About the Physical Acoustics Group
The physical acoustics research program at the University is one of the largest in the United States. Research includes the physical phenomena associated with acoustic waves from less than one hertz to many megahertz, and the use of acoustics as a tool to investigate other phenomena. Most of the activity occurs in the state-of-the-art laboratories of the Jamie Whitten National Center for Physical Acoustics (NCPA) on the Ole Miss campus.
In addition to the core study of physical acoustics, the center's dozen research groups are involved in a great variety of other acoustics fields, including animal bioacoustics, aeroacoustics, psychological acoustics, signal processing, and engineering and agricultural applications of sound.
Aeroacoustics (in association with engineering faculty)
Aeroacoustics is devoted to the study of aerodynamically generated sound. One important area involves the prediction and reduction of sound generated by commercial aircraft. However, the field is quite broad and also involves study of acoustics associated with complex fluid structure interactions of hypersonic vehicles, like that associated with reusable launch vehicles for satellite repair and space station re-supply.
The field involves theoretical, numerical, and experimental efforts to better understand the physics for relating flow field pressures fluctuations to the turbulent dynamics of high speed flows that often contain aero-thermal chemical reactions. Contributions from this field often lead to development of aerospace vehicle systems that meet environmental standards and ones that can be optimized for system performance and reliability.
The propagation of acoustic waves outdoors involves a wide range of complex phenomena. These include microscopic absorption and dispersion, viscous and seismic interaction with the ground, and scattering from turbulence.
The past decade has seen remarkable advances in our ability to theoretically model sound propagation in the real atmosphere. Several state-of-the-art models are currently being developed and used to predict propagation of audible sound over horizontal distances of several kilometers in the lower atmosphere.
Similar work is ongoing for global infrasound propagation, which involves frequencies down to 0.02 Hz and horizontal distances of several thousand kilometers. Infrasound is proving useful in atmospheric and oceanographic research, as well as for the detection of surreptitious nuclear testing. These problems, as well as diffraction due to barriers and refraction due to velocity of sound gradients, are actively investigated.
Understanding the mechanisms of interaction of ultrasonic waves with human is essential for the development of advanced therapeutic and diagnostic applications in biomedicine. Ultrasound is also used to study the fundamental mechanical properties of biologically evolved materials to discover the way nature has solved many of the structural and functional challenges faced by living organisms.
High Intensity Focused Ultrasound (HIFU) for Biomedical Therapeutics. Ultrasound waves in the MHz frequency range can be brought to a tight focus in soft mammalian tissues. The focus occurs remotely on the order of 2 - 20 centimeters from the source. When a focused source is driven at high power, this concentrated sonic energy can be used to therapeutic benefit. The therapeutic effect is delivered without impacting overlying tissues. High temperatures can be generated within the focal region, with surrounding areas remaining largely unaffected. It has been shown that focal peak intensities of 1500 W/cm2 held for 1–2 s can produce temperatures in excess of 56 C (133 F) which leads to instantaneous cell death and coagulative necrosis within the focal zone. Our work is concerned with the deposition of energy via HIFU in regions of large thermal conductivity, such as major vessels.
Tissue Characterization. As a non-invasive probe into the human body, ultrasound is used to acquire diagnostic information non-invasively without inducing significant cellular effects. Interaction of ultrasound with tissues results in scattering, absorption and dispersion, all of which can be linked to tissue microstructure. Determining the fundamental physical and mechanical properties of tissue provides knowledge that is essential for development of advanced diagnostic techniques as well as for understanding the physics of propagation in biological composite materials. Our group is involved in measuring the fundamental ultrasonic properties of various mammalian tissues and finding methods to measure such properties in vivo.
For more details see the Ultrasonics Group page.
Sound propagates in porous granular materials via two compressional modes. The slower of these modes is controlled by the geometry of the pores. By studying this mode, properties of the porous material are determined. Fundamental studies include acoustic scattering from rough porous soils and memory states of pre-strained granular materials.
A major research thrust of this group is the detection of anti-personnel land mines. Airborne sound induces vibration of the ground and the land mine. Differences in the vibrations are detected and imaged using interferometric optical techniques. Measurement techniques include laser Doppler vibrometry and pulsed speckle pattern interferometry.
Resonant Ultrasound Spectroscopy (Gladden)
RUS is an elegant experimental method for determining the full elastic tensor of a single crystal. Elastic constants are a measure of the interaction of the atoms in the crystal lattice and so are sensitive to phase transitions. We specialize in small sample RUS and thin film RUS in which the elastic constants for a thin film on a substrate can be obtained.
Dynamic problems in continuum mechanics: Using high speed video, I have studied the buckling of a thin rod impacted by a projectile — a fancy way of breaking pasta! We have also studied the dynamics of how a rigid rod moves through a viscoelastic gel including transitions from fluid-like flow to solid-like tearing.
Wind Noise (Raspet)
Recent research has concentrated on improving outdoor sound measurements by reducing wind noise in measurement microphones. We are developing quantitative methods of predicting wind noise contributions from measurements of the wind speed fluctuations. This work combines meteorology, fluid dynamics and acoustics to develop and validate theoretical expressions. An extensive measurement program has been very successful in identifying inaccurate and wrong theories that have appeared in the literature.