In addition to subject mastery and the focused effort required to complete a thesis project, graduate students also need to develop a broad understanding of their field and cultivate a familiarity with the larger community of researchers and practitioners. The “summer school” format has been shown to enhance both subject-matter breadth and build community awareness in physical acoustics. Physical Acoustics Summer School (PASS) has been held in late-May, in even-numbered years, since 1992. The format for each day is usually two three-hour lectures followed by evening discussion groups to answer questions and explore extensions of the day’s lecture topics. One lecture session is typically dedicated to acoustics demonstrations. Attendance for the full week is required of all participants who also dine together three times each day. Venues are chosen to provide isolation that minimizes distraction and maximizes interactions among all participants. Typical enrollment has been 10 distinguished lecturers (including many Silver Medal winners in Physical Acoustics), 10 discussion leaders, and 30 graduate students. This format has been successfully extended to one other ASA Technical Committee: the marine bioacoustics community has held their summer school twice (SeaBASS). PASS has now been functioning long enough that former students have become lecturers.

NCPA traces its roots to the Physical Acoustics Research Group which grew from the Department of Physics and Astronomy. The current Department of Physics and Astronomy acoustics effort was initiated with Prof. F. Douglas Shields who received his Ph.D. at Vanderbilt University under the direction of Prof. Robert Lageman. Professor Shields came to the University of Mississippi in 1959. He directed the dissertation of 10 Ph.D. students and seven Masters students. In 1964, Prof. Shields was joined in the department by Prof. Roy Arnold who studied the ultrasonic properties of solids. Professor Arnold directed the dissertation of two Ph.D. students and six Masters students. In 1970, Prof. Randall Peters and Prof. Henry Bass joined the faculty; Bass doing acoustics in gases with a Ph.D. from Oklahoma State under Tom Winter and Peters doing solid state acoustics. The following year, Ron Carter joined the group doing solid state acoustics. Before his passing in 2008, Professor Bass directed the dissertation of nineteen Ph.D. students and eight Masters students. Professor Larry Crum, a student of Prof. Stump at Ohio State, joined the faculty in 1978. In 1986, an act of Congress established the National Center for Physical Acoustics and over a period of two years, PARGUM was transferred to NCPA. Professor Ralph Goodman joined NCPA as Director in 1989. A listing of all additional faculty and scientists to join NCPA and the students directed by all will be updated to 2013 and presented.

The polar coordinate method in You [Ph.D. thesis, The University of Mississippi (1993)] is rederived in differential form and is generalized by defining a local, continuously varying, radius of curvature. The generalization makes it possible to compute sound propagation over arbitrary large-scale terrain where the local radii of curvature are much larger than an acoustic wavelength. For a simple hill, both the original and generalized method are in good agreement with measured low-frequency propagation loss data. The generalized method is applied to more complex terrain in a numerical example. The example demonstrates the utility of the method and also shows that complex terrain can have a significant effect on low-frequency sound propagation, even when the slope angles are small.

More than 100 separate incidents of interactions between aircraft and volcanic ash were documented between 1973 and 2003. Incidents on international flight paths over remote areas have resulted in engine failures and significant damage and expense to commercial airlines. To protect aircraft from volcanic ash, pilots need rapid and reliable notification of ash‐ generating events. A global infrasound array network, consisting of the International Monitoring System (IMS) and other national networks, has demonstrated a capability for remote detection of Vulcanian to Plinian eruptions that can inject ash into commercial aircraft cruise altitudes (approximately 12 kilometers) near the tropopause. The identification of recurring sound signatures associated with high‐ altitude ash injection implies that acoustic remote sensing can improve the reliability and reduce the latency of these notifications.

Cyclonic storms over water radiate a infrasound at about 0.2 Hz. The atmospheric component of this radiation is called the microbarom peak. The seismic component is called the microseism peak. It is believed that the source of the radiation is the non-linear interaction of colliding waves on the ocean surface. Results from our ongoing studies of the radiation mechanism will be reported. These include conjectures about the influence of the finite depth of the ocean on the microbarom/microseism spectrum and about the origin of the colliding waves produced by isolated storms.

Experimental verification of propagation predictions and sensor performance for infrasound frequencies that propagate long distance (greater than 100 km) are difficult to perform due to the rarity of controlled sources. If the source is elevated, the lower atmospheric pressure allows an explosion of a given charge weight to expand further during the supersonic phase, resulting in a lower fundamental frequency. A 50-lb charge exploded at 40 km yields a fundamental frequency similar to a 500-ton explosion at ground level. During the fall of 2005 and the winter and summer of 2006, experiments were conducted at White Sands Missile Range, NM, where an Orion rocket lifted a 50-lb explosive charge to altitudes between 30 and 40 km. Launches occurred near 0200 and 0600 MST. Portable infrasound arrays to record events were deployed in New Mexico, Texas, Arizona, and California to complement permanent arrays. Infrasound recordings were supplemented with extensive meteorological measurements. Data analysis is in a preliminary stage. The High Altitude Infrasound Propagation Experiment resulted from collaboration between several U.S. Army and Navy commands, University of Mississippi, Southern Methodist University, University of Hawaii, University of California at San Diego, University of Alaska at Fairbanks, BBN Technologies, SAIC, and Los Alamos National Laboratory.

In spite of recent technological advances, forecasting the intensity of a hurricane remains a challenging problem. Knowledge of the location and the strength of the storm at the moment where it makes landfall is critical for civil defense planning and public safety. Part of the reason for this is that storms that are well away from land can only be monitored intermittently (e.g., three times per day). Infrasound signals are produced by the interaction of the hurricane with the sea-state and hold promise for characterizing sudden changes in hurricane strength on a time scale not possible with current sensing technologies. The University of Mississippi is leading a US national collaboration to monitor hurricanes using infrasound in the Gulf of Mexico and in the Western Atlantic Ocean. The results of measurements by from our group will be summarized, and the technical challenges associated with these measurements will be discussed.

Current approaches to acoustics in fibrous and porous materials use fitting parameters to match theoretical models to measured values for the material's complex compressibility and wavenumber. In effect, these models treat the material as though it were composed of an array of rigid capillary tubes; they have proven accurate in fitting the model to data for various different porous materials such as wools and foams. However, these models do not address thermoacoustic heat transfer when the material is put under a static temperature gradient. A direct simulation has been performed using a three-dimensional thermal fluid solver to calculate both the acoustic properties and the thermoacoustic properties of a random fibrous material. The results of the simulation will be compared to experimental results for complex compressibility and wavenumber [Tarnow, H., J. Acoust. Soc. Am., 97(4),2272-81] as well as a proposed extension to porous theory that incorporates thermoacoustics [Roh et al., J. Acoust. Soc. Am., 121(3), 1413-22]. [Work supported by U.S. Army Space & Missile Defense Command.]

Infrasound signals in the microbarom band (about 0.2 Hz) generated by hurricanes often do not appear to originate near the eye where the winds are strongest. This paper suggests that conditions conducive to microbarom (and microseism) generation can occur along the trailing periphery of the storm through the interaction of the storm‐generated wavefield with the ambient swell field, resulting in detection bearings that vary from the center of the storm by up to 20 degrees. Infrasound data from Typhoon Usagi (2007) is presented that supports this theory.

Sutherland and Bass (2004) have developed a calculation of the speed of sound and attenuation at altitudes up to 160 km. Dispersion is included in their treatment though emphasis is placed on absorption. That theory is explored in more detail to describe dispersion in the thermosphere. In the lower atmosphere, variations in the speed of sound with altitude are dominated by temperature and wind speed changes; the vibrational and rotational relaxation frequencies, which depend on the ratio of frequency to pressure (f/P), are in the kilohertz to megahertz range. As altitude increases, f/P and the mean free path increase, relaxation frequencies drop significantly, and velocity dispersion due to rotational and translational relaxation intensifies. This relaxation and resulting dispersion cause sound speeds at fixed frequencies to increase significantly with altitude, and to lower the altitude at which sound is refracted downward. Although the most extreme effects of dispersion are masked by simultaneously increased attenuation, decreases in traveltime of several percent are predicted at frequencies slightly below 1 Hz, demonstrating that this dispersion must be taken into account in order to correctly predict atmospheric returns at low frequencies.

As applications of infrasound grow so does the need for reasonably priced microphones and attendant wind noise reduction approaches. Many applications require sensor systems with frequency response between 0.01 and 10 Hz and a dynamic range between a few micropascals and ten Pascals and operate under severe environmental conditions. Sensor systems are often in remote areas with only occasional maintenance visits. Current installations usually employ classic condenser microphones or sliding wire microbarographs along with porous hose wind filters. Next generation sensors use optical fiber sensors long enough to average out wind noise or multiple sensors which allow for electronically canceling wind noise. Some of the advantages and characteristics of these sensors will be described. Calibration of sensors over the range of conditions remains a challenge. Progress on calibration facilities will be included in the discussion. [Work supported by the US Army Space and Missile Defense Command.]

Thermoacoustic theory is extended to stacks made of random bulk media. Characteristics of the porous stack such as the tortuosity and dynamic shape factors are introduced into the thermoacoustic wave equation in the low reduced frequency approximation. Basic thermoacoustic equations for a bulk porous medium are formulated analogously to the equations for a single pore. Use of different dynamic shape factors for the viscous and thermal effects is adopted and scaling using the dynamic shape factors and tortuosity is demonstrated. Comparisons of the calculated and experimentally derived thermoacoustic properties of reticulated vitreous carbon and aluminum foam show good agreement. A consistent mathematical model of sound propagation in a random porous medium with an imposed temperature is developed. This treatment leads to an expression for the coefficient of the temperature gradient in terms of scaled cylindrical thermoviscous functions.