Acoustics Seminars

Acoustic One-way Manipulation by Metamaterials

Friday, March 28, 2014 4:00 p.m. ETC 4.150

Professor Bin Liang
Institute of Acoustics
Department of Physics
Nanjing University
Nanjing, China
http://acoustics.nju.edu.cn/

Acoustic waves have long been thought to propagate as easily along two opposite directions in a given path. It can therefore be expected that the one-way propagation of acoustic waves, if could it be realized successfully, would have deep implications for acoustic devices, acoustic applications and the field of acoustics in general. Here a series of theoretical and experimental works will be reported on how to realize acoustic one-way manipulations by designing various kinds of acoustic metamaterials. The first theoretical model of an “acoustic diode” that enables the rectification of acoustic energy flux was proposed by coupling a nonlinear medium with a superlattice, and the prototype was fabricated experimentally. By introducing a nonlinear resonance cavity, the efficiency of the nonlinear acoustic diode was dramatically enhanced. On the other hand, different schemes of one-way acoustic manipulations have been developed in linear systems, resulting in significant improvements in aspects of transmission efficiency, bandwidth and waveform preservation, etc.

Mitigation of Highway Traffic Noise with Quieter Pavements and Noise Barriers

Friday, March 21, 2014 4:00 p.m. ETC 4.150

Dr. Manuel Trevino
Center for Transportation Research
The University of Texas at Austin
http://www.utexas.edu/research/ctr/

Noise associated with highway transportation has progressively become a nuisance to communities along roads. As transportation of people and goods continues to grow, roads expand, and noise levels rise. Nowadays, transportation agencies have become more environmentally sensitive and deal with pollution problems including noise. A number of factors affect the level of traffic noise, such as vehicle speed, terrain, grade, pavement surface characteristics, and shielding provided by walls, fences, buildings, or even dense vegetation. The most frequently used noise abatement measure has been the construction of noise barriers on the side of the road. The barriers, however, are only effective for receivers in the acoustic shadow of the wall. Other receivers are affected as much as they are without the barrier. In recent years, there has been a growing interest in designing and constructing quieter pavements as a way to abate traffic noise by reducing noise at the source. By modifying some of its properties, pavements have been shown to produce lower noise levels than the “average” pavements. Two case studies exemplify noise mitigation: the first transparent noise barrier in Texas, on IH-30 in Dallas, and the Mopac Improvement Project, in Austin.

Enhanced Ultrasound and Photoacoustic Imaging Using Photoacoustic Nanodroplets

Friday, March 7, 2014 4:00 p.m. ETC 4.150

Alexander S. Hannah
Department of Biomedical Engineering
The University of Texas at Austin
http://www.bme.utexas.edu

Disease detection by noninvasive imaging requires contrast against the surrounding healthy tissue, which is insufficient using many available techniques, including clinical ultrasound (US) and recently emerging photoacoustics (PA). The introduction of exogenous contrast agents highlights selected regions, but these formulations have several shortcomings. Microbubbles used for US contrast are too large to reach tumor neovasculature, and their instability limits the time window for imaging. Contrast agents for PA are also flawed; dyes offer only a modest increase in signal, and metal nanoparticles must undergo regulatory approval before clinical translation. We have developed a dual contrast agent, named photoacoustic nanodroplets (PAnDs), which resolve these issues. These perfluorocarbon droplets are small enough to reach and extravasate from tumor neovasculature, and stable enough to allow for accumulation over several hours. The droplets can be optically triggered to induce particle vaporization, which emits a stronger PA signal than from thermal expansion of metal particles. The resulting gaseous microbubbles are a source of high US contrast as well. We have constructed a dye-loaded droplet comprised of biocompatible materials ready for clinical translation, as well as a droplet which can be triggered using an inexpensive 1064 nm laser source. We characterize the various properties of these nanodroplets and quantify image enhancement from remote triggering of the droplets, while investigating mechanisms of optical droplet vaporization. Additionally, we explore the use of high and low boiling point droplets for specific applications. Lastly we explore droplet formulations to molecularly target specific disease. These PAnDs improve image quality for US and PA modalities, and may encapsulate drugs for image-guided, controlled release of therapeutics.

Multimaterial Piezoelectric Fibers—Fabrics That Can Hear and Sing

Friday, February 28, 2014 4:00 p.m. ETC 4.150

Professor Zheng Wang
Department of Electrical and Computer Engineering
The University of Texas at Austin
http://www.ece.utexas.edu

Active fiber devices that can be electrically modulated at high frequencies promise a wide range of novel applications in energy transduction, sensing, and communications. I will present a novel technique of using thermal drawing to produce monolithic piezoelectric fibers that allow electrical modulation of their acoustic and optical properties. By integrating and poling ferroelectric materials, such as PVDF-TrFE copolymer in the fiber form, one can apply piezoelectric modulation to mitigate the high operational voltages typically required for electro-optical modulation. The fibers can be fabricated over kilometer-long lengths, and electrically actuated from a few Hz through a few MHz, and function as both receivers and transmitters. Beam steering and focusing are demonstrated using a flexible fiber phased array, which paves the way for dynamically reconfigurable acoustic emission and sensing of 3D acoustic fields for a variety of applications.

Design of an Optical Microelectromechanical-System Microphone Toward Sub-15 dBA Noise Floor

Friday, February 21, 2014 4:00 p.m. ETC 4.150

Donghwan Kim
Department of Electrical and Computer Engineering
Microelectronics Research Center
The University of Texas at Austin
https://www.mrc.utexas.edu

Microelectromechanical-system (MEMS) microphones with optical readout have been previously demonstrated. These microphones are similar to capacitive MEMS microphones, but the optical microphone can achieve potentially higher signal-to-noise ratio. The optical microphone measures sound pressure by detecting displacement of a compliant diaphragm. The displacement is measured using a diffraction-grating-based interferometer. Although the optical microphone requires a backplate to support the diffraction grating, the perforation density of the backplate can be much higher as compared to conventional capacitive microphones. Higher perforation density results in lower air damping and lower thermal-mechanical noise. A prototype sensor was fabricated at The Microelectronics Research Center of the University of Texas at Austin. The preliminary test demonstrates a 22.6 dBA noise floor, and shows that the flow resistance near the diffraction grating due to the squeeze-film damping effect between the diaphragm and the backplate is the dominant source of damping. The 22.6 dBA noise floor is approximately 6 dB better than commercially available capacitive MEMS microphones. System modeling suggests a better backplate design is possible which increases the SNR of the optical microphone further by an additional 8 dB, resulting in microphones with 10 dB higher SNR than the current state-of-the-art.

Arbitrary IIR Filter Design for Audio Applications

Friday, February 7, 2014 4:00 p.m. ETC 2.136

Dr. Thomas D. Kite
Audio Precision, Inc.
Beaverton, OR
http://ap.com

Digital infinite impulse response (IIR) filters are popular in audio signal processing applications because they typically have a much lower computational cost than finite impulse response (FIR) filters designed to the same specifications. However, IIR filter design is difficult, particularly for arbitrary frequency responses that span multiple octaves, as is common in audio. A recent technique known as frequency warping offers a new way to design arbitrary IIR filters with much greater accuracy than before.

Existing IIR design methods will be reviewed, and it will be shown how frequency warping is related to the popular bilinear transform.  Using a multi-band approach pioneered by Bank, it will be shown how IIR filters can be designed for the entire audio band with accuracy limited only by filter complexity.  A 30-pole filter designer controlled via GUIs will then be demonstrated, and it will be shown how high-order filters can alleviate common audio headaches such as the ragged frequency responses of loudspeakers.

Beneficial Bubbles: Ultrasound-Mediated Drug Delivery

Friday, January 31, 2014 12:00 p.m. ETC 2.136

Professor Christy K. Holland
Department of Internal Medicine and Biomedical Engineering Program
University of Cincinnati
http://www.ultrasound.uc.edu/people.html

Ultrasound is under development as a potent promoter of beneficial bioeffects for the treatment of cardiovascular disease.  These effects can be mediated by mechanical oscillations of circulating microbubbles, referred to as ultrasound contrast agents, which can also encapsulate and shield a therapeutic agent in the bloodstream.  Oscillating microbubbles can create stresses directly on nearby tissue or induce fluid motion that affect drug penetration into vascular tissue, lyse thrombi, or direct drugs and bioactive gases to optimal locations for delivery.  Ultrasound-triggered release of nitric oxide from echogenic liposomes induces potent vasorelaxation in porcine carotid arteries in an ex vivo system.  Recent in vitro and ex vivo data from a variety of clot and vascular models will be discussed.

This acoustics seminar is offered courtesy of the Chevron Centennial Seminar Series in the Department of Mechanical Engineering, and pizza will be served.

Bubble Pulsation and Translation Near a Soft Tissue Interface

Friday, January 31, 2014 4:00 p.m. ETC 4.150

Daniel R. Tengelsen
Department of Mechanical Engineering and Applied Research Laboratories
The University of Texas at Austin
https://www.arlut.utexas.edu

A Lagrangian formalism used previously to calculate the pulsation of a spherical bubble immersed in liquid and adjacent to a viscoelastic layer is extended here to include bubble translation. Previous models and experiments have shown that the direction of bubble translation near a viscoelastic layer is correlated with the direction of a liquid jet often produced by the bubble during collapse. The liquid jet is an important feature in the interaction between bubbles and neighboring surfaces. In this presentation we describe how to model the pulsation and translation of a spherical bubble near a liquid-solid interface, with emphasis on soft tissue, in order to determine the direction of bubble translation for a broad spectrum of material properties for both the liquid and viscoelastic medium, and for various distances between the bubble and the interface. The force on the bubble due to the presence of the liquid-solid interface is calculated using a Green’s function that takes into account elastic waves, viscosity in the layer, and the viscous boundary layer in the liquid adjacent to the interface.

Synergetic Ablation of Tumors with Focused Ultrasound and Ethanol

Friday, January 24, 2014 4:00 p.m. ETC 4.150

Dr. Damir B. Khismatullin
Department of Biomedical Engineering
Tulane University
https://tulane.edu/sse/bme

Focused ultrasound (FUS) emerges as a powerful technology for noninvasive, or minimally invasive, non-ionizing treatment of cancer and many other diseases. When operated at high intensity, FUS deposits a large amount of acoustic energy at the focal region within the target tissue (e.g., tumor), causing tissue heating and necrosis, the process known as thermal ablation. When operated at low intensity, FUS is capable of increasing the proliferative potential and functionality of living cells such as neurons and bone cells. As such, it can be used as a tool to treat neurodegenerative diseases, nerve injuries, and bone fractures. The major focus of previous studies was on using FUS as a standalone therapeutic method. Several deficiencies of standalone FUS (e.g., small area of treated tissue, lengthy procedure, sophisticating scanning protocol) prevented its widespread use in clinics and approval by the FDA. In this talk, I will present recent in vitro and in vivo data from my laboratory showing that FUS is much more effective in therapy when it is used as an adjuvant to other therapeutic methods. According to our experiments, high-intensity focused ultrasound (HIFU) applied to tumors already exposed to ethanol lead to almost complete destruction of tumor cells even when the ethanol dose is much less than used in percutaneous ethanol injection (PEI), one of FDA-approved methods of chemical ablation. We hypothesize that this synergistic effect of HIFU and ethanol is caused by 1) enhanced delivery of chemical agents into tumor cells via HIFU‑induced acoustic streaming in tumor tissue and reducing the cell membrane permeability by the sonoporation phenomenon, and 2) enhanced tissue heating rate by HIFU due to ethanol-induced localized reduction of the cavitation threshold. Key results are that 1) the HIFU ablation lesion volume increases dramatically and becomes more spherical (note that HIFU focal region is ellipsoidal in shape) with pre-treatment of tissues with ethanol, and 2) ethanol + HIFU but not ethanol alone or HIFU alone can completely eliminate cancer in xenograft mouse models.  In the end of the talk, I will discuss our new project on spinal cord injury treatment by FUS and molecular medicine.

Deciphering the Function of Low-amplitude Songs: Courtship, Aggression, and Hormones

Friday, January 17, 2014 4:00 p.m. ETC 4.150

Dr. Dustin G. Reichard
Department of Evolution and Ecology
University of California, Davis
http://www-eve.ucdavis.edu

Across the animal kingdom, acoustic signals serve a variety of important functions in aggression, mate attraction, courtship, and alarm-related signaling. Previous research on the function of acoustic signals has focused predominantly on high-amplitude (loud), long-range songs (LRS), while largely ignoring the low-amplitude (quiet) songs produced during close-proximity, conspecific interactions. Low-amplitude songs can be divided into two classes: (1) soft, long-range songs (soft LRS), which do not differ structurally from a species’ LRS, and (2) short-range songs (SRS), which can be widely divergent from a species’ LRS in both spectral and temporal characteristics. During my dissertation, I focused on determining the function of low-amplitude songs in a North American songbird, the dark-eyed junco (Junco hyemalis). Male juncos produce a distinct soft LRS and SRS during the breeding season, and SRS is produced at two distinct tempos, slow and fast. I performed a series of experiments involving song playbacks and presentations of live, male and female conspecifics to free-living male juncos to determine the social contexts in which males produce low-amplitude songs and the behavioral and hormonal responses that different songs elicit. The results of these studies provide multiple lines of evidence suggesting that soft LRS may function in both male-male and male-female interactions, while slow SRS functions predominantly in male-female interactions associated with courtship. During simulated courtship interactions in the field, paired and unpaired males appeared to use distinct courtship strategies by differing substantially in LRS production, proximity to the female, and activity, but producing similar amounts of SRS and visual displays. Finally, a meta-analysis of North American breeding birds found that low-amplitude vocalizations are relatively common. Collectively, these results emphasize that future studies of acoustic signaling in any taxon should focus on all components of the acoustic repertoire including both high- and low-amplitude signals.

The Effect of Shape, Shell Thickness, and Fill Material on the Resonance Frequency, Quality Factor and Attenuation of Bubbles

Friday, November 15, 2013 4:00 p.m. in ETC 4.150

Kyle S. Spratt and Gregory R. Enenstein
Applied Research Laboratories and Mechanical Engineering Department
The University of Texas at Austin
http://www.arlut.utexas.edu/
http://www.me.utexas.edu/

The topics discussed are related to the acoustics of air bubbles in water, with applications in underwater noise abatement and the acoustics of fish schools.  The first half of this seminar describes an investigation of the resonance frequency of an arbitrarily shaped ideal bubble using an analytical, lumped-element approach.  The problem of finding the effective mass of the bubble is equivalent to a classical problem in electrostatics, as was first noted by M. Strasberg in 1953 [J. Acoust. Soc. Am. 25, 536–537].  The resonance frequency for various bubble geometries is presented, with special attention paid to the case of a toroidal bubble, and the results are compared with a finite-element numerical model of the corresponding full acoustic scattering problem.  The second half of the seminar explores some details of encapsulated bubble acoustics (shell thickness and fill material) and also describes a simple laboratory measurement technique that is being developed to substitute for expensive open-water tests.  Measurements made in a small (sub-wavelength) laboratory tank of the resonance frequencies and quality factors of single encapsulated bubbles with various shell thicknesses and fill materials are compared to measurements of the attenuation due to arrays of the same encapsulated bubbles measured in open water.

Sound Systems Design for Mass Audiences

Friday, November 8, 2013 4:00 p.m. in ETC 4.150

Craig Janssen
Managing Director
Acoustic Dimensions
Dallas, Texas
http://www.acousticdimensions.com/index.html

Acoustic Dimensions has been fortunate to serve as the acoustics and technology consultant for dozens of major sports venues and hundreds of large assembly event centers worldwide. The technology design at the Circuit of the Americas will be used as a case study to discuss the challenges of communicating both entertainment and emergency messaging to mass audiences. This will be compared briefly with various other stadia and enclosed venues. Issues of emergency evacuation complexities, and technology design to allow this, will also be addressed.

How Language Experience Changes Our Speech Perception

Friday, November 1, 2013 4:00 p.m. in ETC 4.150

Professor Chang Liu
Department of Communication Sciences and Disorders
The University of Texas at Austin
http://csd.utexas.edu/faculty/chang-liu

It is well known that when listening in noise, English non-native listeners have more difficulty to perceive English speech sounds than English-native listeners.  Previous work in our laboratory found that the identification of English phonemes, especially English vowels, was quite challenging in noise for non-native listeners.  The native advantage became greater from quiet to noisy conditions, indicating more negative impacts of noise for non-native listeners than for native listeners.  Moreover, given the same native language background, non-native listeners with more native English experience (e.g., Chinese-native listeners in the US) outperformed their peers with little or no native English experience (e.g., Chinese-native listeners in China) in English phonemic identification in multi-talker babble noise, while the two groups of non-native listeners had similar performance in quiet and long-term speech-shaped noise.  We proposed two possible explanations:  1) native English exposure may help non-native listeners use the cue of the temporal fluctuation in noise more efficiently;  2) native English exposure may improve non-native listeners’ capacity to reduce informational masking of multi-talker babble.  Two present studies are being conducted to test the two hypotheses.  The preliminary results showed that both possibilities may be present.  These studies suggest that when learning English as a second language, listeners may benefit from native English exposures, active or passive, by using acoustic and/or phonetic cues in speech and noise more efficiently.

An Overview of the Combustive Sound Source: History and Recent Developments

Friday, October 25, 2013 4:00 p.m. in ETC 4.150

Andrew R. McNeese
Applied Research Laboratories
The University of Texas at Austin
http://www.arlut.utexas.edu

This seminar describes the development and testing of the Combustive Sound Source (CSS), which is a broadband underwater sound source.  The CSS is being developed as a clean, safe, and cost effective replacement for underwater explosive charges, which present an inherent danger to marine life and researchers using the charges.  The basic operation of the CSS is as follows.  A combustible mixture of gas is held below the surface of the water in a combustion chamber and ignited with an electric spark.  A combustion wave propagates through the mixture and converts the fuel and oxidizer into a bubble of combustion products, which expands due to an increase in temperature, and then ultimately collapses to a volume that is smaller than before ignition, producing a high intensity, low frequency acoustic signal.  The seminar begins by discussing the history and purpose of developing the CSS.  It continues by describing the essential components of the device and convenient features added to recent mechanical designs.  The general operation is discussed along with a description of various experiments conducted to determine the acoustic output and robustness of recent modifications to the CSS.  Results from the experiments are presented to show that the CSS can be deployed from a stationary platform or a towed body throughout the water column, including at the water-sediment interface, to meet various experimental needs.  Future work and plans are discussed to conclude the seminar.

Refinements in the Resonator Sound Speed Technique and Sound Exposure in Motorsports Audiences

Friday, October 18, 2013 4:00 p.m. in ETC 4.150

Craig NDolder
Applied Research Laboratories and Department of Mechanical Engineering
The University of Texas at Austin
http://www.arlut.utexas.edu/
http://www.me.utexas.edu/

This seminar covers two independent topics.  The resonator sound speed technique has been used by researchers at the University of Texas at Austin to determine the acoustic effective medium properties of freely rising bubbles in water, methane hydrates, seagrass, fluid-like gas-bearing sediment, and recently fish schools.  Despite the robust nature of this method for non-dispersive materials, the interpretation of the acoustic field present in the resonators is not intuitive for dispersive media.  However, when measured acoustic resonance frequencies can be correctly associated with the appropriate acoustic mode, interesting insight into highly dispersive systems can be gained.  This presentation walks through recent improvements and insights regarding this technique.  The second topic covers the initial analysis of calibrated sound recordings taken at a recent motorsports event.  These recordings were analyzed in order to provide the noise dosage seen in three different spectator locations.  The results show that spectators are exposed to more noise than is allowed by the OSHA standards for workplace safety.

Sound Concentration, Enhanced Nonlinearities and Giant Nonreciprocal Response in Acoustic Metamaterials

Friday, October 4, 2013 4:00 p.m. in ETC 4.150

Professor Andrea Alù
Department of Electrical and Computer Engineering
The University of Texas at Austin
http://users.ece.utexas.edu/~aalu

In this talk, I will discuss our recent progress and research activity in the field of acoustic metamaterials, focused on the general objective of enhancing sound-matter interactions in artificial materials.  I will show how suitably designed metamaterials may be used to concentrate acoustic energy in small channels, producing giant nonlinear response, enhanced absorption and anomalous impedance matching.  I will also discuss our recent theoretical and experimental results aimed at inducing acoustic isolation and large nonreciprocal sound transmission using metamaterial concepts.  We achieve these effects by producing the acoustic equivalent of the Zeeman effect in a subwavelength meta-molecule consisting of a resonant ring cavity loaded by a circulating fluid.  This concept has been used to realize a compact, fully linear, magnetic-free diode for airborne acoustic waves, which achieves up to 40 dB isolation at audible frequencies in a subwavelength device.  Physical insights into these phenomena will be discussed during the talk.

Product Sound Quality—The Music that Machines Make

Friday, September 27, 2013 4:00 p.m. in ETC 4.150

David A. Nelson, INCE Bd. Cert., PE
Principal Consultant, Nelson Acoustics
Elgin, Texas
http://www.nelsonacoustical.com/

The experience of product noise depends not just on the sound pressure level, but on a host of perceptual factors including loudness, spectral balance, tonality, and modulation.  Overlaid atop these are social factors such as cultural expectations, the context in which the sound is presented, potential for annoyance or distraction, threshold of hearing, and personal preference.  The subjective and objective sides of acoustics are connected through proper selection of objective metrics, development of stimuli, structure of presentations, and statistical analysis methods.  The result is an objective “recipe” for a product sound that is likely to be acceptable in the marketplace, which in turn forms the basis for noise control efforts.  Data from actual product sound quality studies will be used where possible.

Comparison of Models for a Piezoelectric 31-Mode Segmented Cylindrical Transducer

Friday, September 20, 2013 4:00 p.m. in ETC 4.150

Nicholas J. Joseph
Applied Research Laboratories and Department of Mechanical Engineering
The University of Texas at Austin
http://www.arlut.utexas.edu/
http://www.me.utexas.edu/

Piezoelectric transducers with thin-shelled cylindrical geometry are often used to radiate axisymmetric acoustic fields underwater.  This is achieved by generating a uniform electric field across the thickness of the piezoelectric cylinder to develop circumferential strains and, consequently, uniform radial expansion.  The uniform radial expansion is known as the breathing mode and is the desired behavior of these sources.  To tune their performance in a cost-effective way, the cylinders can be constructed of alternating active (piezoelectric) and inactive (non-piezoelectric) segments along their circumference.  Existing lumped parameter models for these transducers reduce the system to a single degree of freedom using effective piezoelectric properties of the composite cylinder.  These models accurately capture the breathing motion of the cylinder but neglect other modes and overestimate the efficiency of the transducer.  Experiments show that segmented transducers may demonstrate a detrimental higher frequency resonance within the operational frequency band.  The parasitic mode associated with this resonance is shown to result from bending motion of the segments and can significantly decrease the radiated acoustic pressure.  Discussed here is the development of a multiple-degree-of-freedom lumped parameter model that captures both the breathing and bending modes of the transducer and provides a more accurate estimate of its efficiency.  Results from the multi-degree-of-freedom model are compared with those from a one-degree-of-freedom model, finite element models, and experimental data.

Hyundai Uses a LabVIEW-Based Portable Sound Camera for Buzz, Squeak, and Rattle Studies

Friday, September 13, 2013 4:00 p.m. in ETC 4.150

Kurt Veggeberg
Business Development Manager, Sound and Vibration
National Instruments
Austin, Texas
http://www.ni.com/soundandvibration/
http://sine.ni.com/cs/app/doc/p/id/cs-15675

Acoustic beamforming involves mapping noise sources using an acoustical array.  It discerns the direction from which the sound originates by the time delays that occur as the sound passes over an array of microphones such as a sound camera.  A sound camera visualizes sound in color contours similar to the way a thermal camera visualizes temperature.  A microphone array, which implements a beamforming method, locates noise sources visually, making it one of the best devices to detect buzz, squeak, and rattle (BSR) noises.  This is a presentation and demonstration of a portable system developed with the cooperation of Hyundai Motors to visualize and identify annoying transient BSR noise sources in Hyundai automobiles.  The solution involves developing a handheld sound camera with National Instruments LabVIEW system design software that identifies and displays noise sources in real time using microelectromechanical system (MEMS) and field-programmable gate array (FPGA) technologies to increase the image update rate and to decrease the total weight of the device.

Finite Element Modeling of Acoustic Scattering from Fluid and Elastic Rough Interfaces for Ocean Acoustics Applications

Friday, September 6, 2013 4:00 p.m. in ETC 4.150

Dr. Marcia J. Isakson
Applied Research Laboratories
The University of Texas at Austin
http://www.arlut.utexas.edu/

Quantifying acoustic scattering from rough interfaces is critical for ocean acoustic applications, especially in shallow water waveguides.  In this scenario, the sound has many interactions with both the air/water interface and the sediment/water interface.  Scattering from these interfaces both reduces the coherent reflected component of the sound as well as produces reverberation.   The air/water interface is often modeled as pressure release while the sediment/water interface must be modeled as penetrable.  Depending on the sediment, it can be modeled as a fluid, visco-elastic solid or poro-elastic solid.  This study concentrates on the sediment/water interface by modeling both a fluid-like sediment and a visco-elastic solid.  Historically, scattering is quantified by approximations to the Helmholtz/Kirchhoff integral.  The two main approximations used are the Kirchhoff approximation and perturbation theory.  The Kirchhoff approximation considers scattering as reflections from planes tangent to the facets of the surface.  Perturbation theory expands the scattered pressure in a Taylor series with respect to the relief of the surface and truncates the series.  Although these approximations are used extensively, there has not been a systematic study of validity especially for realistic rough surfaces.  In this study, the finite element model results for scattering from fluid and visco-elastic solids with rough interfaces will be compared to the approximate methods.  The results illustrate the role of the waves excited at the interface in the scattering process.