitudes are compared to previous work in the literature.

Cylindrically converging nonlinear shear waves

The low shear moduli of soft elastic media permit the generation of shear waves with large acoustic Mach numbers that can exhibit waveform distortion and even shock formation over short distances. Waves that converge onto a cylindrical focus experience significant dispersion, causing waveforms at the focus and in the post-focal region to differ significantly from the source waveform even in the absence of nonlinear distortion. A full-wave model for nonlinear shear waves in cylindrical coordinates that accounts for both quadratic and cubic nonlinearity is developed from first principles. For the special case of an infinite cylindrical source with particle motion parallel to the axis, for which nonlinearity is purely cubic, the nonlinear wave equation is solved numerically with a finite-difference scheme. The full-wave model is compared with a piecewise model based on a generalized Burgers equation for cylindrically converging waves outside of the focal region and linear diffraction theory in the focal regi...

McKinney Fellowship in acoustics at ARL:UT

The McKinney Fellowship in Acoustics is a graduate research fellowship awarded to one student per year by Applied Research Laboratories at The University of Texas at Austin (ARL:UT). Created in 2006 and named after former lab director Chester M. McKinney, the fellowship is meant to foster quality research in acoustics while maintaining the strong connection between ARL:UT and the Graduate Program in Acoustics in the Cockrell School of Engineering. The fellowship provides up to three years of tuition and fees as well as a graduate research assistantship and travel expenses for the purpose of performing work toward a master’s or doctoral degree on a topic related to the acoustics research and development performed at ARL:UT. In this talk, the implementation of the McKinney Fellowship program will be given, as well as an overview of the research that has been done by the 10 McKinney Fellows of the past decade. The first author, a past McKinney Fellow, will also offer some personal perspective on the program.

Champagne bubble acoustics

Sparkling wine, such as the variety coming from the Champagne region of France, is a beverage that is at least partially famous for its carbon dioxide bubbles, a byproduct of the secondary fermentation process that occurs after bottling. A well-known theory, though hardly accepted universally, posits that the quality of a sparkling wine can be ascertained from the characteristics of its bubbles, such as bubble size distribution and rate of production. This talk describes a preliminary investigation to monitor the characteristics of sparkling wine bubbles using passive acoustic measurements, wherein bubble parameters are estimated from the power spectral density of the ambient bubble noise. Measurements made on a variety of sparkling wines will be presented.

Deconvolution methods to obtain the impulse response of acoustic metamaterial samples of finite extent

One complication of characterizing the response of acoustic metamaterials is that models often assume the medium is of infinite extent. On the other hand, material property measurement can only be done with samples of finite size. The result is that acoustic field measurements include the effects of edge diffraction and scattering from fixtures. This work presents deconvolution methods to extract the frequency-dependent reflection and transmission behavior of acoustic metamaterial samples. Measurements using broadband chirp signals are obtained and subsequently post-processed using deconvolution techniques to obtain high-time-resolution impulse responses. Additionally, the chirp signals can be modified to compensate for the frequency response of the transducers being used. Examples will be shown for the transmission behavior through a two-dimensional pentamode gradient index lens and the reflection response of a flat square plate. [Work supported by ONR.]

Low-frequency approximations for the radiation damping of a bubble

The recent work by Ainslie and Leighton [J. Acoust. Soc. Am. 126, 2163 (2009)] pointed out that there exist two different expressions in the literature for the scattering cross-section of a spherical gas bubble in liquid. The difference between the two expressions is contained in the term corresponding to the losses due to acoustic radiation. The more common expression, often attributed to R. Wildt [“Acoustic theory of bubbles,” in Physics of Sound in the Sea], is based on a low-frequency approximation and is thus only accurate when the wavelength of the incident wave is large compared to the circumference of the bubble. The purpose of the present work is to investigate specifically the accuracy of this low-frequency approximation for various physical parameters and frequencies of interest. An alternative low-frequency approximation is also suggested, which is both simpler and more accurate than the present approximation. Finally the case of a nonspherical bubble is considered, and an expression for the s...

Teaching the characterization of performance spaces through in-class measurements at Bates Recital Hall at the University of Texas at Austin

The characterization of a performance space provides an excellent opportunity to provide students with first-hand experience with many fundamental aspects of room acoustics including reverberation, linear time-invariant systems, measurement methods, and post-processing of real-world data to estimate room metrics. This talk reports recent measurements made in Bates Recital Hall at the University of Texas at Austin (UT) as part of the graduate course on architectural acoustics in the acoustics program at UT. For one class period, students participated in acoustical measurements in the 700-seat venue. The measurements consisted of recording the signal at numerous locations within the room resulting from various on-stage excitations including exponential chirps, interrupted pink noise, and balloon-pops. The audio files captured during the experiments were provided to the students for calculation of the impulse response at the measurement positions and associated room metrics such as reverberation time, bass r...

Radiation damping of an arbitrarily shaped bubble

Large encapsulated bubbles have recently been used for abating low-frequency anthropogenic underwater noise [J. Acoust. Soc. Am. 135, 1700–1708 (2014)]. The use of encapsulation allows for the possibility of bubbles that are significantly nonspherical in their equilibrium state. Strasberg [J. Acoust. Soc. Am. 25, 536–537 (1953)] investigated the resonance frequency of an ideal bubble with arbitrary shape and found that the dependence of resonance frequency on the shape of the bubble reduced to a well-known problem in electrostatics. The present work extends that analysis to include the effects of radiation damping on the oscillation of a bubble, and does so by including a loss term due to Ilinskii and Zabolotskaya [J. Acoust. Soc. Am. 92, 2837–2841 (1992)] in the volume-frame dynamical equation for the bubble. An expression is given for the amplitude of the acoustic field scattered from the bubble, and it is shown that radiation damping scales as resonance frequency cubed for arbitrarily shaped bubbles ha...

Nonlinear behavior of heterogeneous materials containing snapping acoustic metamaterial inclusions

This work studies the forced dynamical behavior of a heterogeneous material containing metamaterial inclusions undergoing large deformations. The inclusions exhibit non-monotonic stress-strain behavior, modeled with an expansion to third order in volume strain, where the coefficients of the expansion depend on the metamaterial structure. The resulting constitutive behavior of interest displays regimes of both positive and negative stiffness and the inclusion therefore exhibits hysteretic snapping when forced by an acoustic pressure. Two cases are explored using a generalized Rayleigh-Plesset analysis to model the large-deformation dynamics of the metamaterial inclusion following an approach similar to Emelianov et al. [J. Acoust. Soc. Am., 115, 581 (2004)]. The first case focuses on the forced dynamics of a single inclusion embedded in a weakly compressible elastic medium. The second case broadens the model to analyze the behavior of a heterogeneous material comprised of a low volume fraction of non-inter...