Physical Acoustics

About the Physical Acoustics Group

Faculty: Bass, Breazeale, Church, Gilbert, Gladden, Hickey, Lu, Mobley, Raspet, and Sabatier, and Waxler.
Research Scientists: Lamberton and McPherson.

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.

For more details, see the NCPA website.

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.

Atmospheric Acoustics (Gilbert, Bass)

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.

Nonlinear Acoustics (Breazeale, Raspet)

All physical processes are nonlinear. Recent discoveries have led to acoustical methods of studying non-linearity. In fluids, non-linearity leads to waveform distortion. The point of maximum compression travels at a different velocity than the point of minimum compression. This means that a sinusoidal disturbance, even in an infinite plane wave, gradually approaches a sawtooth waveform. The same phenomenon occurs in a solid. In crystalline solids, experiments are confined to pure mode directions because of interpretational difficulties. The non-linearity, which can be described in terms of third-order elastic constants, has been observed in many crystalline solids, and the third-order elastic constants of many crystalline solids are known.

The greatest non-linearity appears to be in liquids containing voids. Thus cavitation is a subject of considerable interest to nonlinear acoustics.

Porous Media Acoustics (Sabatier, Hickey)

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.

Thermoacoustics (Raspet, Bass)

Thermoacoustics is the generation of sound waves by a temperature gradient or the transfer of heat with sound waves. Thermoacoustic refrigerators can be made that use noble gases as the working fluid rather than CFCs and that have no sliding seals for the potential of extremely high reliability. Our research is focused on developing a better understanding of the physical processes involved, as well as on building practical refrigerators and air conditioners. We can investigate a wide regime of physical parameters and system geometries. Of particular interest are nonlinear processes in thermoacoustic engines, traveling wave systems, utilization of waste heat for refrigeration, and the thermoacoustics of inert gas-vapor mixtures.