Joseph C. Park

Microwave absorption by conductor-loaded dielectrics

A theoretical model to predict the microwave absorption by a conductor-loaded dielectric is developed. The microwave power transmission coefficient of the test material is derived in terms of the effective conductivity and permittivity parameters (of the composite material) as a function of volume fraction of the conductor loading. The absorption/transmission characteristics of the test material versus volume fraction of metal loading are described by three distinct regimes having low-loss, lossy and metal-like characteristics. Theoretical predictions are comparable with the measured data pertinent to an iron-plus-dielectric material. >

The rotary subwoofer: a controllable infrasound source.

The rotary subwoofer is a novel acoustic transducer capable of projecting infrasonic signals at high sound pressure levels. The projector produces higher acoustic particle velocities than conventional transducers which translate into higher radiated sound pressure levels. This paper characterizes measured performance of a rotary subwoofer and presents a model to predict sound pressure levels.

Collective bubble oscillations as a component of surf infrasound

Plunging surf is a known generator of infrasound, though the mechanisms have not been clearly identified. A model based on collective bubble oscillations created by demise of the initially entrained air pocket is examined. Computed spectra are compared to infrasound data from the island of Kauai during periods of medium, large, and extreme surf. Model results suggest that bubble oscillations generated by plunging waves are plausible generators of infrasound, and that dynamic bubble plume evolution on a temporal scale comparable to the breaking wave period may contribute to the broad spectral lobe of dominant infrasonic energy observed in measured data. Application of an inverse model has potential to characterize breaking wave size distributions, energy, and temporal changes in seafloor morphology based on remotely sensed infrasound.

A portable infrasound generator.

The rotary subwoofer is a novel low frequency transducer capable of efficiently generating infrasound from a compact source. A field-deployable version of this device may find application as a calibration source for infrasound arrays of the International Monitoring System (IMS) [(2001). The Global Verification Regime and the International Monitoring System (CTBTO Preparatory Commission Vienna International Centre, Vienna, Austria)]. A prototype tested at the IMS infrasound array I59US demonstrated the ability to insonify all elements of the array from a standoff distance of 3.8 km. Signal-to-noise ratios of continuous wave signals ranged from 5 to 15 dB, indicating the utility of this source to transmit controllable infrasound signals over distances of 5 km.

Acoustic detection, tracking, and characterization of three tornadoes

Acoustic data recorded at 1000 samples per second by two sensor arrays located at ranges of 1-113 km from three tornadoes that occurred on 24 May 2011 in Oklahoma are analyzed. Accurate bearings to the tornadoes have been obtained using beamforming methods applied to the data at infrasonic frequencies. Beamforming was not viable at audio frequencies, but the data demonstrate the ability to detect significant changes in the shape of the estimated power spectral density in the band encompassing 10 Hz to approximately 100 Hz at distances of practical value from the sensors. This suggests that arrays of more closely spaced sensors might provide better bearing accuracy at practically useful distances from a tornado. Additionally, a mathematical model, based on established relationships of aeroacoustic turbulence, is demonstrated to provide good agreement to the estimated power spectra produced by the tornadoes at different times and distances from the sensors. The results of this analysis indicate that, qualitatively, an inverse relationship appears to exist between the frequency of an observed peak of the power spectral density and the reported tornado intensity.

Active control of acoustical beamforming with an electrode array on a piezoelectric rubber composite

An ‘‘active’’ method of controlling acoustical beamforming with a linear array of electrodes configured on a composite piezoelectric rubber sheet is demonstrated. The active control is achieved by applying a dc bias on the electrode elements. The electroded regions of the piezorubber sheet exhibit piezoelectric activity proportional to the magnitude of the bias. By selectively biasing the electrodes, the corresponding locales of the array are forced to be active or inactive (piezoelectrically) for electroacoustic synergism. The ability to control the piezoelectric response of individually electroded regions provides a method to change the wave vector response of the electroacoustic transducer array. Tailored beamforming is therefore, made plausible with appropriate dc bias applied to selective electrodes. This ‘‘active’’ method could be useful in ‘‘smart’’ beamforming strategies and ‘‘active’’ control of vibrations. Test results on a prototype sample are presented as the proof of the concept.

Detection of infrasonic energy from tornado-producing storms

There are numerous reports in the literature on the observation of infrasound emitted from tornadic thunderstorms. Most of these observations have been made from sensors that are several hundreds of kilometers from the location of the storm, and “ground truth” about the tornadic activity is not well established. We report here on a campaign carried out during the summer of 2011 in which 50 infrasound microphones were deployed, as part of an ongoing multi-university program on hazard detection and alert funded by the National Oceanic and Atmospheric Administration. Sensors were placed along the paths of developing tornadic storms. We focus here on a severe weather outbreak that took place near Oklahoma City on May 24, 2011, in which a total of 7 tornados including one F5 and two F2 tornados were produced. Three sensors were located between the paths of an F4 and an F5 tornado, and 11 additional sensors were located northeast of an F4 tornado that generated a 75-km track. Substantial meteorological informat...